Legacy Effects of Biodegradable Mulch and Soil Amendments on Vegetable Crops and the Soil

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Elise V.H. Reid University of Nebraska

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LEGACY EFFECTS OF BIODEGRADABLE MULCH AND SOIL

AMENDMENTS ON VEGETABLE CROPS AND SOIL

Elise V. H. Reid

A THESIS

Presented to the Faculty of

The Graduate College at the University of Nebraska

In Partial Fulfillment of Requirements

For the Degree of Master of Science

Major: Agronomy

Under the Supervision of Professor Sam Wortman

Lincoln Nebraska

November, 2019

LEGACY EFFECTS OF BIODEGRADABLE MULCH AND SOIL

AMENDMENTS ON VEGETABLE CROPS AND THE SOIL

Elise Victoria Helen Reid, M.S.

University of Nebraska, 2019

Advisor: Samuel E. Wortman

Plastic film mulches are used in horticulture to manage weeds, improve water retention, and increase soil temperature. Bioplastics and biofabrics are potentially sustainable alternatives to plastic film; however, they have different rates of in soil degradation. Polylactic acid (PLA) is a 100% biobased polymer that degrades slowly, but could fulfill organic certification to be soil incorporated.

Mater-Bi is a commercially available biodegradable plastic (bioplastic), which degrades quickly, but cannot be incorporated in organic systems. Our objectives were to determine the individual and combined effects of soil amendments and residual mulch on vegetable crop yield and soil fertility. In a two-year study across two climatically diverse locations, a novel biodegradable PLA-based mulch with embedded wood fiber particles was compared to MaterBi bioplastic mulch after soil incorporation. Four soil amendments, compost, compost extract, cover crops, and a combination of the three, were applied with the goal of accelerating the rate of mulch degradation. Compost amendment significantly

increased sweet corn yield by 34-43% and macronutrient availability in 2018 (N-

71%, P-75%, K-16.9%) compared to all other treatments at Scottsbluff.

Biomulch residues in soil did not influence sweet corn crop yield (p<0.05).

Cabbage yield increased in compost treatments in 2019 at Lincoln (p=.0045), and decreased in plots with cover crops (p<0.0001). Soil tensile strength, wet aggregate stability, sorptivity, and compaction were measured 6 and 18 months after soil incorporation of mulch. The PLA biofabric increased water stable macroaggregates compared to the control at both locations in the spring of 2019, whereas the bioplastic decreased macroaggregates compared to the control at

Lincoln. Organic amendments improved soil sorptivity both years at Scottsbluff and in 2019 at Lincoln. Initial results of this study suggest that the effects of biobased mulch residues on soil macronutrients and yield are inconsequential compared to the effects of compost application. Biodegradable PLA based mulches may have a positive effect on soil properties after soil incorporation, although they are slow to degrade even with added amendments.

Acknowledgements:

Thank you to Ben Samuelson for his considerable contributions, as well as Gary

Stone for management at Scottsbluff. Thank you also to fellow graduate students

Chris Chen, Raihanah Hassim, and Eliott Gloeb for help during the field season.

This project is based on research that was partially supported by the Nebraska

Agricultural Experiment Station with funding from the Hatch Act (accession

1014303) through the USDA National Institute of Food and Agriculture (NIFA), and the USDA NIFA Organic Transitions Program (award 2016-51106-25711).

Table of Contents CHAPTER 1, RELATIVE INFLUENCE OF BIODEGRADABLE MULCH

RESIDUE AND ORGANIC SOIL AMENDMENTS ON VEGETABLE CROP

YIELD AND SOIL FERTILITY ...... 9

1.1 INTRODUCTION ...... 9

1.2 MATERIALS AND METHODS ...... 13

1.2.1 SITE CHARACTERISTICS AND EXPERIMENTAL DESIGN ...... 13

1.2.2 CROP MANAGEMENT ...... 14

1.2.3 DATA COLLECTION ...... 16

1.2.4 TREATMENT APPLICATION ...... 17

1.2.5 MULCH DEGRADATION ...... 18

1.2.6 SOIL SAMPLING ...... 19

1.2.7 STATISTICAL ANALYSIS ...... 20

1.3. RESULTS AND DISCUSSION ...... 20

1.3.1 YIELD ...... 21

1.3.2 SOIL MACRONUTRIENTS AND ORGANIC MATTER ...... 23

1.4 CONCLUSION ...... 26

FIGURES AND TABLES ...... 28

FIGURE 1.1. NOVEL POLYLACTIC ACID BASED MULCH...... 28

FIGURE 1.2 SPLIT-SPLIT PLOT DESIGN ...... 29

WHOLE PLOT: PLA OR BIO360 MULCH; SPLIT-PLOT: MULCH ...... 29

FIGURE 1.3. 2018 SWEET CORN YIELD BY SOIL AMENDMENT AT BOTH

LOCATIONS...... 30

FIGURE 1.4. SOIL ORGANIC MATTER (%) AT BOTH LOCATIONS OVER THE

EXPERIMENTAL PERIOD...... 31

TABLE 1.1 AS IS ANALYSIS OF COMPOSTS FOR 2017 AND 2018. YARDWASTE

COMPOST WAS USED AT LINCOLN AND MANURE BASED COMPOST AT SCOTTSBLUFF. 32

TABLE 1.2. CORRELATIONS OF LITTER BAG MULCH RESIDUE AND YIELD

COMPONENTS AND MACRONUTRIENTS...... 33

TABLE 1.3. YIELD OF ‘CARMEN” PEPPERS FOR TWO LOCATIONS

SEPARATED INTO GRADE AND MULCH TYPE...... 33

TABLE 1.4. SWEET CORN YIELD MEASUREMENTS IN 2018...... 35

TABLE 1.5. CABBAGE YIELD MEASUREMENTS FOR 2019...... 36

TABLE 1.6. MACRONUTRIENT LEVELS AT LINCOLN AND SCOTTSBLUFF

FROM THE START OF THE EXPERIMENT IN THE FALL OF 2017 THROUGH

2018...... 37

TABLE 1.7. POTENTIAL CARBON INPUT FROM MULCHES CALCULATED

FROM THE AMOUNT OF MULCH REMAINING IN LITTER BAGS AT FOUR

SAMPLING PERIODS...... 42

TABLE 1.8. CALCULATED CARBON INPUT FROM CORN, RYE, AND OAT. ... 44

REFERENCES ...... 45

CHAPTER 2, RESPONSE OF SOIL PHYSICAL PROPERTIES TO

BIODEGRADABLE MULCH RESIDUE AND SOIL AMENDMENTS IN AN

ORGANIC VEGETABLE SYSTEM ...... 68

2.1. INTRODUCTION...... 68

2.2. MATERIALS AND METHODS ...... 72

2.2.1 SITE CHARACTERISTICS AND EXPERIMENTAL DESIGN ...... 72

2.2.3 TREATMENT APPLICATION ...... 75

2.2.3 MULCH RESIDUE REMAINING ...... 76

2.2.4 SOIL SAMPLING ...... 78

2.2.5 STATISTICAL ANALYSIS ...... 81

2.3. RESULTS AND DISCUSSION ...... 81

2.3.1 ORGANIC MATTER ...... 81

2.3.2 WATER STABLE AGGREGATES ...... 82

2.3.3 SORPTIVITY ...... 85

2.3.4 SOIL TENSILE STRENGTH ...... 86

2.3.5 CONE INDEX ...... 87

2.5. CONCLUSION ...... 88

FIGURES AND TABLES ...... 90

FIGURE 2.1. POLYLACTIC ACID MULCH SPLIT INTO LAYERS...... 90

FIGURE 2.2 SPLIT-SPLIT PLOT DESIGN...... 91

WHOLE PLOT: PLA OR BIO360 MULCH; SPLIT-PLOT: MULCH ...... 91

FIGURE 2.3. MUSTARD COVER CROP IN 2018...... 92

FIGURE 2.4. LINCOLN RYE COVER CROP 14 MAY 2019 PRIOR TO AND AFTER

TERMINATION...... 93

A) SINK AND COVER CROP PLOTS B) DENSITY OF RYE COVER CROP FROM ABOVE VIEW

C) PLOTS ON THE LEFT AFTER TERMINATION, ON THE RIGHT NOT YET TERMINATED D)

RYE AT FULLY EMERGED HEAD STAGE...... 93

FIGURE 2.5. RYE/VETCH COVER CROP PRIOR TO TERMINATION AT

SCOTTSBLUFF 3 JUNE 2019...... 94

FIGURE 2.6. SOIL ORGANIC MATTER (%) AT BOTH LOCATIONS OVER

EXPERIMENTAL PERIOD WITH AMENDMENTS...... 95

FIGURE 2.7. PERCENT OF WATER STABLE MACROAGGREGATES (>250 µM)

IN THE SPRING OF 2019 AT EACH LOCATION...... 96

FIGURE 2.8. SOIL AGGREGATE AT 40X MAGNIFICATION WITH EMBEDDED

PLA FIBERS...... 97

FIGURE 2.9. SORPTIVITY OF SOIL OVER BOTH YEARS AVERAGED OVER

SOIL AMENDMENT TYPE...... 98

FIGURE 2.10. CONE INDEX AT SCOTTSBLUFF FOR MULCH BY TREATMENT

INTERACTION IN 2018...... 99

TABLE 2.1 COLLECTION PERIOD OF SAMPLES FOR SOIL PHYSICAL

PROPERTIES AND ORGANIC MATTER...... 100

TABLE 2.2. CORRELATIONS BETWEEN MULCH RESIDUE REMAINING AND

SOIL PHYSICAL PROPERTIES ...... 101

TABLE 2.3. CORRELATIONS BETWEEN SOIL PROPERTIES...... 100

TABLE 2.4. SOIL TENSILE STRENGTH ANALYZED SEPARATELY FOR EACH

LOCATION AND TIME...... 101

REFERENCES ...... 102 9

CHAPTER 1, RELATIVE INFLUENCE OF BIODEGRADABLE MULCH

RESIDUE AND ORGANIC SOIL AMENDMENTS ON VEGETABLE CROP

YIELD AND SOIL FERTILITY

1.1 Introduction

Vegetable growers often rely on polyethylene films for weed management, increased soil temperature, and water retention (Tarara, 2000). In addition, polyethylene films increase yields for several crops (Moreno and Moreno, 2008; Anzalone et al., 2010; Price et al., 2018; Zhang et al., 2019). In the U.S., 162,000 hectares of land was covered in polyethylene mulch in 2006 (Hayes et al., 2012). As a result, vegetable producers in the

U.S. dispose of 130, 000 metric tons per year of polyethylene (Shogren and Hochmuth,

2004). If polyethylene films or fragments are not completely removed from the field, they can persist for hundreds of years in the soil (Ohtake et al., 1998), release toxins, and adsorb pollutants, which can then could enter water bodies or fauna (Feuilloley et al.,

2005; Ashton et al., 2010; Wang et al., 2013; Briassoulis et al., 2015). However, there are few mulch options that offer similar benefits to polyethylene mulches and comply with National Organic Program certification guidelines. Certified organic mulches include non-synthetic, untreated materials and paper. Plastic films may also be used if they are entirely removed from the field, which is logistically impractical (§205.601,

National Organic Program Federal Regulations).

The organic farming community is increasingly interested in biodegradable mulches, especially 100% biobased mulches. In a survey of Tennessee, Texas, and Washington growers, 50% were interested in research collaboration on biodegradable mulches

(Goldberger et al., 2013). Unpredictable biodegradable mulch degradation during the field season was identified as a problem for almost all growers and 45% of the growers did not believe the mulches were totally biodegradable once incorporated (Goldberger et al., 2013). Biodegradable plastics made from MaterBi material can offer similar benefits to polyethlylene and can be incorporated in conventional, non-organic systems (Caruso et al. 2019; DeVetter et al. 2018; Moore-Kucera et al. 2014; Costa et al. 2014). Although, bioplastic mulches are being increasingly used their short field life and synthetic additives limit their usefulness in organic systems (Wortman et al., 2015). To be certified by the National Organic Program, biodegradable mulches must be microbially degraded

90% or more within two years (ASTM D 5988/ISO 17556) and cannot contain any synthetic or petroleum-derived products. One possible biobased solution for organic farmers is polylactic acid (PLA) based biofabrics that have been prototyped and tested in several crops (Miles et al., 2012; Siwek, Domagała-Świątkiewicz and Kalisz, 2015;

Wortman, Kadoma and Crandall, 2015; Martín-Closas et al., 2016; Ghimire et al., 2018).

PLA fabrics have faced limited adoption due to slow degradation (Wortman, Kadoma and Crandall, 2015; Dharmalingam et al., 2016; Thompson et al., 2019), but material engineering advancements offer promising solutions. Not only does the regular surface of

PLA make it more impervious to microbial attack (Douglas G Hayes et al., 2012), but the microbes responsible for PLA degradation also have a lower abundance in the soil compared to other microbes (Pranamuda et al., 1997). PLA-based mulches deteriorate at varying rates according to how the fibers are produced; spunbond PLA is produced by extruding a polymer melt and spinning it into fibers whereas meltblown PLA is produced when a thermoplastic polymer is put through an extruder die (Dharmalingam et al.,

2016). Other material modifications such as the addition of alfalfa and soy byproducts into the PLA mulch matrix also increase biodegradation (Thompson et al., 2019).

Alternatively, the recent addition of wood particles to PLA mulch may also increase in- soil mulch degradation as well.

A second potential method to increase degradation of biodegradable mulches is the addition of soil amendments. Amendments such as compost facilitate faster degradation of PLA in comparison with soil or sterilized soil (Hakkarainen, Karlsson and Albertsson,

2000; Karamanlioglu and Robson, 2013). Cover crops are also known to stimulate microbial biomass and activity related to degradation of crop residues, therefore they may be another option for speeding the degradation of remaining incorporated PLA (Nivelle et al., 2016; Zheng et al., 2018; Barel et al., 2019). Compost extract, defined as a non- fermented liquid obtained from filtering a compost (Scheuerell and Mahaffee, 2002), has comparable microbial populations to solid compost in studies using incubated compost extracts (Shrestha et al., 2012; Milinkoví et al., 2019). However, there will be some selectivity due to the extraction process and environment. This microbial community may be useful for increasing PLA mulch biodegradation.

Additionally, compost has been shown to increase yields (Nguyen and Wang, 2016;

Long, Brown and Amador, 2017; Wortman et al., 2017; Ceglie et al., 2018; Gheshm and

Brown, 2018). Using manure compost for four or more years increases N, K, P and soil carbon compared to conventional fertilizers (Bedada, Lemenih and Karltun, 2016;

Herencia and Maqueda, 2016). Cover crops are frequently used in organic agriculture due to their benefits if managed well. Rye cover crops promote nutrient cycling benefits by increasing soil inorganic nitrogen during the growing season, likely due to midseason N

12 mineralization of residue, as well as managing nitrogen leaching (Drinkwater and Snapp,

2007; Snapp and Surapur, 2018). Cover crops also provide a large underground input of carbon from roots and root exudates (Angers and Caron, 1998). Additionally, cover crops improve soil infiltration through the creation of macropores and improved aggregate stability, which promotes nutrient uptake by plants (Mitchell et al., 2017; Sindelar et al.,

2019).

The inclusion of agricultural byproducts to PLA can also decrease the C/N ratio of biodegradable mulch, thus minimizing the risk of nitrogen immobilization. PLA alone has a C/N ratio of 1692, but with the addition of alfalfa this drops to 35 (Thompson et al.,

2019). In contrast, Mater-Bi biodegradable plastic mulch has a C/N ratio of 610

(Thompson et al., 2019). C/N ratios can be useful predictors for N immobilization.

Immobilization occurs in high C/N environments where microbial communities are efficient at scavenging for nitrogen and store or immobilize it within their cells

(Mooshammer et al., 2014). Mulches or cover crops with a higher C/N ratio, such as rye or straw, can lead to N immobilization and potentially limit N availability to following cash crops (Parr and Papendick, 1978; Landesman and Dighton, 2010; Reichel et al.,

2018; Sun et al., 2018). For low-input organic systems, awareness of high C/N inputs is particularly important for synchronizing soil nutrient supply with crop physiological demands.

The degradation of PLA or other biodegradable mulches may also offer benefits to soil health and crop yield. A high carbon PLA mulch with embedded wood fibers could provide benefits similar to wood chips or other high carbon organic soil amendments.

Incorporated wood chip mulches can improve soil organic carbon, soil fertility, plant

13 growth, and increase crop yield and quality (Robert et al., 2014; Li et al., 2018a).

Reduced tillage and high carbon inputs such as straw can increase soil organic matter

(Powlson et al., 2008; Xia et al., 2018). An increase in organic matter can then lead to higher yields (Nguyen and Wang, 2016; Mondini et al., 2018).

The objective of this study was to evaluate the effects of soil-incorporated PLA-based biofabric (with embedded wood particles) and Bio-360 bioplastic (Mater-Bi material) mulch residues in combination with organic soil amendments on crop yield and soil fertility. We hypothesized that: 1) Crop yield may be negatively affected by high carbon mulch residues due to low N input in organic production and potential nitrogen immobilization 2) biodegradable mulch residue will increase soil organic matter similar to high carbon, woody soil amendments, 3) soil macronutrients such as nitrate, may be immobilized by the high carbon mulch residues, 4) soil amendments such as compost, cover crops, and compost extract may increase crop yield through improved soil fertility and water infiltration.

1.2 Materials and Methods

1.2.1 Site characteristics and experimental design

The experiment was conducted at two climatically distinct locations in Nebraska between 2017 and 2019 as a randomized complete block, split-split plot design with three replicate blocks. The first site was at University of Nebraska-Lincoln East Campus (40.84

N, 96.65 W elevation=351m). The soil at the site is Zook silty clay loam, which is a fine, smectitic, mesic Cumulic Vertic Endoaquolls. Lincoln has a humid continental climate, average precipitation 735 mm, and an average temperature of 10.8 °C. The second location was at the Panhandle Research and Extension Center in Scottsbluff, Nebraska

(41.89 N, 103.68 W). The soil there is a Tripp very fine sandy loam, which is a coarse- silty, mixed, superactive, mesic Aridic Haplustoll. Scottsbluff has a semi-arid climate with an average precipitation of 399 mm, and an average temperature of 9.3 °C. The research plots were managed according to the United States Department of Agriculture

National Organic Program guidelines (Electronic Code of Federal Regulations, 2019).

The mainplot was mulch type applied; either a Mater-Bi bioplastic (Bio360, St.

Remi, QC, Canada) or a novel polylactic acid based biofabric. The PLA-based mulch is comprised of two layers of spunbond PLA (Fig. 1.1), with a meltblown interior matrix that includes embedded fine-grade wood particles (3M, Saint Paul, MN). The split-plot was incorporation of mulch, where the mulch was either soil-incorporated (INC) via a reciprocating spader implement (Celli; Forli, Italy) or removed from the field after use during one growing season as a control (CTL). The split-split plots (SSP) were treated with soil amendments and included: compost (COM), a cover crop (COV), compost extract (EXT), a combination of all of the amendments (SNK), and a control with no amendments (NA). Each split-split plot was 1.83 m x 5.49 m (FIGURE 2).

1.2.2 Crop management

In spring of 2017, prior to crop planting, both mulch types were field-applied via a raised bed mulch layer (RB448; Nolt’s Produce Supplies) with dripline applied under the mulch for irrigation. There were six rows total; three were covered in the polylactic acid based biofabric (PLA) and three with the Mater-Bi bioplastic (BIO). Peppers

(Capiscum anuum cv. ‘Carmen’) were started from seed in the greenhouse 8 weeks before transplanting into the field 16 May at Lincoln and 6 June at Scottsbluff. Peppers

15 were spaced with 46 cm between plants and 12 plants per SSP. Water-soluble fish emulsion (OrganicGem, NPK 2.9-3.5-0.3) was applied at a rate of 0.3 g N, 0.36 g P, and

0.03 g P per planting hole at planting and twice more throughout the season at the same rate. At the second and third fertilization events blood meal (13-1-0) was added at a rate of 84 kg/ha. Mulch prevented weeds in the rows and weeds were managed with hand- hoeing between rows. Peppers were harvested mid-July through the first week of October and yield data was recorded for each row and mulch type. After the 2017 pepper harvest, the crop residue was shredded and left in the field. Mulch was incorporated and soil amendments applied at this time, as described in detail in sections 2.2 and 2.3.

In the spring of 2018, organic soybean meal fertilizer (Phyta-Grow, 7-1-2) was applied at a rate of 67 kg N/ha prior to soil sampling and to seeding. No mulch was applied and the field was not tilled. Sweet corn (Zea mays cv. ‘Xtra-Tender 2171) was sown on 24 May and 31 May, at Lincoln and Scottsbluff respectively, in two rows per SSP with ~20 cm spacing between seeds and ~0.6 m between rows. Hand-hoeing was implemented for weed management. Sweet corn was harvested 23 July and 30 July in Lincoln and 27-28

August at Scottsbluff. Total and marketable yield was measured for each SSP; five corn ear subsamples from each plot were used to measure marketable quality by corn ear length, width, circumference, lack of disease or damage, similar color characteristics, and maturity (USDA, 1962). Fall residue was shredded 12 Sept. and 21 Sept. for Lincoln and

Scottsbluff, respectively.

In spring 2019, cabbage (Brassica oleraceae var. sabauda cv. ‘Melissa’) was started from seed in the greenhouse on 29 March for Lincoln and on 12 April for

Scottsbluff. At both locations a white on black polyethylene mulch was applied for weed

16 management. The experimental areas were not tilled, nor were beds created, to avoid disturbing biomulch residues incubating in litter bags (see Samuelson, 2019). At both locations fertilizer was applied prior to planting as bloodmeal (Earthworks Health LC,

13-1-0) at a rate of 34 kg N/ha. Cabbages were transplanted 16 May at Lincoln and 5

June at Scottsbluff. At Lincoln and Scottsbluff, cabbages were transplanted with 12 plants per split-split plot and 46 cm spacing. Fish emulsion (Neptune’s Harvest, MA,

USA, 2-3-1) was applied at Scottsbluff after transplant at a rate of 0.46 g N, .69 g P, and

.23 g K per planting hole. Fish emulsion at Lincoln was applied at the same dilution

(1:15) but through an injector. Weeds and grass were mowed in the alleys. Due to imported cabbageworm (Pieris rapae) and cabbage looper (Trichoplusia ni) pressure, Bt

(GardenSafe Spectrum Brands, Middleton, WI), was sprayed once a week at a rate of 3.9 mL Bt/1 L of water from 24 June through harvest at Lincoln and from 30 July through harvest at Scottsbluff. Cabbage was harvested 5 August at Lincoln and 19 August at

Scottsbluff. Cabbage was separated into marketable (dense heads with no insect damage) and cull categories.

1.2.3 Data Collection

In 2017, a SPAD chlorophyll meter (Konica Minolta, Inc, Tokyo) was used to measure leaf greenness (a proxy for chlorophyll content; Markwell, Osterman and

Mitchell, 1995) at three points throughout the season at Lincoln and twice at Scottsbluff.

In 2018, SPAD data was collected at V4 and V8 at Lincoln and at V4 at Scottsbluff. In

2019, SPAD was taken 24 June prior to cabbage head formation at Lincoln only. For all years Watermark (Campbell Scientific, Logan, Utah) soil matric potential readings were

17 taken once per week to manage irrigation with the goal of keeping the matric potential below 60 kPa, or the management allowable depletion (Datta et al., 2017).

1.2.4 Treatment application

After the 2017 pepper harvest, compost extract (EXT) was made by kneading

5.86 kg of compost in a 450 µm nylon mesh bag in 94.6 L of water. Extract was applied at a rate of 3.79 L per SSP with a hand sprayer to the pepper plants and soil. Extract was re-applied every six months at the same rate, to the soil in the spring, and to soil and plant residue in the fall. The entire experimental area was then tilled with two passes by an articulating spader to incorporate the mulches into the INC split-plots and incorporate the crop residue into all plots. Yard waste compost was surface applied at a dry rate of 57

Mg/ha at Lincoln in the fall of 2017, and at a rate of 60 Mg/ha in the fall of 2018. Beef feedlot compost was surface applied at a dry weight of 42 Mg/ha at Scottsbluff in fall

2017 and 51 Mg/ha at Scottsbluff in the fall of 2018. Compost rates were adjusted for an application of 500 kg/ha of N. The very high rate of compost was applied to impact the degradation of mulch (for results, see Samuelson, 2019) and to maximize the chance of seeing an effect due to compost. Chemical analysis of composts is in Table 1.1.

In the spring of 2018 a mustard cover crop (Brassica rapa var. Mighty Mustard ®

Pacific Gold) was sown at 22.4 kg/ha on 22 March at Lincoln, and re-sown 20 April after the cover crop failed to emerge. At Scottsbluff the mustard cover crop was planted on 23

April and compost extract was applied on 26 April. The cover crop was terminated on 23

May at Lincoln by flail mower and on 30 May with hoes at Scottsbluff at 5-6.4 cm height, BBCH stage 14-15 (Commission, 2019), and covered approximately 80% of soil

18 at both locations. In mid-September 2018 cover crop seed was broadcast-planted at a rate of 44.8 kg/ha vetch (Vicia villosa) and 112 kg/ha rye (Secale cereale).

The rye/vetch cover crop was terminated 14 May 2019 by flail mower at Lincoln and 4 June at Scottsbluff. At both locations the rye/vetch cover crop was terminated at the fully emerged head state for rye, BBCH stage 59 (Lorenz et al., 2001), and approximately

0.86m tall; vetch was flowering. Soil coverage from the rye/vetch mixture (dominated by rye) was close to 100% at Lincoln. At Scottsbluff cover crop plots had 90-100% coverage, but with varying percentages of the rye/vetch mixture and weeds

1.2.5 Mulch degradation

Biomulch degradation over time was measured as mulch residue remaining. This data was used to generate an estimate of the potential carbon input of residual mulch and as a potential predictor of changes in yield and macronutrients due to mulch residue. On

29 September 2017 at Lincoln and 5 October 2017 at Scottsbluff mulches were incorporated into the soil with a spading machine implement (Celli Y70 spading machine, Celli, Forli, Italy). In the fall of 2017, the collected control mulches were washed similarly to a process by Ghimire et al. (2017), using running water and 10 and

35 mesh sieves to capture washed mulch fragments prior to taking initial weights and reburial. Cleaned mulches from the control plots were cut into 144 10 × 10 cm squares for each mulch type and weighed on an analytic balance. The first weight measurement in spring of 2018 was taken with an analytical balance and compared to initial weight.

However, due to degradation and the fragility of mulch fragments thereafter, this was not considered the most accurate method. Fall 2018 weight loss measurements were

19 determined by a mass loss on ignition (ashing) process. To ash the samples, a known mass of soil was oven dried at 60C and then heated in a muffle furnace that increased up to 550C for two hours before sustaining that temperature for an additional four hours.

The furnace stopped heating and the temperature then decreased for 8-10 hours to 130C.

Samples were removed and the mass of ash (minerals) remaining was measured. This mass was used to calculate a ratio between ash to dry soil according to the process outlined in Samuelson et al. (2019). The same process was followed with the fresh PLA and Bio360 mulch samples from the litter bags.

The following formula was used to determine grams of mulch in the dry sample of recovered mulch fragments.

푃 − 푆 퐺 × 푀 − 푆

G – mass in grams of 60C oven dried sample of soil and recovered mulch

P – fraction of sample mass remaining after ignition

S – fraction of soil mass remaining after ignition

M – fraction of mulch mass remaining after ignition

1.2.6 Soil Sampling

At each 6-month sampling date, soil chemistry samples were collected as 8 soil cores (1.9 cm diam. by 20 cm) per split-split plot. Soil cores were combined into one sample and a fraction was sent to Ward Labs (Kearney, Nebraska) for KCl extraction of nitrate, Mehlich 3 test for phosphorous and potassium, and loss of weight on ignition for

20 organic matter (Recommended Chemical Soil Test Procedures for the North Central

Region, 1998).

1.2.7 Statistical analysis

A generalized linear mixed model analysis of variance (GLIMMIX procedure in

SAS 9.4; SAS Institute, Cary, N.C.) was used to evaluate main effects and interactions of mulch type, mulch incorporation status, and soil amendments on soil nutrient levels, organic matter, and yield. To analyze mulch weight remaining at each time point, a paired t-test was performed to compare the weight remaining between PLA and BIO samples collected from litter bags. Correlations were performed with rcorr in R (Version

3.6.1, The R Foundation) between the mass of mulch residue from litter bags and yield and macronutrients. The average weights of BIO and PLA were then used to estimate the carbon content of mulch residue. Fixed effects were mulch, management, and soil amendment. Random effects were block, block by mulch, and block by mulch by management. Each location was analyzed separately due to confounding effects of planting dates and location-specific cultural practices. Data was combined across main effects when there were no significant interactions. The Tukey-Kramer multiple comparisons test was used to determine differences between least squares means at significance level of α=0.05. Orthogonal contrasts were used for specific hypotheses.

Pepper yields were analyzed with a t-test as there were only two mulch treatments applied.

1.3. Results and Discussion

1.3.1 Yield

In 2017, pepper yield was compared between the two surface applied mulches

(PLA and BIO). Peppers grown had no significant difference between mulch types for total, marketable, or cull yield (Table 1.1); soil amendments had not yet been applied to the experiment. This is similar to another study where bell pepper yield was statistically similar between three of four PLA biofabrics and two bioplastics (Wortman, Kadoma and

Crandall, 2015).

In 2018, there were no significant ANOVA effects of mulch, mulch incorporation, or soil amendments on sweet corn yield at Lincoln (Fig. 1.3), although there was a significant correlation between PLA and marketable ears at Lincoln (r=0.477, p=0.045).

However, at Scottsbluff, while there were no effects from mulch or incorporation, and no correlations between mulch residue and yield components (Table 1.2), there was a soil amendment effect. The total sweet corn yield increased in COM and SNK amendments in

2018. Total yields were 27%-32% and 21%-27% higher for COM and SNK at

Scottsbluff, respectively, compared to all other soil amendments (p<0.001, Fig. 1.1).

Yield was measured as the weight of all picked ears from a split-split plot. At Scottsbluff,

COM and SNK amendments also increased marketable yield (p<0.0001), ear number

(p=0.0001), marketable ears (0.0003), stand adjusted yield per plant (p<0.0001), ear length (p=0.0023) and ear circumference (p<0.0001, Table 1.3). Stand count was significantly higher in SNK and COM plots compared to NA, EXT, and COV (p<0.0001,

Table 1.2). The yield increase due to compost is consistent with other studies (Nguyen and Wang, 2016; Long, Brown and Amador, 2017; Wortman et al., 2017; Ceglie et al.,

2018; Gheshm and Brown, 2018). Lincoln likely did not have an increase in sweet corn

22 yield, as it had no significant difference in soil nutrients among treatments, unlike at

Scottsbluff (Table 1.6). The soil at Scottsbluff also has a higher pH and lower macronutrients and organic matter compared to Lincoln. However, the different composts used in our study, animal manure based at Scottsbluff and municipal yard waste based at

Lincoln, could partially be responsible for the yield difference between locations as well.

Results from a meta-analysis found animal based composts to increase first-year yield more than yard waste (Wortman et al., 2017). However, we tried to minimize feedstock- specific differences through adjusting compost rates based on N input.

In 2019 at Lincoln, there were no mulch or incorporation effects, nor any significant correlations between mulch and yield components (Table 1.2). Soil amendment did have an effect on yields though. COM increased cabbage marketable yield (kg) 15% compared to the control (p=0.0045) while SNK and COV decreased yield

46% and 49%, respectively, compared to the control (p<0.0001 for both, Table 1.5.).

There were more cull cabbages in SNK compared to COM (p=0.0056, Table 1.5). As for marketable head number, COM, EXT, and NA were statistically equal and higher than

COV and SNK (p<0.0001). The low yield in SNK and COV was due to transplanting into heavy cover crop residue. Similar low yield results in vegetable and grain production have been found in other studies where vegetables were transplanted into cover crop residues, which may be due to nutrient immobilization or allelopathy (Leavitt et al.,

2011; Boydston and Williams, 2017; Abdalla et al., 2019; Domagała-Świątkiewicz et al.,

2019). The rye residue also tore holes in the polyethylene mulch and prevented the mulch from lying flat on the soil, thus shading new transplants. Nitrate levels were also significantly lower in COV and SNK at the spring planting period for cabbage (see

23 section 3.2). There were no significant mulch, incorporation, or treatment effects at

Scottsbluff on marketable yield (kg), cull yield (kg), marketable number of heads, nor number of cull cabbage heads (Table 5). There was no negative effect from cover crops at

Scottsbluff, unlike at Lincoln. The rye was not as dense at Scottsbluff as at Lincoln, rather, there were more weeds in COV plots, which were easier to mulch over than rye stubble. The mulch had a closer fit to the ground because of this, which would have lessened shading of new transplants. And although COV plots had significantly lower N

(section 1.3.2), SNK had similar levels to COM plots. There were no correlations between mulch residue and yield (Table 1.2).

1.3.2 Soil macronutrients and organic matter

At Lincoln, COM and SNK had the greatest impact on macronutrients, while mulch, incorporation, and any interactions did not have any effect. In the spring of 2018

COM was significantly higher in nitrate than COV (p=0.0034), but by the fall of 2018 the difference was no longer significant. In the fall of 2018 SNK was significantly higher than NA (p=0.0016), however, in the spring of 2019 SNK and COV had the lowest nitrate levels of all treatments and COM had the highest (p<0.0001). By the fall of 2019, both COM and SNK were significantly higher in nitrate than other amendments (main effect of amendment p=0.0084). SNK appeared to bounce back in the fall due to the addition of compost compared to COV.

Cover crops decreased phosphorous in the soil, however, the application of compost mitigated the decrease. At Lincoln in the spring of 2019 phosphorous was lowest in COV and NA (Table 1.6). In the fall of 2019, P levels in COV and NA were

28% lower than SNK and 18% lower than COM (Table 1.6). Other studies have shown that oat and vetch cover crops reduce soil phosphorous (Beltrán et al., 2018; Herencia,

2018). However, the addition of compost seemed to mitigate low P in cover crop plots.

Throughout the experiment P was highest in SNK, followed by COM (Table 1.6). The combination of cover crops and manure based composts has been used to limit over- fertilization of phosphorous in hybrid systems (Maltais-Landry and Crews, 2019).

Although Lincoln used a yard waste rather than manure-based compost, based on these results it appeared to add sufficient P to alleviate cover crop use of P.

K was significantly higher at Lincoln in SNK compared to NA and COV in the fall of 2018 (p=0.0036 and p=0.0069). By the fall of 2019, both COM and SNK had significantly more K than the other treatments (Table 1.6). Organic matter was not influenced by mulch, but by spring 2019 organic matter levels started to significantly increase in COM and SNK over other amendments (p<0.0001 main effect of amendment,

Figure 1.6). At the conclusion of the experiment, organic matter content in the soil had increased 0.75% and 0.63% from 2017 for SNK and COM, respectively. There was a slight negative correlation between BIO mulch residue and organic matter in the spring of

2018 (Table 1.2), however, this slight trend did not reappear at later sampling dates.

Compost significantly affected macronutrient levels and organic matter at

Scottsbluff as well, whereas mulch, incorporation, and any interactions had no effect.

There were also no correlations between mulch residue and macronutrients or organic matter at Scottsbluff (Table 1.2). Nitrate levels fluctuated throughout the study, but in

2019 spring and fall COM and SNK had the highest levels of nitrate (p<0.0001 both sampling periods). At both the spring samplings, COV had the lowest nitrate levels

(p<0.0001). Phosphorous levels were the highest in COM and SNK throughout the study

(p<0.0001 for every sampling date, Table 1.6) and by the last sampling date COM phosphorous had increased 2.7 times the 2017 value and SNK 2.6 times (Table 1.6). The effect of P due to cover crops and compost was similar as to Lincoln, although by the fall of 2019 the high P levels of the manure compost increased P levels at Scottsbluff compared to Lincoln. K exhibited a similar response as P, remaining the highest in COM and SNK (p<0.0001 for each sampling date) through the entire study and increasing 39% from 2017, while other amendments were similar to the control throughout the experiment (Table 1.6). Organic matter levels were highest in COM and SNK in 2018;

TEA was statistically equivalent to SNK in the spring, and COV statistically equivalent to COM in the fall (Figure 1.4). By 2019, COM and SNK treatments had the greatest amount of organic matter (p<0.0001) in the spring and the fall.

Cover crops in COV and SNK had a negative effect on nitrate levels in the spring of

2019 at Lincoln and COV only at Scottsbluff (Table 1.6). The nitrate loss is consistent with other studies demonstrating that rye or rye mix cover crops either immobilize nitrogen or have lower levels of nitrogen compared to other cover crop mixtures (Ranells,

Wagger and Ranells, 1996; Williams et al., 2018). While the 2019 rye/vetch cover crop was a mixture, it was dominated by rye; although at Scottsbluff the mix was more even with rye and weeds. Studies have shown rye cover crops without a legume release less nitrogen after termination compared to other cover crops (Ranells et al., 1996; Parr et al.,

2014) There was no evidence that nitrogen (as nitrate) was immobilized with incorporated PLA, which has happened with high C:N amendments such as woodchips

(TerAvest et al., 2010) and rye cereal crops (Williams et al., 2018) and even other PLA

26 based mulches (Samuelson et al., 2019). The lack of N immobilization in our study could be due to the slow degradation and therefore slow release of C from the PLA-based mulch. Based on biomass estimates of corn stover, rye, and oat (Morton, Bergtold and

Price, 2006; Sawyer and Mallarino, 2007; Shinners and Binversie, 2007; Balkcom et al.,

2019), the PLA had a much higher potential carbon input than those crops (Table 1.7,

Table 1.8). Despite a wood-fiber layer, high carbon input, and high C/N ratio, the incorporation and degradation of the PLA-based biomulch did not influence crop yield and quality, soil organic matter, or soil fertility. This is contrast to studies of other woody or high C soil amendments (Powlson et al., 2008; Robert et al., 2014; Li et al., 2018b;

Xia et al., 2018). However, just as importantly, results indicate that the tested biomulches are unlikely to immobilize plant essential macronutrients and contribute to stunted crop growth or reduced yield in organic systems.

1.4 Conclusion

Soil amendment had a greater effect on crop yield than did the incorporated mulch.

Compost increased yields in 2018 at Scottsbluff and in 2019 at Lincoln. Compost also increased macronutrient levels compared to other amendments. Alternatively, cover crops did have some negative effects on macronutrients. Phosphorous levels decreased in 2019 at both locations with cover crops, although if combined with compost this effect was ameliorated. The rye/vetch cover crop also had a detrimental effect on spring nitrate and cabbage yield in 2019 at Lincoln.

After soil incorporation, the novel PLA-based biofabric mulch and the bioplastic mulch had no negative effect on either subsequent sweet corn or cabbage yields. Neither

27 mulch type immobilized nitrate or otherwise affected macronutrients. There were no large, significant correlations between mulch residue remaining and crop yield or soil macronutrients. Therefore, although the mulches may persist in the soil, they do not have any negative effect on yield or soil macronutrients. Novel PLA-based biomulches may be a sustainable option for vegetable production in the future. Although degradation losses failed to reach 90% in the two years required for NOP certification (results in Samuelson,

2019), they may be ideal for perennial systems due to their durability. PLA mulches could withstand multiple years of use, after which, they could be safely tilled into the soil or composted.

Figures and Tables

FIGURE 1.1. NOVEL POLYLACTIC ACID BASED MULCH.

White outer layers consist of spunbond PLA while the black, inner layer is comprised of meltblown PLA embedded with wood particles (3M Corporation, St. Paul, MN).

FIGURE 1.2 SPLIT-SPLIT PLOT DESIGN

Whole plot: PLA or Bio360 mulch; split-plot: mulch management, removed or soil incorporated; split-split plot: soil amendment/treatment, compost, compost tea, cover crop, and a combination of the three “sink.” The upper rectangle would be superimposed on the bottom rectangle in the field. Adapted from Samuelson, 2019.

FIGURE 1.3. 2018 SWEET CORN YIELD BY SOIL AMENDMENT AT BOTH

LOCATIONS.

Each plot was 1.83 m x 5.49 m with two rows of corn. Different letters indicate significant differences as analyzed by the Tukey-Kramer multiple comparisons test at a significance level of α=.05. Error bars represent standard error.

FIGURE 1.4. SOIL ORGANIC MATTER (%) AT BOTH LOCATIONS OVER THE

EXPERIMENTAL PERIOD.

Treatments are NA-control, COM-compost; COV- spring planted mustard 2018 or rye/vetch 2019 cover crop; SNK- compost, cover crops, compost extract; and EXT- compost extract. Different letters indicate significant differences between amendments within each time point as analyzed by the Tukey-Kramer multiple comparisons test at a significance level of α=0.05.

TABLE 1.1 AS IS ANALYSIS OF COMPOSTS FOR 2017 AND 2018. Yardwaste

compost was used at Lincoln and manure based compost at Scottsbluff.

Manure Year Property Yardwaste based 2017 pH 8.2 9.1 Total N 11.9 15.3 P, % P2O5 6.1 12.3 K, % K2O 13.7 26.6 % S 2.1 6 % Ca 31.1 42.5

2018 pH 8.4 8.9 Total N 9.6 13.6 P, % P2O5 6.8 27 K, % K2O 14.3 24.1 % S 2.1 5.6 % Ca 26 47.4 1

TABLE 1.2. CORRELATIONS OF LITTER BAG MULCH RESIDUE AND YIELD

COMPONENTS AND MACRONUTRIENTS.

Significant correlations, p<0.05, highlighted in red

TABLE 1.3. YIELD OF ‘CARMEN” PEPPERS FOR TWO LOCATIONS

SEPARATED INTO GRADE AND MULCH TYPE.

Peppers were grown with Bio360 mulch (BIO) or a novel polylactic acid based mulch

(PLA). Letters indicate significance using a paired t-test at a significance level of α=0.05.

2017 Pepper Yield

Grade Mulch kg SE

Lincoln Marketable BIO 510.1a 21.8

PLA 488.6a 17.5

Cull BIO 57.1a 5.1

PLA 58.8a 4.2

Scottsbluff Marketable BIO 225.1a 31.6

PLA 221.6a 29.6

Cull BIO 22.3a 5.2

PLA 19.8a 4.5

TABLE 1.4. SWEET CORN YIELD MEASUREMENTS IN 2018. Treatments are COM-compost; SNK- compost, cover crops, compost extract; COV- spring planted mustard cover crop; EXT- compost extract; NA-control. Different letters indicate significant differences between amendments within locations as analyzed by the Tukey-Kramer multiple comparisons test at a significance level of α=0.05.

TABLE 1.5. CABBAGE YIELD MEASUREMENTS FOR 2019. Treatments are COM-compost; SNK- compost, cover crops, compost extract; COV- spring planted mustard cover crop; EXT- compost extract; NA-control. Different letters indicate significant differences between amendments within locations as analyzed by the Tukey-Kramer multiple comparisons test at a significance level of α=0.05.

TABLE 1.6. MACRONUTRIENT LEVELS AT LINCOLN AND SCOTTSBLUFF

FROM THE START OF THE EXPERIMENT IN THE FALL OF 2017 THROUGH

2018.

Treatments are COM-compost; SNK- compost, cover crops, compost extract; COV- spring planted mustard cover crop; EXT- compost extract; NA-control; IRR-fallow irrigation. Different letters indicate significant differences between amendments within a sampling event as analyzed by the Tukey-Kramer multiple comparisons test at a significance level of α=0.05.

Macronutrients

Sampling Lincoln Amendment N P K OM date

Fall 2017 COM 12.6a 80ab 394a 4.1a

SNK 14.4a 99a 451a 4.3a

COV 11.0a 81ab 366a 3.7a

EXT 13.2a 87ab 418a 4.0a

NA 12.3a 80b 401a 3.9a

Standard 1.5 7.3 22.7 0.25 Error

Spring COM 17.2a 75a 372a 3.9a 2018

SNK 15.1ab 89a 402a 4.0a

COV 12.9b 74a 339a 3.4a

EXT 14.5ab 80a 371a 3.6a

NA 14.7ab 75a 363a 3.6a

Standard 1.4 6.2 20.1 0.23 Error

Fall 2018 COM 6.9ab 75ab 383a 4.0a

SNK 7.3a 93a 426a* 4.0a

COV 5.9ab 75ab 350a 3.6a

EXT 6.1ab 79ab 382a 3.6a

NA 5.4b 74b 354ab* 3.7a

Standard 0.68 6.5 26.8 0.25 Error

Spring COM 8.2a 92b 431b 4.7ab 2019

SNK 2c 115a 523a 5.1a

COV 1.98c 72c 329.6c 3.9c

EXT 5.2b 78bc 372bc 4.1bc

NA 4.3b 72c 336c 4.1bc

Standard 0.63 6.1 27.1 0.24 Error

Fall 2019 COM 7.5 a 98 ab 438 a 4.8 ab

SNK 7.5 a 111 a 485 a 5.0 a

COV 6.4 ab 80 c 327 b 4.1 b

EXT 5.2 ab 87 bc 354 b 4.2 b

NA 5.0 b 80 c 336 b 4.3 b

Standard 0.79 8.4 22 0.26 Error

Macronutrients

Year Amendment N P K OM

Fall 2017 COM 11.6a 39a 417a 1.5a

SNK 11.8a 39a 423a 1.4a

COV 6.4b 27b 332b 1.3b

EXT 7.1b 26b 354b 1.3b

NA 6.55b 24b 344b 1.3b

Standard Error 0.7 1.5 9.08 0.02

Spring 2018 COM 4.6a 35a 450a 1.6a

SNK 1.75c 37a 452a 1.6a

COV 1.9c 23b 382b 1.5b

EXT 3.7b 23b 398b 1.5ab

NA 3.4b 21b 398b 1.5b

Standard Error 0.30 1.8 9.12 0.03

Fall 2018 COM 4.1a 29a 479a 1.7ab

SNK 5.1a 29a 478a 1.7a

COV 6.2a 14b 396b 1.6bc

EXT 4.7a 14b 413b 1.6c

NA 4.6a 14b 411b 1.6c

Standard Error 0.98 1.2 9.74 0.30

Spring 2019 COM 2.6a 99a 541a 1.8a

SNK 2.3a 103a 523a 1.7a

COV .8c 26b 335b 1.4b

EXT 1.4b 26b 347b 1.5b

NA 1.3bc 27b 350b 1.4b

Standard Error 0.20 6.5 14.3 0.04

138. Fall 2019 COM 7.5ab 582.3a 1.95a 4a

146. SNK 590.5 2.0a 10.7a 1a

33.8 COV 324.7b 1.6b 5.5b b

31.9 EXT 4.7b 323.7b 1.6b b

32.7 NA 4.6b 329.4b 1.6b b

Standard Error 1.4 5.6 13.9 0.04

TABLE 1.7. POTENTIAL CARBON INPUT FROM MULCHES CALCULATED

FROM THE AMOUNT OF MULCH REMAINING IN LITTER BAGS AT FOUR

SAMPLING PERIODS.

PLA= polylactic acid based mulch with two outer spunbond layers and an inner meltblown layer embedded with wood particles; BIO= Bio 360 commercial biodegradable plastic mulch made of Mater-Bi material.

Average litter bag Remaining potential

mulch mass C from mulch

Lincoln Year Mulch remaining (g) (g/m^2)

2018S PLA 1.2021 a 573.6

BIO 0.2086 b 127.2

2018F PLA 1.2021a 573.6

BIO 0.0135 b 8.2

2019S PLA 1.2265 a 585.3

BIO 0.000b 0.0

2019F PLA 1.17 a 558.3

BIO 0.000b 0.0

Scottsbluff 2018S PLA 1.7493 a 834.8

BIO 0.1598 b 97.5

2018F PLA 1.22 a 582.2

BIO 0.208 b 126.9

2019S PLA 1.2822 a 611.9

BIO 0.0897 b 54.7

2019F PLA 1.2431 a 593.2

BIO 0.0922 b 56.2

TABLE 1.8. CALCULATED CARBON INPUT FROM CORN, RYE, AND OAT.

Estimates based off of biomass from other studies (Morton, Bergtold and Price, 2006;

Shinners and Binversie, 2007; Balkcom et al., 2019). Converted to kg/ha of biomass and then grams of C/m2.

Cover crop C input (g/m^2)

Rye 227

Corn Stover 360

Oat straw 204.4

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67 community composition, networks, and potential genes involved with degradation of crop residues in soil’, Biology and Fertility of Soils, 54(6), pp. 743–759. doi: 10.1007/s00374-

018-1298-1.

CHAPTER 2, RESPONSE OF SOIL PHYSICAL PROPERTIES TO

BIODEGRADABLE MULCH RESIDUE AND SOIL AMENDMENTS IN AN

ORGANIC VEGETABLE SYSTEM

2.1. Introduction

Plastic film mulches are frequently used in fruit and vegetable crop production to increase soil temperature, manage weeds, and to retain soil water (Tarara, 2000). Plastic mulches have also been shown to increase yields in several crops (Moreno and Moreno,

2008; Anzalone et al., 2010; Hou et al., 2014). In the United States, plastic mulch covers approximately 162,000 ha (Miles et al., 2012) while in Europe plastic mulch covers

427,000 ha (Bläsing and Amelung, 2018). However, due to the soil and plant residues remaining on the plastic films after field use, most plastic mulches cannot be recycled and are either sent to the landfill or burned (Douglas G. Hayes et al., 2012). As of 2004,

130,000 metric tons per year of plastic films were being disposed of after the field season in the U.S. (Shogren and Hochmuth, 2004). Even after removal of plastic mulches, it has been estimated that up to 11%, or 8.4 kg/ha of the plastic film, remains in the field as residue (He et al., 2018).

Biodegradable mulches offer a potentially sustainable alternative to plastic mulches. Starch based plastics, such as Mater-Bi, are being used as alternatives to polyethylene because they have similar physical properties and agronomic benefits as polyethylene (Martín-Closas, Bach and Pelacho, 2008; Costa et al., 2014; DeVetter et al.,

2018; Caruso et al., 2019b). However, bioplastics often have reduced durability during the field season (Wortman et al., 2015). An increase in microplastics from continually incorporated biodegradable mulches has also been reported (English, 2019; Hayes, 2019).

It is unknown whether these microplastics may have similar negative effects as other microplastics to soil properties such as soil aggregation (Abel De Souza Machado et al.,

2018; Liang et al., 2019).

Bioplastics, even starch-based ones, cannot be used in organic agriculture as they contain petroleum derived components (USDA, 2015). The National Organic Program

(NOP) of the United States Department of Agriculture requires that biodegradable mulches are non-synthetic, untreated material, or paper (National Organic Program

Federal Regulations, 2019). Polylactic acid (PLA) is a 100% biobased polymer, meaning any mulch produced from it has potential for soil incorporation on organic farms. PLA is used to produce biodegradable fabric mulches, but currently PLA mulches degrade slowly compared to bioplastic alternatives (Wortman, Kadoma and Crandall, 2015;

Dharmalingam et al., 2016). However, PLA degradation may be manipulated through material engineering. Spunbond and meltblown PLA biofabrics exhibit different degradation speeds, therefore new prototypes could potentially degrade more quickly

(Dharmalingam et al., 2016).

After soil incorporation, PLA mulches may provide a food source for microbes during decomposition. Fungi are more often found in soils with PLA (Janczak et al.,

2018). After 20 months in the soil, the mass of PLA films had increased 160% due to accumulation of minerals and mycelia during degradation (Karamanlioglu and Robson,

2013; Ghimire et al., 2017). Exudates from microbes or fungal mycelial fragments could promote soil aggregation via inputs of organic matter or physical processes. Increased organic matter in soil improves the water stability of aggregates through increased internal cohesion (Chenu et al., 2000). Even prior to soil incorporation, PLA-based

70 mulches may increase soil aggregation. In one surface applied mulch study, PLA mulch treatments had significantly more small water stable aggregates compared to the control and other mulches (Siwek et al., 2015). The extent to which biodegradable mulches affect soil aggregation is not well understood.

Aggregate stability is a dynamic indicator of soil erosion potential and changes in soil quality; it also is positively correlated with an increase in soil organic matter content

(Blanco-Canqui et al., 2009; Nichols and Toro, 2010; Domagała-świątkiewicz and

Siwek, 2018). The addition of organic amendments can thus increase the water stability of aggregates (Chenu et al., 2000). Macroaggregate promoting practices are useful for balancing the negative impacts of tillage and cultivation in organic vegetable systems.

Intensive tillage in organic vegetable systems reduces the abundance of C-rich macroaggregates (Six et al., 2000; Blanco-Canqui et al., 2006), which could, in turn, reduce soil sorptivity and water infiltration (Taylor and Ashcroft, 1972; Wang et al.,

2017). The sorptivity of a soil affects the level of ponding, root function, and the availability of nutrients to plants (USDA NRCS, no date). Soil sorptivity is a measure of initial water infiltration and is strongly influenced by changes in soil management in the short term (Chan et al., 2001; Leelamanie and Manawardana, 2019).

Tensile strength of aggregates is another important soil structural property that affects crop productivity as it impacts root penetration and development, soil physical processes, soil friability, soil fragmentation, and soil tilth (Utomo and Dexter, 1981; Guerif, 1990;

Kay et al., 1994; Munkholm, 2015; Blanco-Canqui, 2017). Tensile strength is especially affected by tillage and subsequently is responsible for how much force needs to be applied in future tillage to break the soil. An increase in soil organic matter content often

71 reduces the tensile strength of aggregates, contributing to better soil tilth or workability

(Blanco-Canqui, 2017).

Addition of organic soil amendments could potentially improve degradation of biodegradable mulches while enhancing soil physical quality. For example, the addition of compost increases soil organic matter, and compared to non-organic amendments, may decrease soil tensile strength and improve wet aggregate stability (Abdollahi et al., 2014).

Similarly, the inclusion of cover crops has been shown to increase soil aggregation and water infiltration rates, particularly in the long term (Mitchell et al., 2017; Williams et al., 2017). Cover crops provide additional biomass input such as roots, which increase soil organic matter, activate soil fauna, and provide organic binding agents, enhancing soil aggregation (Six et al., 2004, Comin et al., 2018; Williams et al., 2018). Root exudates from cover crops can stimulate soil microbes; and even without microbial influence exudates will adhere soil particles together (Morel et al., 1991; Six et al.,

2004). Compost extracts are another amendment of increasing interest to organic farmers, in part for their potential disease-suppressive properties (Scheuerell and Mahaffee, 2002;

Pant et al., 2011; Hanc et al., 2017). Compost extracts have similar microbial communities as solid compost (Shrestha et al., 2012; Milinkoví et al., 2019). However, there will be some selectivity due to the extraction process and environment. We were particularly interested in using extract to speed the degradation of the mulches (results reported in Samuelson, 2019). However, their effects on soil aggregation and other soil physical processes have not been widely studied.

In this study we compared a novel polylactic acid biofabric (PLA) to a starch- based bioplastic, Bio360 (BIO). In addition, four soil amendments, compost, compost

72 extract, cover crops, and all three combined, were added to influence mulch degradation.

Soil incorporation of the high carbon PLA could yield benefits similar to high carbon amendments such as wood chips or straw: improved soil water status, increased organic matter, and enhanced microbial activity (Powlson et al., 2008; Prober et al., 2014; Li et al., 2018a).

The objective of this study was to determine the effects of biodegradable mulch residues with and without soil amendments on soil physical properties in organic vegetable systems. We hypothesized that: 1) the incorporated PLA mulch will increase soil organic matter content, soil aggregation, and thus increase soil sorptivity and reduce the tensile strength of soil aggregates; 2) incorporated Bio360 may decrease soil aggregation based on the effects of other microplastics; 3) addition of soil amendments such as compost and cover crops will increase soil aggregation, sorptivity, organic matter content, and reduce tensile strength; and 4) the combination of mulches with compost and/or cover crops will improve soil properties more than mulch or organic amendment alone.

2.2. Materials and methods

2.2.1 Site characteristics and experimental design

The experiment was conducted at two climatically distinct locations in Nebraska between 2017 and 2019. The first site was at University of Nebraska-Lincoln East

Campus (40.84 N, 96.65 W, elevation=351 m, humid continental climate). The soil at the site is Zook silty clay loam, which is a fine smectitic, mesic Cumulic Vertic Endoaquoll.

The second site was the Panhandle Research and Extension Center in Scottsbluff,

Nebraska (41.89 N, 103.68 W, elevation=1198 m, semi-arid climate). The soil at the second site is a Tripp very fine sandy loam, which is a coarse-silty, mixed, superactive, mesic Aridic Haplustoll.

The experimental design at each site was a randomized complete block split-split- plot design with three replicate blocks at both locations. The main plot was biomulch type: either a Mater-Bi bioplastic (BIO) (Bio360, St. Remi, QC, Canada) or a novel polylactic acid (PLA)-based biofabric. The biofabric was composed of two outer layers of spunbond polylactic acid (PLA, FIGURE 2.1) with a middle meltblown layer loaded with fine wood particles (3M Company; St. Paul, Minnesota, USA). The split-plot biomulch incorporation; either a control where biomulch was removed from the field after one growing season (CTL) or incorporated in soil after the initial growing season

(INC) with a reciprocating spading machine (Celli; Forli, Italy). The split-split-plots

(SSP) were comprised of five soil amendments: no amendment (NA), compost (COM), cover crop (COV), compost extract (EXT), and a combination of compost, cover crops, and compost extract (SNK). Each SSP was 1.8m x 5.5m (FIGURE 2).

2.2.2 Crop management

Both mulches were field-applied in spring 2017 via a raised bed mulch layer

(RB448; Nolt’s Produce Supplies) with drip irrigation lines under the mulch. Three rows were covered with PLA and three with BIO. Pepper (Capsicum anuum L. cv. ‘Carmen’) seedlings were grown from seed in a heated greenhouse for 8 weeks prior to transplanting. Peppers were transplanted 16 May 2017 at Lincoln and 6 June 2017 at

Scottsbluff with 46 cm between plants and 12 plants per split-split-plot. Water-soluble

74 fish emulsion (OrganicGem, NPK 2.9-3.5-0.3) was applied at a rate of 0.3 g N, 0.36 g P, and 0.03 g P per planting hole at planting and twice more throughout the season with the addition of blood meal (13-1-0, 84 kg/ha). Watermark readings were taken once a week to determine soil matric potential and manage irrigation by keeping the matric potential below the management allowable depletion, or 60 kPa (Datta et al., 2017). Weeds were controlled with mulch within the rows and hand-hoeing in the between-row alleys.

Pepper residue was shredded post-harvest and remained in the field. Mulch was incorporated and soil amendments applied at this time, as described in detail in sections

2.2 and 2.3.

In spring of 2018 no mulch was applied. Soybean meal fertilizer (Phyta-Grow, 7-

1-2) was applied at a rate equivalent to 164 kg N/ha. The experimental area was not tilled prior to seeding to prevent disturbance of litter bags buried in each plot for measuring biomulch degradation (data not included in this paper). On 24 May 2018 at Lincoln and

31 May 2018 at Scottsbluff, sweet corn (Zea mays cv. ‘Xtra-Tender 2171) was planted in two rows per plot with approximately 20 cm between seeds. Weeds were maintained by hand hoeing and irrigation was applied to keep the matric potential below 60 kPa as measured by Watermark readings. Sweet corn was harvested on 23 July and 30 July 2018 at Lincoln and 27-28 August 2018 at Scottsbluff.

Cabbage (Brassica oleraceae var. sabauda cv. ‘Melissa’) was started in the greenhouse 29 March 2019 for Lincoln and 12 April 2019 for Scottsbluff transplants.

Prior to transplanting the experimental area was neither tilled nor were raised beds made.

Blood meal (Earthworks Health LC, 13-1-0) was applied prior to transplanting at a rate of

34 kg N/ha. Cabbage was transplanted on 16 May at Lincoln and on 5 June at Scottsbluff

75 with 46 cm spacing and 12 plants per split-split-plot. Due to the much lower nitrate at

Scottsbluff, fish emulsion (Neptune’s Harvest, MA, USA, 2-3-1) was applied immediately after transplant at a rate of 0.46 g N, 0.69 g P, and 0.23 g K per planting hole. Weeds were managed with polyethylene mulch within rows and mowing the weeds in the alleys. Bt (GardenSafe, Spectrum Brands) was sprayed at a rate of 3.9 mL/L of water weekly at Lincoln starting 24 June and starting 30 July at Scottsbluff to control

Pieris rapae (imported cabbageworm) and Trichoplusia ni (cabbage looper). Cabbage was harvested 5 August at Lincoln and 19 August at Scottsbluff. All polyethylene mulch was removed from the experimental area.

2.2.3 Treatment application

Upon completion of the 2017 harvest, mulch was removed by hand from the control split-plots. Compost extract (EXT) was then made by kneading 5.86 kg of a yardwaste compost at Lincoln and beef feedlot manure compost at Scottsbluff in a 450

µm nylon mesh bag in 95.6 L of water. EXT was applied at a rate of 3.79 L per split- split-plot and re-applied every six months for a total of four applications. The experimental area was tilled with two passes of an articulating spader to incorporate pepper residue as well as mulch into the incorporated split-plots. Yard waste compost was applied at a dry weight rate of 57 Mg/ha at Lincoln and a beef feedlot manure compost was applied at a dry weight rate of 42 Mg/ha at Scottsbluff. Compost rates were adjusted at each location to achieve an application rate of approximately 500 kg/ha of total N. The extremely high rate of compost was applied to impact the degradation of the

76 mulch (for results see Samuelson, 2019) and to maximize the chance of seeing an effect due to compost.

In the spring of 2018, a mustard cover crop (Brassica rapa var. Mighty Mustard

® Pacific Gold) was sown at 22.4 kg/ha on March 22nd at Lincoln, and re-sown 20 April after the cover crop failed to emerge. The mustard cover crop was planted and compost extract applied on 23 April and on 26 April respectively at Scottsbluff. The cover crop was terminated on 23 May at Lincoln by flail mower and on 30 May with hoes at

Scottsbluff. The mustard was approximately 5.0-6.4 cm tall, at BBCH stage 14-15

(Commission, 2019) and covered approximately 80% of soil at both locations (Figure

2.3). In mid-September 2018 cover crop seed was broadcast-planted at a rate of 44.8 kg/ha vetch (Vicia villosa) and 112 kg/ha rye (Secale cereale). Compost was applied at a dry weight of 60 Mg/ha at Lincoln and 51 Mg/ha at Scottsbluff in September. Compost rates were again adjusted for an application of 500 kg total N/ha.

At both locations the rye/vetch cover crop was terminated at the fully emerged head state for rye, BBCH stage 59, and approximately 0.86m tall (Fig. 2.4); vetch was flowering. Soil coverage from the rye/vetch mixture (dominated by rye) was close to

100% at Lincoln. At Scottsbluff cover crop plots had 90-100% soil coverage, but with varying percentages of the rye/vetch mixture and weeds (Fig. 2.5). Termination was 14

May 2019 by flail mower at Lincoln and 4 June 2019 at Scottsbluff.

2.2.3 Mulch residue remaining

Biomulch degradation over time was measured as mulch residue remaining. This data was used as a potential predictor of changes in soil physical properties and organic

77 matter over time. On 29 September 2017 at Lincoln and 5 October 2017 at Scottsbluff mulches were incorporated into the soil with a spading machine implement (Celli Y70 spading machine, Celli, Forli, Italy). In the fall of 2017 we washed the collected control mulch similarly to a process by Ghimire et al. (2017), using running water and 10 and 35 mesh sieves to capture washed mulch fragments prior to taking initial weights and reburial. The cleaned mulches from the control plots were cut into 144 10 × 10 cm squares for each mulch type and initial weights taken with an analytical balance. The first weight measurement after incorporation was in the spring of 2018. Mulch was again washed, weighed with an analytical balance, and compared to initial weight. However, due to degradation and the fragility of mulch fragments thereafter, this was not considered the most accurate method. Fall 2018 weight loss measurements were determined by a mass loss on ignition (ashing) process. Mulch squares from the mesh bags were oven dried at 60C and then heated in a muffle furnace that increased up to

550C for two hours before sustaining that temperature for an additional four hours. The furnace stopped heating and the temperature then decreased for 8-10 hours to 130C.

Samples were removed and the mass of ash (minerals) remaining was measured. Ash content of PLA was 0.45% and 0.17% for BIO. Due to the variability in soil ash content, mulch-free soil ash content was determined for each mesh bag. Further details on the methods are reported in Samuelson et al., 2019.

The following formula was used to determine grams of mulch in the dry sample of recovered mulch fragments.

푃 − 푆 퐺 × 푀 − 푆

G – mass in grams of 60C oven dried sample of soil and recovered mulch

P – fraction of sample mass remaining after ignition

S – fraction of soil mass remaining after ignition

M – fraction of mulch mass remaining after ignition

2.2.4 Soil sampling

Soil samples were collected every six months as 8 soil cores (1.9 cm diam.) per split-split plot to a depth of 20 cm (Table 2.1). They were combined into one sample per

SSP and approximately 200 g of the combined sample was sent to Ward Labs (Kearney,

NE) for measurements of organic matter by loss of weight on ignition (Recommended

Chemical Soil Test Procedures for the North Central Region, 1998).

Soil sorptivity was measured 6 months (spring of 2018) and 18 months (spring of

2019), after soil incorporation of biomulch as outlined by Smith (1999). Metal rings (10.5 cm height, 9.8 cm diameter) were carefully pressed into undisturbed soil and 75 mL of water was poured into the ring along the edge to minimize soil disturbance. Time until infiltration of the 75 mL of water was recorded. Four total readings per split-split plot were taken. For SNK plots, two measurements were taken with a plant within the ring, and two without a plant within the ring. Soil sorptivity was calculated with:

S=H/(t)1/2

H – head of water in cm t – time in seconds.

Soil tensile strength was measured for all plots in the spring of 2018 and spring of

2019 with a force meter using a rounded, flat head (Wagner FDX Force Ten, USA) following the methods of Öztas et al. (1999) and Dexter and Kroesbergen (1985). Due to variability, 20 air-dried aggregates between 8 mm and 4.75 mm were crushed individually between a flat metal disk sitting on a metal plate and the force meter head.

The force needed to crush each aggregate was recorded. Tensile strength was calculated from

Y= 0.576 (F/D2)

Y – tensile strength, (kPa)

0.576 – the coefficient of proportionality between the applied compressive load and the inner tensile strength of the aggregate

F – force required to break the aggregate, (N)

D – diameter of the aggregate, (m)

(Dexter and Kroesbergen, 1985)

Soil penetration resistance as an indicator of compaction was measured in the spring of 2018 and spring of 2019 using a hand cone penetrometer (Wagner FDX Force

Ten, 8 mm cone diameter). Penetration resistance was measured per SSP to a depth of approximately 5 cm with a speed of approximately 1 cm/s. The readings were converted to cone index based on the basal cone area (Rakkar et al., 2017). (Rakkar et al., 2017). At

80 the Lincoln site, soil water content was measured in spring 2018 with a TDR probe

(FieldScout TDR 300, Specturm Technologies, Inc, IL, USA) to evaluate the relationship between the cone index and soil moisture and to make corrections for the effects of soil moisture on cone index (Busscher et al., 1997). At the Scottsbluff site, penetration resistance was measured 24 hours after a rain (3.2 cm) when the field was near field capacity in spring 2018. In 2019, we measured water content at both locations with a

TDR probe along with penetration resistance measurements.

Wet aggregate stability was determined by the wet sieving method to determine the proportion of macroaggregates (> 0.25 mm diameter) and microaggregates (< 0.25 mm diameter; (Nimmo and Perkins, 2002; Rakkar et al., 2017). Samples were collected from all plots in the spring of 2018 and only the extremes, SNK and NA, in the spring of

2019, as other amendments had no trends the previous year. Air-dried soil samples were dry sieved through an 8 mm sieve and 50g +/- 1g were placed on top of a stack of sieves.

The three sieves had openings of 2 mm, 1 mm, and 0.25 mm. Soil samples were wetted by capillary action for 10 minutes and then oscillated up and down 3 cm for ten minutes in a water tank at 30 strokes per minute. Aggregates remaining on each sieve were collected into pre-weighed beakers, which were then oven-dried at 105 ºC and weighed again. For the Scottsbluff samples, the amount of soil aggregates was corrected for sand content (Nimmo and Perkins 2002). Sand correction accounts for loose sand accumulated from sieving, which can cause inaccuracies in the ratio of total stable aggregates and make it difficult to distinguish differences in macroaggregates (Márquez et al., 2004).

Sand and aggregates were collected from each sieve, dried and weighed, then treated with

~30 mL of 0.5% (NaPO3)6 to break up aggregates. Samples were then washed in a 53-

81 micrometer sieve. The remaining sand was dried at 105 ºC, weighed, and used to correct the original aggregate mass collected on each sieve.

2.2.5 Statistical analysis

A generalized linear mixed model analysis of variance (GLIMMIX procedure in

SAS 9.4; SAS Institute, Cary, N.C.) was used to evaluate main effects and interactions of mulch, incorporation status, and soil amendments on soil organic matter and soil physical properties. Fixed effects were mulch, incorporation, and soil amendment. Data were analyzed by site due to confounding effects of planting dates and location-specific cultural practices. The same model was used for repeat measure analysis to compare the increase in tensile strength and soil compaction from 2018 to 2019. Data was combined across main effects when there were no significant interactions. The Tukey-Kramer multiple comparisons test and orthogonal contrasts were used to determine differences between least squares means and test hypotheses at significance level of α=0.05.

Correlations were performed with rcorr in R (3.6.1, The R Foundation).

2.3. Results and Discussion

2.3.1 Organic Matter

There were no mulch or mulch incorporation effects on organic matter, but there were significant soil amendment effects. At Lincoln in 2019, organic matter was significantly higher in COM and SNK treatments (Fig. 2.6). Organic matter increased in

COM and SNK from 4.15% and 4.25% to 4.8% and 5%, respectively, from fall 2017 to spring 2019, (p=0.0003 and p=<0.0001). The high organic matter content in COM and

SNK was expected given the compost input. BIO mulch residue had a weak negative correlation with organic matter (TTABLE 2.2). There was a weak positive correlation between organic matter, soil tensile strength, macroaggregate stability, and sorptivity at

Lincoln in 2018 and 2019 and a weak negative correlation with microaggregate stability

(TABLE 2.3). Soil organic matter at Scottsbluff was significantly greater (p<0.001) in

COM and SNK compared to other treatments in 2018 and 2019 (FIGURE 2.6). At

Scottsbluff in 2018, there was a weak positive correlation between organic matter and sorptivity, and a weak negative correlation with cone index (TABLE 2.3). In 2019, there was a slightly stronger positive correlation with organic matter and macroaggregates and a stronger negative correlation with organic matter and microaggregate stability (TABLE

2.3). Compost is known to increase soil organic matter in as few as one or two years, which improves soil aggregation (De León-González et al., 2000; Bronick and Lal, 2005;

Abdollahi et al., 2014; Scotti et al., 2016).

2.3.2 Water stable aggregates

There were no significant mulch, mulch incorporation, or soil amendment effects on water stable aggregates in spring 2018. However, in spring 2019 there was a mulch by incorporation by treatment interaction at Lincoln. Soil-incorporated PLA combined with all possible amendments (SNK; compost, compost extract, plus cover crops) had 13.3% more water stable macroaggregates than the SNK treatment without any soil-incorporated mulch (CTL) (p=.0495) (FIGURE 2.7). However, soil incorporated PLA with no amendment had significantly fewer macroaggregates than soil-incorporated PLA with

SNK (p=0.0278); indicating that both the amendment and mulch were important for

83 aggregation. In contrast to the PLA, soil-incorporated BIO mulch with SNK had 13.2% fewer macroaggregates compared to the SNK control where BIO was removed from the field (p=.0499). There was also a negative correlation between BIO mulch residue and macroaggregates at Lincoln (TABLE 2.2).

The role of PLA mulch in aggregate formation was evident at Scottsbluff where an increase in macroaggregates was found for PLA plots in both NA and SNK treatments. Incorporated PLA plots had 79% more water stable macroaggregates than control plots where PLA was removed (p=0.0392). At Scottsbluff NA plots had a significantly higher proportion of microaggregates (p=0.0283) than SNK. Also, plots with no incorporated PLA had significantly more microaggregates than plots with incorporated PLA (p=0.0392). BIO INC trended to have more microaggregates than PLA

INC (p=0.0379). There was a negative correlation between the percent of microaggregates and soil sorptivity in 2019 at Scottsbluff (Table 2.3). A higher percentage of microaggregates can increase soil erodibility, surface crusting, and soils with more microaggregates have lower organic carbon (Elliot, 1986; Munkholm, 2015).

Therefore, it is desirable to increase macroaggregation over microaggregation.

The increase in the amount of macroaggregates in PLA relative to BIO can be attributed to the increased fungal communities in soil with PLA (Karamanlioglu and

Robson, 2013; Janczak et al., 2018, Samuelson, 2019). It is well recognized that fungi contribute to macroaggregate formation (Beare et al., 1997; Bossuyt et al., 2001;

Väisänen et al., 2005). Therefore, an increase in fungi due to PLA presence could have promoted macroaggregate formation and stability in incorporated PLA plots. Particles of

PLA itself was visually observed (FIGURE 2.8) in aggregates, which possibly

84 contributed to adsorption of mineral particles and aggregation (Elliot, 1986; Six et al.,

2000; Domagała-świątkiewicz and Siwek, 2018). The increase in macroaggregates in

SNK plots over NA may reflect higher organic matter in SNK plots, as well as the use of cover crops. In both SNK and COV plots, the mustard cover crop in 2018 was only 5-

6.5cm high at BBCH stage 14-15, which likely explains the lack of effect in 2018.

However, the rye/vetch cover crop in 2019 reached approximately 0.86 meters in height and the fully emerged head stage in SNK and COV. Our results were similar to greenhouse studies that have found that ryegrass, vetch (Vicia villosa), and cover crops with a large root mass, can increase macroaggregate stability; often due to vesicular- arbuscular mycorrhizal hyphae which promote aggregate formation (Tisdall and Oades

1980; Linsler et al. 2016; Haynes and Beare 1997; Lucas et al., 2014). In our study there was a significant increase in soil organic matter in COM and SNK plots, more so than in the cover crop plots (Fig. 2.6).

Tilled fields are more prone to erosion, have fewer stable aggregates, and more microaggregates than conservation or no-till organic systems (Wright et al. 1999;

Tebrügge and Düring 1999; Blanco-Canqui et al. 2009; Tisdall and Oades 1980). Tillage is still frequently used in organic production due to yield losses from weed pressure in no-till (Schonbeck and Morse, 2007; Shirtliffe and Johnson, 2012; Larsen et al., 2014;

Vincent-Caboud et al., 2017). Therefore, aggregation is especially important in organic vegetable systems where tillage is employed. PLA-based mulch could potentially reduce the need for tillage in vegetable production by reducing weeds during the growing season. After the growing season, incorporated PLA mulch may continue to promote aggregate formation through fungal growth, potentially reducing the impact of tilling in

85 the future. BIO did show some slightly negative effects on soil aggregation; however, during two years they did not differ significantly from macroaggregate percentages of soil with incorporated PLA. Future research may be needed to see if incorporated BIO has a negative and cumulative effect on decreasing macroaggregates. At Scottsbluff, regardless of mulch type or incorporation status, SNK plots had a significantly higher proportion of macroaggregates than NA plots (13.3% compared to 9.2%, respectively).

The difference at Scottsbluff is likely due to the low organic matter of the soil; therefore, organic amendments had high potential to improve soil aggregation.

2.3.3 Sorptivity

There were no effects of mulch or mulch incorporation on sorptivity; however, soil amendments did have a significant effect. SNK treatments had higher sorptivity compared to NA treatments. In Lincoln in 2019, sorptivity in SNK increased by 60%

(p<0.0001) compared to the NA control at Lincoln (FIGURE 2.9). At Scottsbluff, the

SNK treatment had 18.8% and 38.7% higher sorptivity than the NA control in 2018 and

2019 (p=0.0089 and p=0.0057), respectively. Cover crops, especially with high residue amounts, can increase surface soil sorptivity (Shaver et al. 2013; Nouri et al. 2019;

Alvarez et al. 2017), but there are also examples where cover crops have no effect on soil sorptivity (Chan et al., 2001; Blanco-Canqui and Jasa, 2019; Sindelar et al., 2019).

The SNK treatment increased soil organic matter content due to repeated compost applications. Increased organic matter can lead to increased sorptivity (Butler and Muir,

2006), as well as soil aggregation (see section 3.1). Macroaggregation is an important factor in sorptivity. There was a positive correlation between macroaggregation and

86 sorptivity at Scottsbluff in 2019 (Table 2.3). Macroaggregates have been found to explain up to 73% of variability in sorptivity and to decreases in bulk density and increases in total porosity, which enhances infiltration (Shaver et al., 2013). Increased aggregate stability can improve sorptivity by decreasing the likelihood of pores clogging due to slaking or crusting (Mitchell et al., 2017).

2.3.4 Soil Tensile Strength

Mulch and mulch incorporation had no effect on tensile strength in either year or location. Soil amendments influenced soil tensile strength at Lincoln in 2019, but there was no effect at Scottsbluff in either year. In 2019 at Lincoln, compost reduced tensile strength the most, and significantly reduced tensile strength compared to NA (p=0.0229),

SNK (p=0.0355), and EXT (p=0.0009) treatments (TABLE 2.4). Tensile strength had a weak, positive correlation to cone index at Lincoln in 2019. The lack of change at

Scottsbluff is not entirely surprising as tensile strength is less easily manipulated by management compared to aggregate stability (Kay et al., 1994). One likely reason tensile strength increased at Lincoln, but not at Scottsbluff is that there is a positive relationship between tensile strength and soil clay content, and Lincoln has much higher clay content in the soil (Bilson Obour et al., 2018). Both locations did have a significant increase in tensile strength from 2018 to 2019. Lincoln plots increased from 123 kPa in 2018 to 157 kPa in 2019 (p=0.0017) across all main effects. Scottsbluff plots had an increase across all main effects of 64 kPa in 2018 to 157 kPa in 2019 (p<0.0001); however, there was also an interaction effect of incorporation by time. In 2018 at Scottsbluff, both CTL and

INC were statistically similar for tensile strength, but in 2019, INC had an average soil tensile strength of 176 kPa, significantly higher than CTL at 137 kPa (p=.0438).

INC plots at Scottsbluff had higher tensile strength for aggregates; this could indicate that incorporated mulches promote soil structure. However, it must also be considered that soil tensile strength has a complicated relationship with numerous interacting factors such as organic matter and clay content. Our study is consistent with others demonstrating high organic matter, like COM plots, is associated with a decrease in tensile strength (Guerif, 1990; Abdollahi et al., 2014). High organic matter is important for the production of aggregates, their stability, and friability (Macks et al.,

1996; Munkholm, 2015). However, organic matter may be less influential in the dry aggregate state, in which clay can be the dominating factor (Blanco-canqui and Lal,

2016). In highly cultivated systems, it is generally desirable to decrease the tensile strength of soils.

2.3.5 Cone Index

At Lincoln there were no significant differences in the cone index of the soil in

2018 or 2019 from either mulch, incorporation or soil amendment. From 2018 to 2019 compaction increased significantly across main effects from 0.72 MPa to 2.6 MPa

(p<.0001). In 2019, the soil strength reached 2.0 MPa, at which point plant growth can be inhibited (Kaspar and Taylor, 1994; Lin, He and Chen, 2016). Scottsbluff soil compaction also increased significantly from 0.75 MPa in the first year to 1.5 MPa in the second year (p<0.0001). In 2018 at Scottsbluff, BIO plots with COM had a lower cone index than the NA control (p=0.0468 and p=0.0227, respectively), and cone index was

88 trending lower in PLA NA plots compared to PLA SNK plots (FIGURE 2.10, p=0.0528).

From orthogonal contrast analyses, in Scottsbluff in 2019, cone index trended higher in

COV rather than SNK plots (p=0.0598). The lower compaction in plots with compost follows other studies where organic matter decreases compaction (Guerif, 1990;

Abdollahi et al., 2014). However, the trend with compaction in PLA plots being lower in

NA than SNK and the 2019 COV could be due to the rye cover crop. Other studies have found that rye can slightly increase compaction measured by penetration resistance

(Acuña and Villamil, 2014; Chen, Weil and Hill, 2014; Welch et al., 2016). Similar to these studies, the rye was still growing when we took compaction measurements. Plant uptake of water can increase wet-dry periods which in turn increases soil strength (Horn and Dexter, 1989; Angers and Caron, 1998). Our two year study also may not have provided sufficient time to see a decrease or difference in compaction due to a cover crop

(Jokela et al., 2009; Ren et al., 2019). The overall increase in compaction between 2018 and 2019 could be due to the no-till methods employed in our study to prevent mulch litter bags being destroyed or disturbed. No-till has been shown to increase surface compaction, especially in silty clay loam soils like at Lincoln (Lars J. Munkholm et al.

2003; Skaalsveen et al. 2019; Burgos Hernández et al. 2019; Hill and Cruse 1985).

2.5. Conclusion

Biodegradable mulches can improve some measures of soil physical properties post-incorporation, although this is contingent upon the composition of the mulch. The incorporated novel polylactic acid mulch in this study increased the proportion of water

89 stable macroaggregates after incorporation, especially in combination with organic amendments at a humid continental location. Macroaggregates are highly beneficial to soil structure, the prevention of erosion, and improved sorptivity. Bio 360 decreased macroaggregates, which may be something for growers to consider and researchers to further investigate in the future. Mulch type did not have an effect on any other measured soil property, rather soil amendments had the most significant short-term effects.

Compost decreased soil tensile strength and soil compaction, which can improve root growth and crop productivity. Organic amendments such as compost, and compost in conjunction with cover crops also increased soil organic matter during the experimental period. Organic matter can improve crop yields and soil structure. The rye cover crop increased surface compaction, but in combination with compost sorptivity and macroaggregation were improved. While there is not yet an ideal biodegradable mulch on the market, this study elucidates how two biodegradable mulches affect soil physical properties after incorporation. Growers can use this information to decide on which mulches or organic amendments to use in their operations.

Figures and Tables

FIGURE 2.1. Polylactic acid mulch split into layers.

Polylactic acid (PLA) based mulch split into layers. The two white layers are, spunbond

PLA sandwiching a black meltblown inner layer with embedded wood fiber particles.

FIGURE 2.2 SPLIT-SPLIT PLOT DESIGN.

Whole plot: PLA or Bio360 mulch; split-plot: mulch incorporation, removed or soil incorporated; split-split plot: soil amendment/treatment, compost, compost tea, cover crop, and a combination of the three “sink.” The upper rectangle would be superimposed on the bottom rectangle in the field. Adapted from Samuelson, 2019.

FIGURE 2.3. MUSTARD COVER CROP IN 2018.

Scottsbluff (left) and close view for size at Lincoln (right). Mustard was 5-6.5 cm tall at termination and varied around 80% coverage.

FIGURE 2.4. LINCOLN RYE COVER CROP 14 MAY 2019 PRIOR TO AND AFTER

TERMINATION.

A) Sink and cover crop plots B) Density of rye cover crop from above view C) plots on the left after termination, on the right not yet terminated D) Rye at fully emerged head stage.

FIGURE 2.5. RYE/VETCH COVER CROP PRIOR TO TERMINATION AT

SCOTTSBLUFF 3 JUNE 2019.

FIGURE 2.6. SOIL ORGANIC MATTER (%) AT BOTH LOCATIONS OVER

EXPERIMENTAL PERIOD WITH AMENDMENTS.

Amendments are NA-control, COM-compost; COV- spring planted mustard 2018 or rye/vetch 2019 cover crop; SNK- compost, cover crops, compost extract; and EXT- compost extract. Different letters indicate significant differences between amendments within each time point as analyzed by the Tukey-Kramer multiple comparisons test at a significance level of α=0.05.

FIGURE 2.7. PERCENT OF WATER STABLE MACROAGGREGATES (>250 µm) IN

THE SPRING OF 2019 AT EACH LOCATION.

At Lincoln there was an interaction with the sink amendment. BIO=Bio360 biodegradable plastic mulch PLA=polylactic acid biofabric mulch. INC=mulch was incorporated into the soil in the fall of 2017. CTL=no mulch in or on the soil since 2017.

SNK=compost, compost extract, and cover crop applied. Error bars represent standard error of means from lsmeans pairwise comparison; letters represent significant differences at α=0.05.

FIGURE 2.8. SOIL AGGREGATE AT 40X MAGNIFICATION WITH EMBEDDED

PLA FIBERS.

Courtesy of Ben Samuelson.

FIGURE 2.9. SORPTIVITY OF SOIL OVER BOTH YEARS AVERAGED OVER

SOIL AMENDMENT TYPE.

SNK=compost, compost extract, and cover crop applied; NA= control, no amendment applied. Error bars and letters represent significant differences analyzed by Tukey-

Kramer multiple comparison test at α=0.05.

FIGURE 2.10. CONE INDEX AT SCOTTSBLUFF FOR MULCH BY TREATMENT

INTERACTION IN 2018.

BIO=Bio360 biodegradable plastic mulch PLA=novel polylactic acid non-woven fabric mulch. NA=no soil amendment applied, COM= compost amendment, SNK= compost, compost extract, and mustard cover crop.

100

TABLE 2.1 COLLECTION PERIOD OF SAMPLES FOR SOIL PHYSICAL

PROPERTIES AND ORGANIC MATTER.

S-spring, F-Fall of either 2018 or 2019. COM-compost; COV- spring planted mustard

2018 or rye/vetch 2019 cover crop; EXT- compost extract; SNK- compost, cover crops, compost extract; and NA-control.

Mulch

Sample collection Measurement Plots incorporation

F17, S18, F18, S19, Soil organic COM, COV, EXT, Incorporated and

F19 matter SNK, NA control

Incorporated and

S18, S19 Soil sorptivity SNK, NA control

Soil tensile COM, COV, EXT, Incorporated and

S18, S19 strength SNK, NA control

COM, COV, EXT, Incorporated and

S18, S19 Soil compaction SNK, NA control

Wet aggregate COM, COV, EXT, Incorporated and

S18 stability SNK, NA control

Wet aggregate Incorporated and

F19 stability SNK, NA control

101

TABLE 2.2. CORRELATIONS BETWEEN MULCH RESIDUE REMAINING AND SOIL PHYSICAL PROPERTIES

101

100

TABLE 2.3. CORRELATIONS BETWEEN SOIL PROPERTIES.

Correlations highlighted in red are significant at α=0.05.

101

TABLE 2.4. SOIL TENSILE STRENGTH ANALYZED SEPARATELY FOR

EACH LOCATION AND TIME.

Different letters indicate significant differences between amendments within locations using the Tukey-Kramer multiple comparisons test at a significance level of α=0.05.

Soil Tensile Strength Location Year Mulch (kPa) Lincoln 2018 PLA 115.8a BIO 129.8a SE 12.8

2019 PLA 165.6a

BIO 129.8a SE 34.3

Scottsbluff 2018 PLA 70.4a BIO 57.7a SE 3.2

2019 PLA 153.0a BIO 159.8a SE 26.5

102

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