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10-2008 A comparison of food webs beneath C3- and C4-dominated Mathew Dornbush University of Wisconsin-Green Bay

Cynthia A. Cambardella United States Department of , [email protected]

Soil Foodweb, Inc.

James W. Raich IOWA STATE UNIVERSITY LIBRARY, [email protected]

Follow this and additional works at: http://lib.dr.iastate.edu/eeob_ag_pubs Part of the Ecology and Evolutionary Biology Commons, Commons, and the Sustainability Commons The ompc lete bibliographic information for this item can be found at http://lib.dr.iastate.edu/ eeob_ag_pubs/221. For information on how to cite this item, please visit http://lib.dr.iastate.edu/ howtocite.html.

This Article is brought to you for free and open access by the Ecology, Evolution and Organismal Biology at Iowa State University Digital Repository. It has been accepted for inclusion in Ecology, Evolution and Organismal Biology Publications by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. A comparison of soil food webs beneath C3- and C4-dominated grasslands

Abstract Soil food webs influence organic matter mineralization and availability, but the potential for to capitalize on these processes by altering soil food webs has received little tta ention. We compared soil food webs beneath C3- and C4-grass plantings by measuring bacterial and fungal and protozoan and abundance repeatedly over 2 years. We tested published expectations that C3 and (low /N) favor bacterial-based food webs and root-feeding , whereas C4 detritus (high lignin/N) and greater production favor fungal decomposers and predatory nematodes. We also hypothesized that seasonal differences in plant growth between the two types would generate season-specific differences in soil food webs. In contrast to our expectations, bacterial biomass and abundance were greater beneath C4 grasses, and we found no differences in fungi, amoebae, , or nematodes. Soil food webs varied significantly among sample dates, but differences were unrelated to aboveground plant growth. Our findings, in combination with previous work, suggest that preexisting soil properties moderate the effect of plant inputs on soil food webs. We hypothesize that high levels of provide a stable environment and energy source for soil and thus buffer soil food webs from short-term dynamics of plant communities.

Keywords , Fungi, Grasslands, Nematodes,

Disciplines Ecology and Evolutionary Biology | Soil Science | Sustainability

Comments This article is published as Dornbush, Mathew, Cynthia Cambardella, Elaine Ingham, and James Raich. "A comparison of soil food webs beneath C3-and C4-dominated grasslands." Biology and fertility of 45, no. 1 (2008): 73-81. doi: 10.1007/s00374-008-0312-4. Posted with permission.

Rights Works produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The onc tent of this document is not copyrighted.

This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/eeob_ag_pubs/221 Biol Fertil Soils (2008) 45:73–81 DOI 10.1007/s00374-008-0312-4

ORIGINAL PAPER

A comparison of soil food webs beneath C3- and C4-dominated grasslands

Mathew Dornbush & Cynthia Cambardella & Elaine Ingham & James Raich

Received: 20 August 2007 /Revised: 17 June 2008 /Accepted: 23 June 2008 /Published online: 16 July 2008 # Springer-Verlag 2008

Abstract Soil food webs influence organic matter miner- unrelated to aboveground plant growth. Our findings, in alization and plant nutrient availability, but the potential for combination with previous work, suggest that preexisting soil plants to capitalize on these processes by altering soil food properties moderate the effect of plant inputs on soil food webs has received little attention. We compared soil food webs. We hypothesize that high levels of soil organic matter webs beneath C3- and C4-grass plantings by measuring provide a stable environment and energy source for soil bacterial and fungal biomass and protozoan and nematode organisms and thus buffer soil food webs from short-term abundance repeatedly over 2 years. We tested published dynamics of plant communities. expectations that C3 detritus and root chemistry (low lignin/N) favor bacterial-based food webs and root-feeding Keywords Bacteria . Fungi . Grasslands . Nematodes . nematodes, whereas C4 detritus (high lignin/N) and greater Protozoa production favor fungal decomposers and predatory nemat- odes. We also hypothesized that seasonal differences in plant growth between the two grassland types would generate Introduction season-specific differences in soil food webs. In contrast to our expectations, bacterial biomass and ciliate abundance Soil microbial activity is responsible for the decomposition were greater beneath C4 grasses, and we found no differences of detritus and the conversion of organic into in fungi, amoebae, flagellates, or nematodes. Soil food webs inorganic forms available for plant uptake and growth. varied significantly among sample dates, but differences were However, decomposition and mineralization processes carried out by soil bacteria and fungi are also influenced by higher trophic levels within the soil . For * M. Dornbush ( ) example, microcosm experiments altering soil food webs Department of Natural and Applied Sciences, University of Wisconsin-Green Bay, have repeatedly shown that the inclusion of higher-level 2420 Nicolet Drive, microbial grazers enhances plant growth, N mineralization, Green Bay, WI 54311, USA plant N uptake, and CO2 evolution rates relative to e-mail: [email protected] microcosms lacking them (Anderson et al. 1981; Parker C. Cambardella et al. 1984; Clarholm 1985; Ingham et al. 1985; Setälä and USDA/ARS National Soil Tilth Laboratory, Huhta 1991; Lenoir et al. 2007). Thus, both microbial Ames, IA 50011, USA activity and soil food webs influence plant growth via their effect on plant nutrient availability. At the same time, plants E. Ingham Soil Foodweb, Inc., directly influence microbes through differences in the Corvallis, OR 97333, USA quality and quantity of detrital inputs and indirectly through plant-mediated effects on microclimate (Paul and Clark J. Raich 1996). This inter-relationship has stimulated renewed Department of Ecology, Evolution and Organismal Biology, – Iowa State University, research into understanding plant soil Ames, IA 50011, USA interactions and has increased emphasis on linking above- 74 Biol Fertil Soils (2008) 45:73–81

and belowground systems (Wardle 2002; De Deyn and Van during midsummer beneath C4 grasslands, coinciding with der Putten 2005). maximum plant growth periods. This hypothesis is based on

Cool-season (C3) and warm-season (C4) grasses are two an expectation that belowground inputs in the form of root -studied plant functional groups that differ in many exudation and fine-root turnover coincide with maximum ecologically important ways. Relative to C3 grasses, C4 aboveground growth on a seasonal basis (Fitter et al. 1998, grasses generally have greater use efficiencies 1999;Craineetal.1999). (NUE) and water use efficiencies and reach optimal photosynthetic rates at higher temperatures (Long 1999). These fundamental differences result in divergences in Materials and methods (Ehleringer 1978; Pearcy and Ehleringer 1984), tissue chemistry (Long 1999; Craine et al. 2005), This study was conducted from May 29, 2001 to October 21, herbivory patterns, litter accumulation rates, frequencies 2002 at a private farm in Story County, IA, USA. (42°11′ N,

(Wedin 1995), and soil N mineralization rates (Wedin and 93°30′ W). Three cool-season (C3) and two warm-season Tilman 1990; Vinton and Burke 1997). For example, in (C4) grass stands were repeatedly sampled for 2 years. An central Iowa, USA, aboveground net in additional C4 stand was repeatedly sampled in 2002. Cool- planted C4 grasslands was 300% that of planted C3 season stands were dominated by a mixture of the C3 grasses grasslands (Dornbush and Raich 2006). In addition, Bromus inermis Leysser (smooth brome), Dactylis glomerata differential responses to temperature and moisture produce L. (orchardgrass), and Phalaris arundinacea L. (reed notable differences in seasonal growth and litter production canarygrass). All C3 stands were former pastures >30 years patterns. Maximum C3 production occurs during the cooler of age that have not been grazed since 1990. Warm-season spring and fall seasons, while maximum C4 growth occurs stands were virtual monocultures of Panicum virgatum L. during hotter and drier mid-summer conditions (Ode and (switchgrass) planted onto previously under row-crop

Tieszen 1980; Dornbush and Raich 2006). Differences in agriculture. Two of the C4 grasslands were planted in 1990, NUE generally produce higher tissue C/N in C4 grasses while the stand added in 2002 was planted in 1994; all three than in C3 grasses (Wedin and Tilman 1990; Wilsey et al. C4 stands were well-established, dense swards of tall grasses. 1997), which is in turn often correlated with differences in All warm-season stands were burned in 2001 prior to study soil processes, such as N mineralization rates (Wedin and initiation following typical management practices for C4 Tilman 1990; Vinton and Burke 1997). We suggest that grasslands in the Midwestern USA. The C3- and C4- differences in production, seasonal growth patterns, resource grasslands we studied differed in annual aboveground use, and tissue chemistry between C3 and C4 grasses should production, standing fine-root biomass, and tissue quality create conditions that differentially affect the . ( 1). All plots were located on a Coland , a We utilized planted grass filter strips, widely employed for fine-loamy, mixed, mesic Cumic Haplaquoll (DeWitt 1984), and nutrient retention in Midwestern USA agricul- with 3.0% C, 0.26% N, a of 1.07 g cm−3,anda tural systems (Schultz et al. 2000) to investigate differences in pH of 7.1 (unpublished data). the soil food web beneath C3-andC4-dominated grasslands. Mean annual temperature at our study site is 9.2°C, with We tested two main hypotheses related to known differences mean monthly temperatures ranging from 23.2°C in July to between C3 and C4 grasses. First, we tested the hypothesis −7.5°C in January. Approximately 80% of annual precip- that general soil food web structure would differ significantly itation falls from April through October. Growing season beneath C3 and C4 grasslands. Specifically, we tested Wardle precipitation (April through October) in 2001 was 61% of and Yeates’ (1993) hypothesis that greater production of C4 the average (69.6 cm), while precipitation in 2002 was grasslands should produce bottom–up effects, increasing normal (97%; Midwestern Regional Center 2008). fungal biomass and predatory nematode abundance, while Seasonal precipitation patterns were similar in both years, the higher quality of C3-grass detritus (e.g., lower lignin/N) with maximum precipitation occurring in spring, with lower should favor bacteria, resulting in greater bacterial biomass summer and fall levels. or a higher abundance of bacterial grazers (Porazinska et al. Sampling dates were selected to capture the range of

2003). Likewise, higher C3-root quality (lower root lignin/N) temperature, moisture, and plant phenological differences should support a higher abundance of root-feeding nemat- occurring throughout the growing season. Soil food web odes (Yeates 1987). Our second main hypothesis was that samples were taken in late May (spring), July (summer), and temporal differences in aboveground plant growth between October (fall) in both years, with an additional early April

C3 and C4 grasses would result in seasonal differences in soil (early-spring) sample taken in 2002. On each sampling date, food web structure. Specifically, we expected that the ten 1.8-cm diameter soil cores (0 to 10-cm depth) were greatest biomass and abundance of soil food web trophic randomly collected from each plot and then homogenized groups would occur in spring beneath C3 grasslands and together to form one composite sample per plot (n=3 plots Biol Fertil Soils (2008) 45:73–81 75

Table 1 Aboveground productivity, fine root biomass, and tissue beneath planted grasslands were analyzed using a general quality for cool-season (C3) and warm-season (C4) dominated grass- linear model repeated measures analysis of variance for each lands in central Iowa, USA soil food web category. The model included the following

Cool-Season Warm-Season terms: type (C3 vs. C4), time (seven distinct dates), (C3)(C4) and vegetation-by-time interaction. We analyzed 15 soil food

−2 −1 web variables in total, including active and total bacterial and ANPP (g OM m year ) 700 1,800 Aboveground biomass fungal biomass; active and total microbial biomass; and the (%) 25.4 32.7 abundances of soil flagellates, amoebae, , bacterial-, Lignin (%) 4.5 8.9 fungal-, fungal-root, and root-feeding nematodes, predatory N (%) 1.2 0.8 nematodes, and total nematodes. All values were standard- −2 Root Biomass (g OM m ) 800 1,000 ized on a per gram soil basis, then square root Cellulose (%) 25.8 27.3 transformed as needed to meet equal variance assumptions. Lignin (%) 10.1 13.1 N (%) 0.9 0.7

Aboveground productivity (ANPP) values are from Dornbush and Results Raich (2006), and root biomass values are from Tufekcioglu et al. ≤ (2003) and include all 2 mm diameter from 0 to 35 cm deep. General soil food web structure All production and biomass values have been rounded to the nearest 100 g organic matter (OM). Cellulose (%) and lignin (%) concen- trations were determined using a modified Van Soest forage fiber Both C3 and C4 grasslands supported larger active bacterial technique (Goering and Van Soest 1970; Anonymous 1997). Plant N than active fungal biomass (Fig. 1), but active biomass (%) was determined using a Carlo Erba NA 1500 elemental analyzer generally represented a small proportion of total biomass in (CE Instruments, Milan, Italy). both grassland types. Active fungal biomass averaged 6.5%

and 8.4% of total fungal biomass in C3 and C4 grasslands, per grass type). Samples were immediately returned to the respectively, and never fell below 2.2% or above 13.2% in laboratory, soil aggregates were broken apart by hand, large either grassland type. Active bacterial biomass averaged roots were removed, and the soils were thoroughly mixed, 35.6% and 34.8% of total bacterial biomass in C3 and C4 packed on blue ice, and mailed express overnight to Soil grasslands, respectively. However, active bacterial biomass Foodweb, Corvallis, OR, USA, for soil food web analyses. was a more variable proportion of total bacterial biomass,

Soil Foodweb processed all samples following the methods ranging from a low of 3.9% in C4 stands in spring of 2001 described by Ingham et al. (1985). Total bacterial biomass to 95.9% of total bacterial biomass in C4 stands in fall of was determined by fluorescein isothiocyanate , 2002; proportional abundance of active bacterial biomass followed by measurement of length and width using was virtually identical in C3 grasslands on the same dates epifluorescent microscopy (Babiuk and Paul 1970). Active (Fig. 1). Total bacterial-to-fungal biomass ratios were bacterial and active fungal biomass were determined by similar in both grasslands (Fig. 1), and relative abundance staining for 3 min with fluorescein diacetate, followed by among the protozoan groups were nearly identical. Specif- measurement of bacterial length and width and hyphal length ically, amoebae were the most abundant protozoan group, and width using epifluorescent microscopy (Lundgren 1981; followed by flagellates, then ciliates (Fig. 2). Strict Ingham and Klein 1984). Total fungal biomass was bacterial- and strict fungal-feeders were the most abundant determined using differential interference contrast microscopy nematode groups, accounting for 69.3% and 61.2% of all to measure hyphal length and width (Ingham and Klein 1984). nematodes in C3 and C4 grasslands, respectively (Fig. 3). Microbial biovolume calculations were converted to biomass C3 and C4 grasslands supported a (mean±standard error) following the equations of Bottomley (1994). Soil , bacterial-to-fungal feeding nematode ratio of 1.1±0.2 and , and ciliate abundances were determined using 1.4±0.2, respectively, mirroring similarities in total bacterial most probable number enumeration following growth on soil and total fungal biomass (Fig. 3). Root-feeding and fungal extract agar (Ingham 1994a). Nematodes were isolated using root-feeding nematodes were the next most abundant a modified Baermann funnel (Ingham 1994b). Following nematode groups, while predatory nematodes were the extraction, all nematodes were placed within one of five least abundant nematode group in both grasslands (Fig. 3). trophic groups (bacterial-feeding nematode, fungal-feeding nematode, fungal-root-feeding nematode, root-feeding nem- Addressing hypothesis 1 that C3 and C4 soil food web atode, and predatory nematode) based on Yeates et al. (1993) structure differs and Ingham et al. (1989), and counted. All data were analyzed using JMP version 5.0.1a (SAS Active bacterial biomass and active microbial biomass were Institute, Cary, NC, USA). Differences in soil food webs significantly different (P<0.05), and total bacterial biomass 76 Biol Fertil Soils (2008) 45:73–81

Fig. 1 Temporal patterns of 12 bacterial and fungal biomass 6 (per gram of soil C) in planted 10

C3 and C4 grasslands in central ) ) -1 Iowa, USA. Samples were taken -1 8 4 between May 2001 and October 2002. Open triangles are C 6

4 (mg gC grasslands; filled circles are C (mg gC 2 3 Bacteria Active grasslands. Error bars represent Bacteria Total 4 standard error of the mean for 2 0 C3 and C4 grasslands 15 2.0

12 1.6 ) ) -1 -1 9 1.2

6 0.8 (mg gC (mg gC Total Fungi Total Fungi Active Fungi Active 3 0.4

0 0.0

5 20

4 15

3 10 2

5 1 Total Bacteria : Fungi Total Bacteria Active Bacteria : Fungi Bacteria Active 0 0 May Aug Nov Feb May Aug Nov May Aug Nov Feb May Aug Nov

(P<0.07) and soil ciliate abundance (P<0.06) were Addressing hypothesis 2 that temporal patterns of the soil marginally significantly different between C3 and C4 grass- food web reflect phenological differences in C3 and C4 lands (Figs. 1 and 2, Table 2). However, in contrast to our grasslands expectations, both bacterial biomass and ciliate abundance were higher under C4 than C3 vegetation (Figs. 1 and 2). Despite notable differences in growth phenologies between Vegetation-by-time interactions were significant (P<0.05) C3 and C4 grasslands, temporal differences in trophic group for bacterial-feeding nematodes, root-feeding nematodes, abundances were largely synchronized beneath C3 and C4 and total nematodes and marginally significant (P<0.08) plantings (Figs. 1, 2, and 3). For example, of the 12 groups for fungal-feeding nematodes (Fig. 3, Table 2). Active affected by time, two thirds of these responses were bacterial biomass, total bacterial biomass, active fungal independent of vegetation type. However, this pattern did biomass, total fungal biomass, active microbial biomass, total not hold true for the nematode groups, as bacterial-feeding, microbial biomass, and the abundance of fungal-root feeding fungal-feeding, root-feeding, and total nematodes were all nematodes varied significantly (P<0.05), and ciliate abun- affected by the interaction between vegetation and sample dance was marginally significantly different (P<0.06) among date (Fig. 3, Table 2). In these cases, significant interactions sample dates, independent of vegetation identity (Table 2). appear to have resulted from either higher nematode

Thus, of the 15 variables compared statistically, four groups abundance under C3 grasses in April 2002 or from greater differed between vegetation types independent of sample nematode abundances under C4 grasses during the remainder date, while eight trophic groups differed among sample dates of 2002 (Fig. 3). Irrespectively, while vegetation appears to independent of vegetation type. Vegetation and sample date have affected temporal trends in nematode abundance, we interacted to affect four trophic groups. Significant inter- found no evidence that aboveground plant phenology actions aside, approximately twice as many groups were influenced belowground trophic structure for nematodes or influenced by sample date as by vegetation type, and in any other trophic groups. Specifically, we saw no evidence general, sampling date appeared more important than of a peak in soil food web groups in May under C3 grasses vegetation type in structuring the soil food web in this study. nor under C4 grasses in July as we originally hypothesized. Biol Fertil Soils (2008) 45:73–81 77

800

) to bacterial biomass (i.e., active and total bacterial biomass -1 700 and active microbial biomass). However, in contrast to our 600 production-based hypothesis, neither fungal biomass nor 500 predatory nematode abundance were significantly greater 400 under the more productive C4 grasslands, although fungal 300 biomass did trend higher under C4 stands (Fig. 1). In 200 contrast to our tissue quality hypothesis, both bacterial

Flagellates (X1000 gC 100 biomass and ciliate abundance were significantly greater, 0 and amoebae and flagellate abundances trended higher

1600 under the lower-quality C4 grasses. We found no difference

) in either active or total bacterial-to-fungal biomass ratios -1 1200 between C3 and C4 grasslands, nor was root-feeding nematode abundance higher under the higher-quality C3 800 grasses, as we predicted. In summary, we found two main

differences between C3 and C4 grassland soil food webs 400 related to bacterial biomass and ciliate abundance. However, observed differences were opposite to our expectations Amoebae (X1000 gC 0 based on differences in plant tissue chemistry and produc- tion, and the remaining 11 comparisons did not differ 600 significantly between planted C3 and C4 grasslands.

) While our results do not support our initial hypotheses, -1 400 they are in line with the results of several other field studies evaluating the effects of vegetation identity on soil food webs. Wardle et al. (1999) studied the response of the 200 decomposer food web in a New Zealand grassland to the

removal of either all plants, C4 grasses, C3 annual grasses, all Ciliates (X100 gC C3 grasses, or dicotyledonous weeds. They found that only 0 May Aug Nov Feb May Aug Nov the complete removal of all plants produced a significant change in microbial biomass and nematode functional group Fig. 2 Temporal patterns of protozoan abundances (per gram of soil abundances. Porazinska et al. (2003) measured soil food C: flagellates, amoebae, and ciliates) in planted cool-season (C3) and warm-season (C4) grasslands in central Iowa, USA. Open triangles are webs under six grass species growing in native tallgrass C4 grasslands; filled circles are C3 grasslands. Error bars represent prairie. They found no effect of C3 or C4 photosynthetic standard error of the mean for C3 and C4 grasslands pathways on microbial C or N, soil amoeba, flagellate, or ciliate abundances, or nematode functional group biomass. In fact, most patterns appeared rather idiosyncratic from Grass identity also had no effect on amoebae, flagellates, season to season and year to year, with the exception of total ciliates, or on root-associated, bacterial-feeding, fungal- bacterial biomass and active fungal biomass, which had feeding, omnivorous, or predatory nematodes. Plant-parasitic consistent seasonal trends in both grasslands across both nematode abundance differed for one of the six grasses years (Fig. 1). measured. Wardle et al. (1999) suggested that the high soil organic C content of their site (13.0%) may have buffered the soil food web from changes in vegetation. The same Discussion explanation seems applicable to both Porazinska et al.’s (2003) study (soil C, 4.0%) and ours (soil C, 3.0%). In Soil food webs in planted C3 and C4 grasslands further agreement, recent modeling by Moore et al. (2004) identified an important, yet neglected, role of organic matter We compared soil food web structure between planted C3 in stabilizing and structuring the soil food web, and Albers and C4 grasslands differing in production (energy inputs), et al. (2006) recently used stable isotopes in an agro- phenology (timing of inputs), and tissue chemistry (quality to document that soil meso- and macrofauna of inputs) (Table 1), and we found significant differences in depended primarily upon plant residues that were at least bacterial biomass and ciliate abundance. Specifically, of the 2 years old. Thus, while it is unclear as to why bacterial four statistically significant differences in soil food web biomass and ciliate abundance were larger under C4 than C3 groups that we identified between C3 and C4 grasslands; grasslands, we suggest that the large number of similarities one was related to ciliate abundance, and three were related in the soil food web between these distinct grasslands likely circles C h enfrC for mean the bars a (C cool-season planted in soil of cm 10 top the from groups web food soil of nematodes) 2 Table (C cool-season planted from C) soil of gram (per abundance nematode total and predatory, feeding, fungal-root root-feeding, fungal-feeding,terial-feeding, 3 Fig. 78 infcn,tecretdgeso reo s6 1frtetm anteteteffect. treatment main time the for 21 6, is freedom of degrees correct the significant, n amsao (C warm-season and triangles USA. Iowa, central in ru eeainTm Vegetation-by-time Time Vegetation Group oa eaoe ..44<.131<0.03 <0.03 3.1 <0.08 N.S. N.S. <0.05 3.2 <0.01 N.S. N.S. 2.3 N.S. 2.6 <0.01 4.4 N.S. <0.06 N.S. N.S. N.S. <0.01 N.S. <0.0001 <0.01 4.4 N.S. 2.3 <0.001 N.S. N.S. <0.0001 N.S. <0.0001 N.S. 4.9 8.5 <0.06 4.7 N.S. 6.2 <0.0001 13.8 <0.0001 72.6 N.S. 7.1 seasons. growing 2002 28.6 N.S. and 2001 N.S. the during occasions seven <0.05 on 86.0 taken N.S. were Samples N.S. N.S. <0.07 nematodes Total N.S. N.S. nematodes N.S. Predatory <0.03 8.5 nematodes Root-feeding N.S. nematodes Fungal-root-feeding 6.1 nematodes Fungal-feeding nematodes Bacterial-feeding 12.1 Ciliates Amoeba Flagellates microbes Total microbes Active fungi Total fungi Active bacteria Total bacteria Active osgiiatvgtto-ytm neato em ( terms interaction vegetation-by-time Nonsignificant 4 grasslands ersn tnaderrof error standard represent r C are eprlpten fbac- of patterns Temporal r C are eetdmaue nlsso aineeautn ifrne nteboas(atraadfni n bnac pooon and (protozoans abundance and fungi) and (bacteria biomass the in differences evaluating variance of analysis measures Repeated 3 grasslands. 4 3 grasslands; and 4 grasslands ) Open Error 3 filled )

Root-feeders Fungal-feeders Bacterial-feeders (# gC-1) (# gC-1) (# gC-1) F 100 1,4 20 40 60 80 20 40 60 80 20 40 60 80 0 0 0 a u o e ayAgN v No Aug y Ma Feb Nov Aug May P 01 eermvdfo h oe.I h orcssweeteitrcinwas interaction the where cases four the In model. the from removed were >0.1) PF 6,27 a 3 n amsao (C warm-season and )

PF Total Nematodes Predators Fungal-root feeders (# gC-1) (# gC-1) (# gC-1) 100 150 200 250 300 20 40 60 80 50 0 2 4 6 8 0 0 ayAgN e ayAgN v No Aug y Ma Feb v No Aug y Ma 4 ilFri ol 20)45:73 (2008) Soils Fertil Biol rslnsi eta oa USA Iowa, central in grasslands ) 6,21 P – 81 Biol Fertil Soils (2008) 45:73–81 79 resulted from a stronger effect of soil properties than current date did not correspond to differences in aboveground plant vegetation identity in structuring the soil food web in our growth, as we observed no increases in soil food web fertile grasslands. biomass or abundance that were attributable to peaks in

In contrast to our results and those described above, either C3 (May) or C4 (July) aboveground production. In Belnap and Phillips (2001) and Belnap et al. (2005) general, all trophic group abundances were quite variable measured the soil food web in dry rangeland soils (215 mm through time and tended to change in synchrony under both annual precipitation) and found differences in active and vegetation types, suggesting that temporal changes to the total bacterial biomass, active fungal biomass, total fungal- soil food web were not controlled by vegetation or climatic to-bacterial biomass, active fungal-to-total fungal biomass, seasons. Sample date was significant for 12 of the 15 active bacterial-to-total bacterial biomass, and fungal-root groups compared, with no clear explanation for the feeding, bacterial-feeding, and total nematode abundances observed increases and decreases apparent from our data. among sites differing in Stipa, Hilaria,orBromus grass Seasonal trends in total bacterial biomass and active fungal biomass. However, their dry rangeland sites had lower soil biomass provide a notable exception, as both showed organic C contents than our site, ranging from 0.52% to consistent seasonal patterns across both years (Fig. 1). 0.75% organic C (Belnap et al. 2005; assuming 1.0 g Viewed collectively, it is clear from our data that multiple- organic matter is 0.58 g C). Wardle et al. (2003), De Deyn date sampling is required to adequately describe the soil et al. (2004), and Viketoft et al. (2005) also reported food web. The lack of a correlation between plant growth significant plant identity effects on soil food web structure. and soil food web structure lends further support to our These studies examined the effects of multiple grasses, position that factors other than plant inputs were the , or forbs on the soil food web and thus included a primary determinants of soil food web structure at our site. broader range of plant tissue chemistries and functional For example, if the soil food web was strongly dependent groupings than our study. In their studies, the largest upon the energy in seasonal plant inputs, then we would differences in soil food webs were associated with expect a strong correlation between plant growth and soil comparisons among different plant functional groupings. food web structure. This pattern was not observed. For example, grasses tended to favor herbivorous nemat- odes, while nitrogen-fixing legumes favored the bacterial energy channel (i.e., bacteria and bacterial feeding nemat- Conclusion odes). We argue that our results, in combination with others, suggest that preexisting soil properties may moder- We draw two main conclusions from our work. First, we ate the effect of plant inputs on soil food web structure and found a strong temporal effect on the soil food web that was that the relative strength of the moderating effect may not associated with either seasonal climatic trends or plant change as a function of relative differences in the tissue growth patterns. Future sampling protocols will need to quality and quantity of plant inputs. We suggest that soil account for this temporal variability when assessing organic matter content is the most likely soil property average soil food web structure. Second, we interpret the candidate, as it affects both soil structural properties, such modest effect of vegetation identity on the soil food web to as bulk density, and soil resource pools used by microbes, reflect a dominant influence by pre-existing soil properties. such as energy, water, and nutrients (Paul and Clark 1996). The most likely candidate is soil organic matter content, as Thus, we hypothesize that differences in plant tissue it simultaneously influences multiple soil properties including chemistry or production may result in observable changes , available energy, and soil resource pools. Stated to the soil food web in low organic C soils, but the same directly, we hypothesize that high levels of soil organic matter differences may have no observable effect in a higher provide a stable environment and energy source for soil organic C soil. This is clearly a preliminary hypothesis organisms and thus buffer soil food webs from short-term requiring direct testing. dynamics of plant communities. As such, we put forth the Our study is unique in providing one of the most testable hypothesis that plant-species effects on soil food webs complete evaluations of the soil food web by documenting are a function of the interaction between differences in tissue soil food web structure for two consecutive growing quality among the plant species compared and the size or seasons under two phenologically and physiologically quality of the existing soil organic matter pool. distinct vegetation types growing on the same soil series, in the same region, and at the same topographic position. Acknowledgements We thank R. and S. Risdal and L. Strum for Despite these designed controls, we found no evidence that allowing access to their land. R. Schultz, T. Isenhart, J. Nelson, and E. Stuaffer, and the Department of Ecology, Evolution, and Organismal differences in aboveground plant phenology resulted in Biology at Iowa State University and the Department of Natural and predictable, seasonal changes to the soil food web. Applied Sciences at the University of Wisconsin-Green Bay provided Significant interactions between vegetation type and sample valuable logistical and technical support during this project. M. 80 Biol Fertil Soils (2008) 45:73–81

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