Controlled Drainage As a Targeted Mitigation Measure for Nitrogen and Phosphorus

Published May 3, 2019

Journal of Environmental Quality technical reports Surface Water Quality

Controlled Drainage as a Targeted Mitigation Measure for Nitrogen and Phosphorus

Mette V. Carstensen,* Christen D. Børgesen, Niels B. Ovesen, Jane R. Poulsen, Søren K. Hvid, and Brian Kronvang

ubsurface drained and intensively farmed agricultural Abstract land is associated with high amounts of water and nutrients, − Drainage systems provide a more or less direct conduit for excess especially NO3 –N, which are transported directly from the water and nutrients from fields to surface water. High nutrient Sroot zone of the fields to nearby streams. This minimizes the natural loads to streams and lakes are known to adversely affect water retention capacity of the landscape such as NO −–N reduction via quality and may potentially cause algae blooms. Therefore, 3 in-field as well as edge-of-field mitigation measures that can denitrification. This enhanced nutrient loss further entails the risk assist in reducing the loss of nutrients are needed. The aim of of eutrophication of freshwater and coastal water bodies, which may this study was to investigate the effectiveness and possibility of lead to hypoxia (Rivett et al., 2008). Therefore, there is an urgent need using controlled drainage during the drainage season to reduce for introduction of mitigation measures that can reduce the loss of N nutrient losses while growing a winter crop in a temperate and P from agriculture (Bouraoui and Grizzetti, 2014, Schoumans climate. The 3-yr-long (2012–2015) study was conducted on four experimental field plots on loamy soil. The impacts of controlled et al., 2014). One such measure is controlled drainage (CD), which drainage on groundwater levels, drain flow, and water quality at is a groundwater management technique where the groundwater regulation levels of 50 and 70 cm above the conventional drain is elevated either during specific seasons (e.g., winter) or the entire pipe level were determined by using a before-after control- year. Controlled drainage has received much attention during the impact study design. A regulation level of 70 cm was required last decade, especially in the United States (Skaggs et al., 2012) and to significantly elevate groundwater levels and reduce the drain outflow and N and P loss, which decreased by 37 to 54%, the Nordic countries (e.g., Sweden and Finland; Wesström and 38 to 51%, and 43 to 46%, respectively, relative to conventional Messing, 2007). Most of the conducted studies ascribe the decreased − drainage levels. Denitrification in the root zone, as measured with NO3 –N loss to reduced drain flow, but a few studies ascribe it to stable isotopes, was not markedly enhanced at the plots with a combination of reduced drain flow and enhanced denitrification controlled drainage, except on a few occasions. Resetting the (Gilliam et al., 1979; Lalonde et al., 1996; Wesström et al., 2001). groundwater level to conventional levels in early spring only had a marginal influence on water and nutrient losses. Thus, potential Denitrification, which requires anoxic conditions, can potentially be water quality tradeoffs (e.g., increased N loss to groundwater) enhanced by CD as an elevated groundwater table in a field leaves need to be more thoroughly investigated before implementing a larger proportion of the soil column saturated, promoting anoxic controlled drainage as a mitigation measure in Denmark. conditions (Knowles, 1982). Denitrification can be detected using 15 14 18 16 − the two stable isotope ratios N/ N and O/ O of NO3 –N as Core Ideas microbes prefer light isotopes over heavy isotopes (Kendall et al., 2007). Thus, denitrification creates a distinct relationship between • Controlled drainage reduced the N and P loss from drainage d15N and d18O around 0.5 to 1 in the remaining pool of NO −–N systems. 3 • The reduction of N and P loss was mainly due to reduced drain (Aravena and Robertson, 1998; Mengis et al., 1999; Fukada et al., flow. 2003; Sigman et al., 2005; Granger et al., 2008; Li et al., 2014). • Denitrification was evident both at field plots with and without Another less intensively studied aspect of CD is the effect on controlled drainage. P (Ross et al., 2016), and studies have shown promising results • Controlled drainage is not officially accepted as a mitigation with P retention from 40 to 95% compared with conventional measure in Denmark. drainage levels. However, it is a concern that CD might increase 3− 3− dissolved PO4 loss as reduced conditions can lead to PO4 desorption from hydrous iron oxides if Fe3+ is reduced to Fe2+ (Kadlec and Wallac, 1996). The aims of this study were to investigate (i) if CD can assist in reducing N and P losses to surface waters during the winter period

© 2019 The Author(s). This is an open access article distributed under the CC BY- NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). M.V. Carstensen, N.B. Ovesen, and B. Kronvang, Dep. of Bioscience, Aarhus Univ., Vejlsøvej 25, 8600 Silkeborg, Denmark; C.D. Børgesen, Dep. of Agroecology, J. Environ. Qual. 48:677–685 (2019) Blichers Allé 20, 8830 Tjele, Denmark; J.R. Poulsen, EnviDan A/S, Vejlsøvej 23, 8600 doi:10.2134/jeq2018.11.0393 Silkeborg, Denmark; S.K. Hvid, SEGES, Agro Food Park 15, 8200 Aarhus N, Denmark. Supplemental material is available online for this article. Assigned to Associate Editor Thomas Harter. Received 6 Nov. 2018. Accepted 13 Mar. 2019. Abbreviations: BACI, before-after control-impact; CD, controlled drainage; d.l., *Corresponding author ([email protected]). detection limit; TN, total nitrogen; TP, total P.

677 when growing winter wheat (Triticum aestivum L.), (ii) the factors Field management practices during the 3-yr study period responsible for N and P reduction, and (iii) if lowering of ground- involved cropping of winter wheat sown in mid-September and water levels to conventional levels to enable fields operations will harvested the following year in late August (Supplemental Table result in excess loss of N and P from the field plots with CD. S4). The yields of each plot were recorded by a combined har- vester. Mineral fertilizer was applied twice, in March–April (103 Materials and Methods to 200 kg ha−1) and in April–May (100 to 215 kg ha−1), followed Study Area by spraying of pig slurry in the beginning of May (18 to 36 tons ha−1). Plots CDP1, CDP2, and RD1 were part of the same field The experimental site was located in Odder municipality, and were treated similarly, whereas RD2 was part of another field Denmark (55°55¢01.2¢¢ N, 10°10¢54.5¢¢ E), and consisted of and was treated only slightly different during the study period. In four adjacent systems, which each drained ?1 ha with three to the year prior to the experiment (2011), winter wheat was grown five lateral drainage pipes placed at ?15-m intervals at 1.1-m at RP2, whereas spring barley (Hordeum vulgare L.) was grown depth (Fig. 1). A description of the study can also be found in at the other plots. This implied that N concentrations were con- Carstensen et al. (2016). siderably lower at RP2 in Y0 and the N measurements from this A measuring well and a regulation well with a riser determin- plot were therefore omitted from the analysis in Y0, and the plot ing the regulation level (the targeted elevation of the groundwater could not be used as a reference. level above the conventional drainage level) were installed on each plot. The study was conducted during the main drainage periods Sampling and Chemical Analysis (September to April) in 2012 to 2015. Controlled drainage was Weekly grab samples of drain water were taken from each not applied during the first period, 2012–2013 (Y0), which served measuring well and filtered through a 0.45-mm filter within as a reference (Supplemental Table S1). In 2013–2014 (Y1) and 24 h. The water samples were stored in the dark at 3°C and ana- 2014–2015 (Y2), CD was introduced at two plots (CDP1 and − lyzed within 2 wk. Concentrations of NO3 –N, total N (TN), CDP2), whereas the two remaining plots served as references + 2− NH4 –N, and SO4 were determined by ion chromatography (RP1 and RP2). In Y1 on 22 Nov. 2013, the regulation levels of − 2− + using a Dionex ICS-1500. The NO3 –N, SO4 , and NH4 –N the wells at CDP1 and CDP2 were set to 50 cm above the con- were analyzed according to ISO 10304/1 (ISO, 2009; detection ventional drain depth; however, at CDP1, the regulation level was limit [d.l.] = 0.050 mg L−1) and ISO 11732 (ISO, 2005; d.l. = increased to 70 cm on 28 Jan. 2014 after a midway evaluation had 0.010 mg L−1), respectively. Total N analysis followed ISO 12260 revealed no significant effect by CD on the groundwater level. To (ISO, 2003) and was conducted using a total organic C analyzer, ensure field trafficability, the groundwater levels were reset to the Shimadzu TOC-L, with a TN measuring unit. Total P (TP) and conventional level by removing the riser in the regulation well in 3− PO4 were analyzed according to ISO 6878 (d.l. = 0.0005 mg spring (11 Mar. 2014). In Y2 on 27 Oct. 2014, the regulation level L−1), and the spectrophotometric analysis was conducted on a was set to 70 cm at both CDP1 and CDP2, and the regulation Shimadzu UV-1700. During the first hour after the regulation level was reset to the conventional level on 9 Mar. 2015. 3− level was reset in Y1 (11 Mar. 2014), samples of PO4 and TP The soil texture analysis showed that the root zone (1-m depth) were taken every 20 to 30 min with an ISCO sampler at CDP1 of the plots was characterized by silty loam and loam (USDA and CDP2. In the following week, daily composite samples of classification) with an average clay, silt, and sand content of 14.9 PO 3− and NO −–N consisted of a sample collected hourly. ± 3.7, 21.6 ± 6.0, and 59.5 ± 7.8%, respectively (Supplemental 4 3 Table S2). The soil C content was 1.7 to 1.8% C in the AP horizon Water Monitoring (0–27 cm) on all plots (Supplemental Table S3). At CDP2, the C Groundwater levels were monitored continuously at one site at content was low (<0.16%) in the remainder of the profile, whereas each plot by a pressure transducer, MadgeTech Level 2000 (accuracy RP1 and RP2 had a C horizon (95–110 cm) with 2% C and a of ±0.3%), located in the piezometer pipes placed closest to the reg- high chalk content (7.3 to 15.5%). At CDP1, an interglacial lake ulation well (Fig. 1). The piezometer pipes had no filter pack, which deposit with a high humus and C content (6% C) was found at caused periodical clogging of screens, especially at CDP2, and the approximately 120- to 130-cm depth.

Fig. 1. Overview of the four drainage systems at the experimental site located in Odder, Denmark, with two controlled drainage plots (CDP1 and CDP2) and two reference plots (RP1 and RP2). The drainage systems covered ?1 ha each and had a depth of 110 cm. 678 Journal of Environmental Quality continuously measured groundwater levels at CDP2 consequently analysis at the Stable Isotope Facility at the University of California, had to be omitted from the analysis. Groundwater levels were also Davis, according to the denitrifier method (d.l. of 0.4‰ for 15N/14N monitored in the rest of the plot in eight to nine piezometer pipes and 0.5‰ for 18O/16O; Sigman et al., 2001; Casciotti et al., 2002; (Supplemental Fig. S1). In these piezometer pipes (equipped with Stable Isotope Facility, 2015). Isotope ratios of 15N and 18O were filter packs), groundwater levels were monitored manually with a measured using a ThermoFinnigan gas bench and a PreCon trace water level meter, Solinst 101 (accuracy of ±0.3 cm), one to four gas concentration system interfaced to a ThermoScientific Delta times per month. Drain flow was monitored using an electromag- V Plus isotope-ratio mass spectrometer. The results were cali- netic flow meter, Waterflux 3000 (accuracy of ±0.3%), recording brated with the international reference standards USGS 32, 34, every 10 min, and a converter, IFC 100, placed in each measuring and 35 from the National Institute of Standards and Technology, well. Groundwater storage change was calculated as the difference Gaithersburg, MD. Values were reported in parts per thousand between the groundwater level at the beginning and at the end of (‰) relative to atmospheric N2 and Vienna Standard Mean Ocean the each drainage season multiplied by the drainable porosity of the Water (VSMOW) for d15N and d18O: soil. Weather data were obtained from the Danish Meteorological

Institute as a 10-km ´ 10-km grid covering the experimental site (RRsample- standard ) dd15N or 18 O( ‰) = ´1000 [3] and were corrected following Allerup and Madsen (1980). ( )sample Rstandard Statistical and Data Analysis where R is the ratio of the heavy to light isotope (e.g., 15N/14N A before-after control-impact (BACI) design was used to or 18O/16O). determine if CD had a significant impact on drain flow, ground- Linear regression analysis of d18O and d15N was conducted for water levels, and water quality (Stewart-Oaten et al., 1986). The each plot and year. Rayleigh fractions could not be used to differen- statistical model for the BACI analysis was fitted using a repeated tiate fractionation from mixing due to the narrow data range, and measures generalized linear mixed model in SAS-STAT 9.4 (SAS − we therefore used additional tracers such as time series of NO3 –N, Institute, 2013). Plots and date were considered random effects. + 2− NH4 –N, and SO4 concentrations and the relationship between Data were log transformed before analysis and drain flow was NO −–N and d15N to support the stable isotope analyses. transformed by log(y + 1) due to the presence of null values (Box 3 and Cox, 1964). The model to be fit was Results log(Y) = Treatment Plot( RR) Period Date( ) Hydrology [1] ´´Treatment Period Plot ´ Date(R) The annual precipitation in Y0, Y1, and Y2 was 783, 680, and 823 mm yr−1, which were within ±12% of the 25-yr average of 733 ± 8 mm yr−1 (1989–2014). The drain flow and groundwater tables where Period is before or after, Treatment is control or impact, fluctuated in response to precipitation (Fig. 2A and 2B). In Y0 and Plot is the random (R) site effect within treatment, and Date is Y1, precipitation events occurred primarily in November and the the sampling dates within each period. beginning of December, whereas in Y2, precipitation occurred For each test, the BACI effect (BE), indicating the size of the during the entire drainage season (Fig. 2B). Thus, the groundwater difference, was calculated as tables were higher and fluctuated more in Y2 than in Y0 or Y1. BE = (X − X ) − (X − X ) [2] The groundwater table was naturally shallow and above drain- CA CB IA IB age depth at all plots during the major part of the study period where X is the mean of the constituent analyzed, C is controlled, (Fig. 2B). The implementation of CD significantly elevated the B is before, A is after, and I is impacted. groundwater table within 25 m from the regulation well at CDP1 Daily values of nutrient concentrations were obtained by linear in both Y1 and Y2 (Table 1). However, in Y1, the groundwater interpolation of the weekly measured values. Daily loss was calcu- table rose significantly only after adjusting the regulation level lated by multiplying the daily drain flow by daily concentrations. from 50 to 70 cm on 28 Jan. 2014. The difference between the The percentage changes in drain outflow and nutrient con- groundwater table at CDP1 and RP1 was more pronounced in Y2 centrations and losses were calculated by first establishing the than in Y1 (Table 1). The greatest differences (up to 39 cm) were ratios between impacted (CDP1 and CDP2) and reference plots measured during dry periods, whereas during precipitation events, (RP1 and RP2) in Y0. These ratios and the measured values at the groundwater table at RP1 reached the same level as at CDP1. RP1 and RP2 in Y1 and Y2 were used to predict the drain out- The spatial monitoring of groundwater levels within the plots flow and nutrient loss at CDP1 and CDP2 in Y1 and Y2. The showed that the extent of the area with significantly elevated percentage change at CDP1 and CDP2 was calculated as the dif- levels was larger in Y2 than in Y1, now including the most north- ference between the predicted and the observed loss. ern and southern parts of CDP1 and CDP2 (Supplemental Table S5, Supplemental Fig. S1). However, the average differ- 15 18 Isotope Analysis of d N and d O in Nitrate in Drain Water ences in groundwater levels at plots with and without CD were 15 18 − Stable isotope analysis of d N and d O in NO3 was conducted generally very small, and the groundwater levels showed a high to detect denitrification. For the analysis, five and six grab samples degree of variability within the plots (Supplemental Fig. S2–S5). of drain water from Y1 and Y2 were selected from each plot, rep- The groundwater storage change was positive during all drain- resenting scenarios with low and high water flow. The samples were age seasons, and no difference could be traced between CD and stored in the dark at 3°C and filtered within 24 h (0.45-mm filter). reference plots (Table 2). The harvest yield of winter wheat from Thereafter, the samples were stored frozen using dry ice until isotope plots with CD was more or less the same as the harvest yield from Journal of Environmental Quality 679 − 3− Fig. 2. (A) Drain flow, (B) continuously monitored groundwater levels, (C) NO3 –N, and (D) PO4 concentrations in drain water at plots with controlled drainage (CDP1 and CDP2) and reference plots (RP1 and RP2) during Y0 (2012–2013), Y1 (2013–2014), and Y2 (2014–2015). b.s. = below surface. plots without CD in both Y1 and Y2; thus, no influence of CD and Y2 (Table 1). The reduction was 11 and 5% (17 and 8 mm, on yields could be documented (Supplemental Table S4). respectively) at CDP1 and CDP2 in Y1 and 37 and 54% (90 and The drain flow was similar in magnitude and duration at all 124 mm) at CDP1 and CDP2 in Y2 (Table 2). The implementa- plots in Y0, reflecting the high degree of similarity between the tion of CD also resulted in a delay of the drain flow of ?1 mo plots (Fig. 2A). Compared with the reference plots, the drain from at the plots with CD compared with the reference plots flow from the CD plots was significantly reduced in both Y1 (Fig. 2A). The drain flow from the plots with CD also tended

680 Journal of Environmental Quality to cease during periods with low precipitation, while water still in Y0, and the proportion was 85 to 93% in Y1 to Y2 due to a ran from the drains at the reference plots. During precipitation higher fraction of organic N. events, the peaks in drain flow at the CD plots were almost simi- 18 15 lar to those at the reference plots. Isotope Analysis of d O and d N of Drain Water Nitrate Thed 15N values were within the same range at the CD and the Drain Water Nitrogen Concentrations and Losses reference plots during Y1 and Y2, except for one enriched value of − The mean concentration of NO3 –N in drainage water ranged 13.8‰ recorded at CDP1 on 4 Feb. 2014 (Fig. 3). On this date, from 6.8 ± 2.7 to 8.2 ± 1.4 mg L−1 in Y0 (Fig. 2C, Supplemental also the d18O value was enriched (4.3‰). Otherwise, the d18O − Table S6). There was no significant difference between NO3 –N values (0.7–2.6‰) were within the same range at the CD and concentrations in drainage water from the CD plots compared reference plots in Y1 and Y2, although they tended to be slightly with the reference plots (Table 1). more enriched in Y2 than in Y1. The linear relationship between − 18 15 During Y1, the loss of NO3 –N with drainage water decreased d O and d N ranged from 0.54 to 1.01 at CDP1–2 and RDP1, − −1 − −1 by 23% (3.1 kg NO3 –N ha ) and 1% (0.1 kg NO3 –N ha ) indicating that denitrification was the dominant fractionation pro- at CDP1 and CDP2, respectively, relative to the ratio between cess, whereas the relationship ranged from 0.41 to 1.27 (Fig. 3) at CD and reference plots during Y0 (Table 2). During Y2, the RP2. However, omitting the particularly enriched values measured − 18 15 − NO3 –N loss via drain water declined by 38 and 51% (6.7 and on 4 Feb. 2014 at CDP1, the d O and d N of NO3 –N were not − −1 2 8.5 kg NO3 –N ha ) at CDP1 and CDP2. distinctively enriched (slope = 0.13, R = 0.02, p = 0.85) in Y1. + 2− In Y1, the loss of TN via drainage water decreased by 26% On this date, an increase of both NH4 –N and SO4 concentra- and increased by 1% at CDP1 and CDP2, respectively, whereas tions in the drainage water from CDP1 was observed, while the − in Y2, the reduction of TN was 34 and 45%. About 95 to 98% of NO3 –N concentration simultaneously decreased (Supplemental − 15 the TN loss via drainage flow occurred in the form of NO3 –N Fig. S6 and S7). The strong negative correlation between d N

Table 1. Before-after control-impact (BACI) test of the treatment ´ period interaction for hydrological parameters and nutrients in drain water for Y1 (2013–2014) and Y2 (2014–2015) compared with the reference period Y0 (2012–2013) monitored at plots with controlled drainage (CDP1 and CDP2) and reference plots (RP1 and RP2). Only dates with measurements from all plots are included. Parameter Plots Period Unit of BE† BE n‡ df F value p Drain flow CDP1–2, RP1–2 Y1 mm 0.14 8 536 65.3 <0.0001* CDP1–2, RP1–2 Y2 mm 1 8 545 395.7 <0.0001* Groundwater level§ CDP1, RP1 Y1 cm 0.6 4 140 6.4 <0.05* CDP1, RP1 Y2 cm 8.7 4 205 49.9 <0.0001* Total N CDP1–2, RP1 Y1 mg L−1 1 6 69 2.1 0.15 CDP1–2, RP1 Y2 mg L−1 −0.5 6 78 2.8 0.10 - CDP1–2, RP1 Y1 mg L−1 0.6 6 69 1.7 0.20 NO3 -N CDP1–2, RP1 Y2 mg L−1 −0.4 6 80 0.8 0.36 + −1 NH4 -N CDP1–2, RP1 Y1 mg L −0.012 6 63 0.5 0.50 CDP1–2, RP1 Y2 mg L−1 −0.006 6 78 0.2 0.64 3- CDP1–2, RP1–2 Y1 mg L−1 b.d.l.¶ 8 101 0.1 0.82 PO4 CDP1–2, RP1–2 Y2 mg L−1 b.d.l. 8 116 1 0.33 * Significant at the 0.05 probability level.

† BE, BACI effect. BE = (XCA − XCB) − (XIA – XIB), where X is the mean of the relative constituent, B is before, A is after, C is control (reference plots), and I is impact (plots with controlled drainage). ‡ n, number of samples. § Continuously monitored groundwater levels. ¶ b.d.l., below detection level.

Table 2. Precipitation, drain flow, groundwater storage change D( GW), and total losses of nutrients via drainage water during Y0 to Y2 at plots with controlled drainage (CDP1 and CDP 2) and reference plots (RP1 and RP2). Y0 (2012–2013) Y1 (2013–2014) Y2 (2014–2015) Parameter Unit CDP1 CDP2 RP1 RP2 CDP1 CDP2 RP1 RP2 CDP1 CDP2 RP1 RP2 Precipitation mm 263 263 263 263 226 226 226 226 307 307 307 307 Drain flow mm −166 −157 −163 −174 −135 −136 −150 −158 −151 −104 −245 −244 DGW mm 34 36 30 20 5 12 26 4 27 13 18 7 Total N kg ha−1 14 13 15 11† 11 14 16 13† 13 9 22 20† - kg ha−1 14 13 15 11† 10 12 14 12† 11 8 19 20† NO3 -N + −1 NH4 -N g ha 13 27 26 28† 8 5 6 6† 8 5 16 13† Total P g ha−1 44 13 23 46 9 4 6 6 46 13 53 61 3- g ha−1 16 9 12 19 6 5 7 8 13 6 16 27 PO4 Particulate P g ha−1 25 3 10 2 5 1 1 1 30 7 29 32 † Values were omitted from the data analysis, as RDP2 could not be used as a reference due to different field management practices in 2011–2012. Journal of Environmental Quality 681 to allow fields operations. When resetting the groundwater level, water and nutrients stored in the soil might be released. In Y1 and Y2, the water stored in the root zone and drain pipes was released within 10 min (Fig. 4). In Y1, the drain flow was 1 mm higher from the CD plots than from the reference plots during the first week (Supplemental Table S7). In Y2, the drain flow was the same for CDP1 and the reference plots, although it was 2 mm higher from CDP2. During the first week after resetting the drainage level in Y1, − 3− the losses of NO3 –N, TP, and PO4 from the CD plots were higher than from reference plots (Supplemental Table S7). In 3− Y2, the losses of TP and PO4 were also higher from the CD − plots than from the reference plots, whereas for NO3 –N, the loss was slightly lower from CDP1. Discussion Impact of Controlled Drainage on Hydrology The groundwater level was naturally shallow at all plots; therefore, a regulation level of 70 cm was required to produce a significant groundwater level increase at plots with CD. The difference in groundwater level between plots with and without CD was largely driven by the climate and was greatest during dry periods. However, the average difference was small, even adjacent to the regulation well (on average 8 cm). In addition, the difference diminished with increased distance to the regulation well. However, it must be kept in mind that

18 15 − the piezometer pipes were located between drain pipes, and it Fig. 3. d O vs. d N of NO3 –N in drain water with 95% confidence intervals at plots with controlled drainage (CDP1 and CDP2) and is possible that the effect of CD was greater just above the tile reference plots (RP1 and RP2) during Y0 (2012–2013), Y1 (2013–2014), drain pipes, as the groundwater level increases with increasing and Y2 (2014–2015). * Significant relationship. distance to the drain pipes at fields with traditional drainage − 2 systems (Twitty and Rice, 2001). and NO3 –N (R = 0.57–0.98) at CD plots and RP1 in both Y1 − The implementation of CD did not affect the harvest yield of and Y2 supported the suggestion that the NO3 –N reduction was caused by denitrification, although the signal was somewhat winter wheat, which corroborates the findings of other studies on weaker in Y2 (R2 = 0.39) at RP2. CD, although different crops were investigated in these studies (corn [Zea mays L.] and soybean [Glycine max (L.) Merr.]; Tan et Drain Water Phosphorus Concentrations and Losses al., 1999; Fausey, 2005; Drury et al., 2009). However, in a Swedish 3− study, Wesström and Messing (2007) found a 2 to 18% higher The mean concentration of PO4 in the drain water was generally very low, ranging from 0.005 ± 0.002 to 0.008 ± 0.003 mg yield of spring crops (barley, oats [Avena sativa L.], and wheat) due −1 3− to greater N uptake when applying CD during all seasons. L during Y0 (Fig. 2D, Supplemental Table S6). The PO4 concentrations were not significantly affected by CD in either Y1 or The drain flow reduction of 6 to 54% found in this study is in Y2 (Table 1). agreement with the findings of 8 to 64% reductions in other field 3− studies conducted on silt and clay loam (Tan et al., 1999; Gaynor During Y1, the total loss of PO4 via drainage water decreased by 26% (1.7 g P ha−1) at CDP1 and increased by et al., 2002; Drury et al., 2009; Fang et al., 2012; Sunohara et 9% (0.6 g P ha−1) at CDP2 relative to the ratio between CD al., 2014; Williams et al., 2015). The effect of CD was clearly 3− strongest at the highest regulation level (70 cm), which agrees and reference plots during Y0 (Table 2). In Y2, the PO4 loss from CDP1 and CDP2 decreased by 41 and 51% (9.6 and 7.2 g with earlier studies (Evans et al., 1995; Wesström and Messing, ha−1), respectively. In Y1, the loss of TP increased by 5 to 57% 2007). In our study, only 50 to 75% of the plot was influenced from the CD plots compared with the reference plots, whereas by CD when the regulation level was 50 cm, whereas 100% of in Y2, the TP loss declined by 43 to 46% at the CD plots. The the plot was influence by CD when the level was 70 cm. Besides loss of PP was higher from both CD plots compared with the the regulation level, climate had a strong impact on the effect of reference plots in Y1, whereas in Y2, it was only higher from CD on the drain flow, which was smaller or even absent in wet 3− periods during Y2. Wesström and Messing (2007) also found a CDP2. During Y0, PO4 and PP constituted 36 to 69% and 23 to 57%, respectively, of TP. lower effect of CD in wet than in dry years. Thus, even though Y2 was drier than Y1 in our study, the effect of CD on drain Resetting the Regulation Level water reduction was greater in Y2 (37 to 54%) than in Y1 (5 to The elevated groundwater levels at plots with CD had to be 11%) due to the higher regulation level in Y2. reset to conventional drainage depth during spring and summer We expected a considerable release of drainage water and dissolved nutrients when resetting the regulation level at the CD

682 Journal of Environmental Quality − 3− Fig. 4. Drain flow and NO3 –N, total P (TP), and PO4 concentrations during the week after resetting the regulation well on 11 Mar. 2014 in Y1 and a 3− snapshot of the drain flow and TP and PO4 concentrations on 11 Mar. 2014. plots in spring due to the higher storage capacity. However, this used in heterotrophic denitrification or, possibly, dissimilatory − + − was not the case, the outflow from the CD plots being either only NO3 –N reduction to NH4 . Also NO3 –N reduction (23%) slightly higher or similar to the reference plots in Y1 and Y2. was higher than the drain flow reduction (11%), and the mean − However these results support the finding that the effect of CD NO3 –N concentration of drain water was considerably lower on groundwater levels was not as great or extensive as expected. in CDP1 than in to the reference plots. Thus, the results for − The water not leaving the field as drain flow can be lost Y1 suggest that denitrification contributed to NO3 –N reduc- through evaporation, surface runoff, storage, lateral subsurface tion at one of the plots. Only a few studies have found reduc- − flow, vertical seepage, and/or groundwater storage (Wesström et tion of the NO3 –N concentration in drain water (Lalonde et al., 2001; Skaggs et al., 2010). The importance of these pathways al., 1996; Drury et al., 2009; Ramoska et al., 2011, Sunohara depends on climate, soil properties, site conditions, drainage et al., 2014). Ramoska et al. (2011) recorded a 5 to 13% lower − system design and management, and crops (Skaggs et al., 2010). annual mean NO3 –N concentration in drainage water, and Only a few studies have investigated how CD alters the water Drury et al. (2009) found a 28% decrease in flow-weighted − pathways (Sunohara et al., 2014; Rozemeijer et al., 2016). In this mean NO3 –N during low N fertilizer application and a 7% study, we expected no increase in evaporation due to the study increase at high N fertilizer application. − season (winter), and surface runoff was not observed during In contrast with Y1, the reduction of NO3 –N loss was pro- the entire study period. Using the three-dimensional model portional to the reduction of drain flow in Y2, indicating that − DRAINMOD, Sunohara et al. (2014) found that after imple- the most important factor controlling the NO3 –N loss reduc- mentation of CD on a silt loam in Ontario, the lateral flow to tion this year was drain flow, which corroborates the findings of an adjacent field was the flow component increasing the most, the vast majority of studies on CD (Skaggs et al., 2012). Our followed by flow directly to the ditch and vertical percolation. results from the stable isotope analysis also showed that the presence of CD had no impact on denitrification, as denitrification Factors Affecting Nitrogen Loss via Drainage Water was the dominant fractionation process at all plots in Y2. Thus, − The loss of NO3 –N via drainage water can either be low- our results clearly demonstrate that to evaluate if enhanced deni- − ered by reducing the drain flow or the NO3 –N concentration. trification was a singular event or a general trend, longer study − The NO3 –N concentration in drainage water was not signifi- periods are needed. Additionally, the overall effect of CD as a − cantly reduced at the plots with CD regardless of the regula- mitigation measure reducing the NO3 –N loss to surface water tion level. However, in Y1, the stable isotope analysis and time could not be assessed in our study, which did not allow determi- − + 2− − series of NO3 –N, NH4 and SO4 concentrations provided nation of alternate flow routes of the water and NO3 –N bypass- evidence of enhanced denitrification at CDP1. The decrease ing the drain pipes. Thus, to evaluate potential water quality − 2− of NO3 –N and increase of SO4 concentrations indicated tradeoffs, CD must be more thoroughly investigated. Therefore + autotrophic denitrification, whereas the increase of NH4 CD could not be accepted as an official mitigation measure eli- + might be ascribed to NH4 release from the organic matter gible for subsidy in Denmark. Journal of Environmental Quality 683 d15 d18 − Impact of Controlled Drainage on Drain Water The analysis of N and O of NO3 –N in drain water Phosphorus Concentrations and Loss indicated that denitrification was the dominant fractionation process at both plots with and without CD. However, denitri- The impact of CD on P concentrations in drain water has not fication was markedly enhanced for a short, dry period at one of been as widely studied as the impact on N according to a recent the CD plots. Thus, to fully assess the overall effectiveness of CD, review by Ross et al. (2016). A concern related to CD is that the more investigations into the fate of N and P in the interactions elevation of the groundwater level will create conditions suffi- between groundwater and surface water are needed. 3− ciently reduced for Al- or Fe-bound PO4 to be released when 3− mineral oxides are reduced. However, in our study, the PO4 Conflict of Interest concentration in drain water was not significantly influenced by The authors declare no conflict of interest. CD. Other studies have also reported an insignificant effect of 3− CD on the PO4 concentration in drain water (Wesström and Supplemental Material Messing, 2007; Wesström et al., 2014; Williams et al., 2015). As The supplemental material includes texture analysis of the suggested by Williams et al. (2015), this might be due to the cir- soil, BACI analysis of the spatially distributed piezometer pipes, cumstance that groundwater levels are often only increased for agricultural management activities, loss of drain water and nutri- a short period of time, which prevents the occurrence of suffi- ents during the first week after opening of the regulation well, ciently reduced conditions. and time series of the chemical constitutes (SO 2− and NH +) The total losses of TP and PO 3− from the CD plots were 4 4 4 not presented in the paper. lower than from the reference plots in both Y1 and Y2, corresponding with the reduction of drain outflow. Other studies Acknowledgments have also reported a reduction of the P loss more or less identical The authors are grateful for assistance from the technicians at Aarhus with the reduction of the drain flow (, Wesström and Messing, University. The authors acknowledge Anne Mette Poulsen and Gabor 2007; Feset et al., 2010; Williams et al., 2015). Thus, a study by Löwei for proofreading and Tinna Christensen for graphical design. Rozemeijer et al. (2016) on sandy soil in the Netherlands also The research project was financed by the Green Development and Demonstration Programme (GUDP) of the Danish Ministry of found a decrease in P loss via drain pipes, most likely at the Environment and Food and supported by the Nordic Centre of Excellence expense of higher surface runoff and shallow groundwater flow BIOWATER, funded by NordForsk under Project no. 82263. to neighboring fields or direct flow to the ditch. When the regulation level was reset to the conventional References 3− drainage depth, the losses of TP and PO4 , although negligible, Allerup, P., and H. Madsen. 1980. Accuracy of point precipitation measure- were greater from the plots with CD than from the reference ments. Nord. Hydrol. 11:57–70. doi:10.2166/nh.1980.0005 plots. This indicates that resetting of the regulation level did not Aravena, R., and W.D. Robertson. 1998. Use of multiple isotope tracers to evalu- 3− ate denitrification in ground water: Study of nitrate from a large-flux septic entail a substantial risk of TP and PO4 release, as the losses system plume. Ground Water 36:975–982. doi:10.1111/j.1745-6584.1998. were small relative to the annual losses due to the very low release tb02104.x of soil water from the plots. Nevertheless, the results did show a Bouraoui, F., and B. Grizzetti. 2014. Modelling mitigation options to reduce 3− diffuse nitrogen water pollution from agriculture. Sci. Total Environ. 468- tendency toward higher TP and PO4 losses at the CD plots. 469:1267–1277. doi:10.1016/j.scitotenv.2013.07.066 Therefore this is an important aspect to consider when imple- Box, G.E.P., and D.R. Cox. 1964. An analysis of transformations. J. R. Stat. Soc. menting CD, as it is a common management practice to switch B 26:211–252. off CD several times in spring or autumn to improve the traffic- Carstensen, M.V., J.R. Poulsen, N.B. Ovesen, C.D. Børgesen, S.K. Hvid, and B. Kronvang. 2016. Can controlled drainage control agricultural nutrient emis- ability of the field. Thus, if CD is implemented at greater scale in sions? Evidence from a BACI experiment combined with a dual isotope ap- catchments upstream lakes or P limited estuaries, a springtime proach. Hydrol. Earth Syst. Sci. Discuss. doi:10.5194/hess-2016-303 pulse of P-rich water could have a negative impact on the water Casciotti, K.L., D.M. Sigman, M.G. Hastings, J.K. Böhlke, and A. Hilkert. 2002. Measurement of the oxygen isotopic composition of nitrate in seawater quality and, with it, the ecology of the water body. In conse- and freshwater using the denitrifier method. Anal. Chem. 74:4905–4912. quence, more investigations of this aspect are needed. doi:10.1021/ac020113w Drury, C.F., C.S. Tan, W.D. Reynolds, T.W. Welacky, T.O. Oloya, and J.D. Conclusion Gaynor. 2009. Managing tile drainage, subirrigation, and nitrogen fertil- ization to enhance crop yields and reduce nitrate loss. J. Environ. Qual. In this study, the regulation level of CD had to be at least 70 cm 38:1193–1204. doi:10.2134/jeq2008.0036 above the conventional drainage level to obtain a significant increase Evans, R., R. Wayne Skaggs, and J. Wendell Gilliam. 1995. Controlled ver- of the groundwater level and thus a decreased drain flow. Climate sus conventional drainage effects on water quality. J. Irrig. Drain. Eng. 121:271–276. doi:10.1061/(ASCE)0733-9437(1995)121:4(271) had a strong impact on CD, with more profound effect on drain Fang, Q.X., R.W. Malone, L. Ma, D.B. Jaynes, K.R. Thorp, T.R. Green, et al. flow and nutrient losses during dry periods. The total outflow of 2012. Modeling the effects of controlled drainage, N rate and weather on drain water from plots with CD was reduced by 37 to 54%, and a nitrate loss to subsurface drainage. Agric. Water Manage. 103:150–161. doi:10.1016/j.agwat.2011.11.006 decrease of similar magnitude was found for TN and TP losses via Fausey, N.R. 2005. Drainage management for humid regions. Int. Agric. Eng. drain pipes when using a regulation level of 70 cm. When resetting J. 14:209–214. the groundwater level to the conventional drain pipe level in March, Feset, S., J. Strock, G. Sands, and A. Birr. 2010. Controlled drainage to improve there was only a negligible extra loss of P and N from plots with edge-of-field water quality in southwest Minnesota, USA. In: 9th Interna- tional Drainage Symposium held jointly with CIGR and CSBE/SCGAB CD. However, if CD is implemented at large scale, there could be a Proceedings, Quebec City, Canada. 13–16 June 2010. ASABE, St. Joseph, potential risk of excess release of N and P when resetting the ground- MI. p. 299–308. doi:10.13031/2013.32137 water level to conventional levels. Controlled drainage had no effect Fukada, T., K.M. Hiscock, P.F. Dennis, and T. Grischek. 2003. A dual isotope ap- proach to identify denitrification in groundwater at a river-bank infiltration on harvest yields when winter wheat was cropped. site. Water Res. 37:3070–3078. doi:10.1016/S0043-1354(03)00176-3 684 Journal of Environmental Quality Gaynor, J.D., C.S. Tan, C.F. Drury, T.W. Welacky, H.Y.F. Ng, and W.D. Reyn- Rozemeijer, J.C., A. Visser, W. Borren, M. Winegram, Y. van der Velde, J. Klein, et olds. 2002. Runoff and drainage losses of atrazine, metribuzin, and meto- al. 2016. High-frequency monitoring of water fluxes and nutrient loads to lachlor in three water management systems. J. Environ. Qual. 31:300. assess the effects of controlled drainage on water storage and nutrient trans- doi:10.2134/jeq2002.3000 port. Hydrol. Earth Syst. Sci. 20:347–358. doi:10.5194/hess-20-347-2016 Gilliam, J.W., R.W. Skaggs, and S.B. Weed. 1979. Drainage control to diminish SAS Institute. 2013. SAS/STAT software. Release 9.4. SAS Inst., Cary, NC. nitrate loss from agricultural fields. J. Environ. Qual. 8:137. doi:10.2134/ Schoumans, O.F., W.J. Chardon, M.E. Bechmann, C. Gascuel-Odoux, G. Hofman, jeq1979.00472425000800010030x B. Kronvang, et al. 2014. Mitigation options to reduce phosphorus losses Granger, J., D.M. Sigman, M.F. Lehmann, and P.D. Tortell. 2008. Nitrogen and from the agricultural sector and improve surface water quality: A review. Sci. oxygen isotope fractionation during dissimilatory nitrate reduction by Total Environ. 468–469:1255–1266. doi:10.1016/j.scitotenv.2013.08.061 denitrifying bacteria. Limnol. Oceanogr. 53:2533–2545. doi:10.4319/ Sigman, D.M., K.L. Casciotti, M. Andreani, C. Barford, M. Galanter, and lo.2008.53.6.2533 J.K. Böhlke. 2001. A bacterial method for the nitrogen isotopic analy- ISO. 2003. ISO 12260. Water quality: Determination of nitrogen—Determina- sis of nitrate in seawater and freshwater. Anal. Chem. 73:4145–4153. tion of bound bnitrogen (TNb), following oxidation to nitrogen oxides. doi:10.1021/ac010088e Int. Org. Stand., Geneva. Sigman, D.M., J. Granger, P.J. DiFiore, M.M. Lehmann, R. Ho, G. Cane, et al. ISO. 2005. ISO 11732. Water quality: Determination of ammonium nitrogen— 2005. Coupled nitrogen and oxygen isotope measurements of nitrate Method by flow analysis (CFA and FIA) and spectrometric detection. Int. along the eastern North Pacific margin. Global Biogeochem. Cycles 19(4). Org. Stand., Geneva. doi:10.1029/2005GB002458 ISO. 2009. DS/EN ISO 10304-1:2009 Water quality: Determination of dis- Skaggs, R.W., N.R. Fausey, and R.O. Evans. 2012. Drainage water management. solved anions by liquid chromatography of ions—Part 1: Determination J. Soil Water Conserv. 67:167A–172A. doi:10.2489/jswc.67.6.167A of bromide, chloride, fluoride, nitrate, nitrite, phosphate and sulfate Dansk Skaggs, R.W., M.A. Youssef, J.W. Gillliam and R.O. Evans. 2010. Effect of con- standard. Int. Org. Stand., Geneva. trolled drainage on water and nitrogen balances in drained lands. Trans. Kadlec, R.H., and S.D. Wallac. 1996. Treatment wetlands. Taylor & Francis ASABE 53:1843–1850. doi:10.13031/2013.35810 Group, Didcot, UK. Stable Isotope Facility. 2015. Nitrate (NO3) analysis by bacteria denitrification. Kendall, C., E.M. Elliott, and S.D. Wankel. 2007. Tracing anthropogenic inputs Stable Isotope Facility, Univ. California, Davis. of nitrogen to ecosystems. In: R. Michener and K. Lajtha, editors, Stable Stewart-Oaten, A., W.W. Murdoch, and K.R. Parker. 1986. Environmental isotopes in ecology and environmental science. 2nd ed. Blackwell Publ., impact assessment: “Pseudoreplication” in time? Ecology 67:929–940. Oxford, UK. p. 375–449. doi:10.1002/9780470691854.ch12 doi:10.2307/1939815 Knowles, R. 1982. Denitrification. Microbiol. Rev. 46:43–70. Sunohara, M.D., E. Craiovan, E. Topp, N. Gottschall, C.F. Drury, and D.R. La- Lalonde, V., C.A. Madramootoo, L. Trenholm, and R.S. Broughton. pen. 2014. Comprehensive nitrogen budgets for controlled tile drainage 1996. Effects of controlled drainage on nitrate concentrations in fields in eastern Ontario, Canada. J. Environ. Qual. 43:617. doi:10.2134/ subsurface drain discharge. Agric. Water Manage. 29:187–199. jeq2013.04.0117 doi:10.1016/0378-3774(95)01193-5 Tan, C.S., C.F. Drury, J.D. Gaynor, and H.Y.F. Ng. 1999. Effect of controlled drain- Li, C., Y. Jiang, X. Guo, Y. Cao, and H. Ji. 2014. Multi-isotope (15N, 18O and 13C) age and subirrigation on subsurface tile drainage nitrate loss and crop yield indicators of sources and fate of nitrate in the upper stream of Chaobai Riv- at the farm scale. Can. Water Resour. J. 24:177. doi:10.4296/cwrj2403177 er, Beijing, China. Environ. Sci. Process. Impacts 16:2644. doi:10.1039/ Twitty, K., and J. Rice. 2001. Water table control. Part 624. Drainage Natl. Eng. C4EM00338A Handb. USDA-NRCS, Washington, DC. Mengis, M., S.L. Schif, M. Harris, M.C. English, R. Aravena, R.J. Elgood, et al. Wesström, I., A. Joel, and I. Messing. 2014. Controlled drainage and subirriga- 1999. Multiple geochemical and isotopic approaches for assessing ground tion: A water management option to reduce non-point source pollution − water NO3 elimination in a riparian zone. Ground Water 37:448–457. from agricultural land. Agric. Ecosyst. Environ. 198:74–82. doi:10.1016/j. doi:10.1111/j.1745-6584.1999.tb01124.x agee.2014.03.017 Ramoska, E., N. Bastiene, and V. Saulys. 2011. Evaluation of controlled drain- Wesström, I., and I. Messing. 2007. Effects of controlled drainage on N and P age efficiency in Lithuania. Irrig. Drain. 60:196–206. doi:10.1002/ird.548 losses and N dynamics in a loamy sand with spring crops. Agric. Water Rivett, M.O., S.R. Buss, P. Morgan, J.W.N. Smith, and C.D. Bemment. 2008. Ni- Manage. 87:229–240. doi:10.1016/j.agwat.2006.07.005 trate attenuation in groundwater: A review of biogeochemical controlling Wesström, I., I. Messing, H. Linnér, and J. Lindström. 2001. Controlled drain- processes. Water Res. 42:4215–4232. doi:10.1016/j.watres.2008.07.020 age: Effects on drain outflow and water quality. Agric. Water Manage. Ross, J.A., M.E. Herbert, S.P. Sowa, J.R. Frankenberger, K.W. King, S.F. Chris- 47:85–100. doi:10.1016/S0378-3774(00)00104-9 topher, et al. 2016. A synthesis and comparative evaluation of factors in- Williams, M.R., K.W. King, and N.R. Fausey. 2015. Drainage water manage- fluencing the effectiveness of drainage water management. Agric. Water ment effects on tile discharge and water quality. Agric. Water Manage. Manage. 178:366–376. doi:10.1016/j.agwat.2016.10.011 148:43–51. doi:10.1016/j.agwat.2014.09.017

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