Insights on agricultural nitrate leaching from soil block mesocosms.

Quantifying nitrate leaching in agricultural fields is often complicated by inability to capture all water draining through a specific area. We designed and tested undisturbed soil monoliths (termed "soil block mesocosms") to achieve complete collection of drainage. Each mesocosm measures 1.5 m × 1.5 m × 1.2 m and is enclosed by steel on the sides and bottom with a single outlet to collect drainage. We compared measurements from replicate mesocosms planted to corn (Zea mays L.) with a nearby field experiment with tile-drained plots ("drainage plots"), and with drainage from nearby watersheds from 2020 through 2022 under drought conditions. Annual mesocosm drainage volumes were 6.5-24.6 cm greater than from the drainage plots, likely because the mesocosms were isolated from the subsoil and could not store groundwater below the drain depth, whereas the drainage plots accumulated infiltration as groundwater. Thus, we obtained consistent nitrate leaching measurements from the mesocosms even when some drainage plots yielded no water. Despite drainage volume differences, mean flow-weighted nitrate concentrations were similar between mesocosms and drainage plots in 2 of 3 years. Mesocosm annual drainage volume was 8.7 cm lower to 16.7 cm higher than watershed drainage, likely due to lagged influences of groundwater. Corn yields were lower in mesocosms than drainage plots in 2020, but with irrigation, yields were similar in subsequent years. Mean 2020 surface soil moisture and temperature were similar between the mesocosms and nearby fields. Based on these comparisons, the mesocosms provide a robust method to measure nitrate leaching with lower variability than field plots.

Impact of nitrogen fertilizer timing on nitrate loss and crop production in northwest Iowa.

In the U.S. Midwest, nitrate in subsurface tile drainage from corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] systems is detrimental to water quality at local and national scales. The objective of this replicated plot study in northwest Iowa, performed in 2015-2020, was to investigate the influence of nitrogen (N) fertilizer timing on crop production and NO3 load in subsurface (tile) drainage discharge. Four treatments applied to corn included fall anhydrous ammonia with a nitrification inhibitor (F), spring anhydrous ammonia (S), split-banded urea at planting and mid-vegetative growth (SS), and no N fertilizer (0N). Across crops and years, NO3 -N concentration in subsurface drainage discharge was the same at 11.7 mg L-1 for F and S applied anhydrous ammonia (AA). The NO3 -N concentration was statistically lower with SS urea (10 mg L-1 ) than F and S, and 0N was lower than SS at 8.3 mg L-1 . Average annual NO3 -N loads were not different between any treatments due to plot variability in drainage discharge. Corn responded to N application, with overall mean yield the same for F, S, and SS. There were no agronomic or water quality benefits for applying AA in spring compared with fall, where the F included a nitrification inhibitor and was applied to cold soils. Split-applied urea had a small positive water quality impact but no crop yield enhancement. This study shows that there were improvements to NO3 -N concentration in subsurface drainage discharge, but more nutrient reduction practices are needed than fertilizer N management alone to reduce nitrate load to surface water systems.

Midwestern cropping system effects on drainage water quality and crop yields.

Grain producers are challenged to maximize crop production while utilizing nutrients efficiently and minimizing negative impacts on water quality. There is a particular concern about nutrient export to the Gulf of Mexico via loss from subsurface drainage systems. The objective of this study was to investigate the effects of crop rotation, tillage, crop residue removal, swine manure applications, and cereal rye (Secale cereale L.) cover crops on nitrate-N (NO3 -N) and total reactive phosphorus (TRP) loss via subsurface drainage. The study was evaluated from 2008 through 2015 using 36 0.4-ha plots outfitted with a subsurface drainage water quality monitoring system. Results showed that when swine manure was applied before both corn (Zea mays L.) and soybean [Glycine max (L.) Merr.], drainage water had significantly higher 8-yr-average flow-weighted NO3 -N concentrations compared with swine manure applied before corn only in a corn-soybean rotation. The lowest NO3 -N loss was 15.2 kg N ha-1  yr-1 from a no-till corn-soybean treatment with rye cover crop and spring application of urea-ammonium nitrate (UAN) to corn. The highest NO3 -N loss was 29.5 kg N ha-1  yr-1 from swine manure applied to both corn and soybean. A rye cover crop reduced NO3 -N loss, whereas tillage and residue management had little impact on NO3 -N loss. Losses of TRP averaged <32 g P ha-1  yr-1 from all treatments. Corn yield was negatively affected by both no-till management and cereal rye cover crops. Results showed that cropping management affected N leaching but impacts on P leaching were minimal.

Influence of no-till and a winter rye cover crop on nitrate losses from tile-drained row-crop agriculture in Iowa.

Artificial subsurface drainage is necessary to maintain agricultural production in the soils and climate of north-central Iowa. However, it can result in adverse environmental impacts, because it intercepts and diverts some water and soluble NO3 -N directly to streams. We investigated the impact of no-till and a winter rye cover crop (Secale cereale L.) on seasonal and annual NO3 -N concentration and loading in leachate from a corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation. The eight treatments are chisel plow (CT), chisel plow with winter cereal rye (CTr), no-till (NT), and no-till with winter cereal rye (NTr), with "-C" indicating corn and "-S" indicating soybeans. Plots with artificial subsurface drainage were monitored for water quality from 2011 to 2015. The NT and CTr treatments consistently decreased NO3 -N loss on the seasonal and annual scales compared with CT. Compared with NT, NTr did not reduce NO3 -N loading nor concentration in leachate, probably because of low NO3 leaching potential from NT combined with low rye cover crop biomass throughout the study with NT. The 5-yr average annual NO3 -N concentrations were: 16.9 mg L-1 with CT-S, 16.7 mg L-1 with CT-C, 12.6 mg L-1 with NT-S, 12.0 mg L-1 with CTr-S, 11.8 mg L-1 with CTr-C, 11.4 mg L-1 with NTr-S and NTr-C, and 11.1 mg L-1 with NT-C. Overall, both no-till and a cover crop showed potential for improving N management for water quality.

Corn stover harvest increases herbicide movement to subsurface drains - Root Zone Water Quality Model simulations.

BACKGROUND: Crop residue removal for bioenergy production can alter soil hydrologic properties and the movement of agrochemicals to subsurface drains. The Root Zone Water Quality Model (RZWQM), previously calibrated using measured flow and atrazine concentrations in drainage from a 0.4 ha chisel-tilled plot, was used to investigate effects of 50 and 100% corn (Zea mays L.) stover harvest and the accompanying reductions in soil crust hydraulic conductivity and total macroporosity on transport of atrazine, metolachlor and metolachlor oxanilic acid (OXA). RESULTS: The model accurately simulated field-measured metolachlor transport in drainage. A 3 year simulation indicated that 50% residue removal reduced subsurface drainage by 31% and increased atrazine and metolachlor transport in drainage 4-5-fold when surface crust conductivity and macroporosity were reduced by 25%. Based on its measured sorption coefficient, approximately twofold reductions in OXA losses were simulated with residue removal. CONCLUSION: The RZWQM indicated that, if corn stover harvest reduces crust conductivity and soil macroporosity, losses of atrazine and metolachlor in subsurface drainage will increase owing to reduced sorption related to more water moving through fewer macropores. Losses of the metolachlor degradation product OXA will decrease as a result of the more rapid movement of the parent compound into the soil. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.

Subsurface Drainage Nitrate and Total Reactive Phosphorus Losses in Bioenergy-Based Prairies and Corn Systems.

We compare subsurface-drainage NO-N and total reactive phosphorus (TRP) concentrations and yields of select bioenergy cropping systems and their rotational phases. Cropping systems evaluated were grain-harvested corn-soybean rotations, grain- and stover-harvested continuous corn systems with and without a cover crop, and annually harvested reconstructed prairies with and without the addition of N fertilizer in an Iowa field. Drainage was monitored when soils were unfrozen during 2010 through 2013. The corn-soybean rotations without residue removal and continuous corn with residue removal produced similar mean annual flow-weighted NO-N concentrations, ranging from 6 to 18.5 mg N L during the 4-yr study. In contrast, continuous corn with residue removal and with a cover crop had significantly lower NO-N concentrations of 5.6 mg N L when mean annual flow-weighted values were averaged across the 4 yr. Prairies systems with or without N fertilization produced significantly lower concentrations below <1 mg NO-N L than all the row crop systems throughout the study. Mean annual flow-weighted TRP concentrations and annual yields were generally low, with values <0.04 mg TRP L and <0.14 kg TRP ha, and were not significantly affected by any cropping systems or their rotational phases. Bioenergy-based prairies with or without N fertilization and continuous corn with stover removal and a cover crop have the potential to supply bioenergy feedstocks while minimizing NO-N losses to drainage waters. However, subsurface drainage TRP concentrations and yields in bioenergy systems will need further evaluation in areas prone to higher levels of P losses.

Impact of system management on vegetative treatment system effluent concentrations.

Beef feedlots of all sizes are looking for more cost-effective solutions for managing feedlot runoff. Vegetative treatment systems are one potential option, but require performance evaluation for use on concentrated animal feeding operations. The performance of six vegetative treatment systems on open beef feedlots throughout Iowa was monitored from 2006 through 2009. These feedlots had interim, National Pollution Discharge Elimination System permits that allowed the use of vegetative treatment systems to control and treat runoff from the open feedlots. This manuscript focuses on making within site comparisons, i.e., from year-to-year and component-to-component within a site, to evaluate how management changes and system modifications altered performance. The effectiveness, in terms of effluent concentration reductions, of each system was evaluated; nutrient concentration reductions typically ranged from 60 to 99% during treatment in the vegetative components of the vegetative treatment systems. Monitoring results showed a consistent improvement in system performance during the four years of study. Much of this improvement can be attributed to improved management techniques and system modifications that addressed key performance issues. Specifically, active control of the solid settling basin outlet improved solids retention and allowed the producers to match effluent application rates to the infiltration rate of the vegetative treatment area, reducing the occurrence of effluent release. Additional improvements resulted from system maturation, increased operator experience, and the addition of earthen flow spreaders within the vegetative treatment area to slow flow and provide increased effluent storage within the treatment area, and switching to active management of settling basin effluent release.

Nitrate-nitrogen losses through subsurface drainage under various agricultural land covers.

Nitrate-nitrogen (NO3-N) loading to surface water bodies from subsurface drainage is an environmental concern in the midwestern United States. The objective of this study was to investigate the effect of various land covers on NO3-N loss through subsurface drainage. Land-cover treatments included (i) conventional corn ( L.) (C) and soybean [ (L.) Merr.] (S); (ii) winter rye ( L.) cover crop before corn (rC) and before soybean (rS); (iii) kura clover ( M. Bieb.) as a living mulch for corn (kC); and (iv) perennial forage of orchardgrass ( L.) mixed with clovers (PF). In spring, total N uptake by aboveground biomass of rye in rC, rye in rS, kura clover in kC, and grasses in PF were 14.2, 31.8, 87.0, and 46.3 kg N ha, respectively. Effect of land covers on subsurface drainage was not significant. The NO3-N loss was significantly lower for kC and PF than C and S treatments (p < 0.05); rye cover crop did not reduce NO3-N loss, but NO3-N concentration was significantly reduced in rC during March to June and in rS during July to November (p < 0.05). Moreover, the increase of soil NO3-N from early to late spring in rS was significantly lower than the S treatment (p < 0.05). This study suggests that kC and PF are effective in reducing NO3-N loss, but these systems could lead to concerns relative to grain yield loss and change in farming practices. Management strategies for kC need further study to achieve reasonable corn yield. The effectiveness of rye cover crop on NO-N loss reduction needs further investigation under conditions of different N rates, wider weather patterns, and fall tillage.