Nitrate losses and nitrous oxide emissions under contrasting tillage and cover crop management.

Agroecosystems in the upper Mississippi River Basin are highly productive but often contribute to deterioration of water quality and greenhouse gas emissions. Cover cropping and no-till are conservation strategies implemented to reduce the environmental impact of these agroecosystems. However, using multiple strategies can lead to systemwide interactions that are not fully understood. These interactions can affect not only environmental quality metrics, such as subsurface drainage nitrate losses or nitrous oxide (N2 O) emissions, but also may influence crop production potential. A field trial was initiated comparing nitrate losses, N2 O emissions, and crop production under systems with fall chisel plow tillage, fall chisel plow tillage with an oat (Avena sativa L.) cover crop (CP-oat), no-till (NT), no-till with a rye (Secale cereale L.) cover crop (NT-rye), and NT with zero N fertilizer. Pathways for nitrate losses and N2 O emissions did not appear linked and were not tied to cover crop or tillage practices. Nitrate losses were linked with drainage volumes, and cover crops and tillage had limited effect on cumulative drainage volumes. Notably, NT-rye altered the relationship between drainage volume and nitrate losses by reducing nitrate concentrations, lowering nitrate losses by 59 ±9% compared with CP-oat and 67 ± 9% compared with NT. Neither cover crop nor tillage consistently affected N2 O emissions or crop yield. Rather, N2 O emissions were closely tied with fertilizer N application and seasonal weather patterns. These findings indicate that nitrate leaching and N2 O emissions are regulated by separate mechanisms, so conservation management may require stacking multiple practices to be effective.

Both subsurface nitrate losses and nitrous oxide emissions were linked with weather. Subsurface nitrate losses were linked with cumulative annual drainage. Nitrous oxide emissions were linked with fertilizer N applications. Rye cover crop with no-till reduced nitrate losses with no yield declines.

Nitrous Oxide Emissions from Saturated Riparian Buffers: Are We Trading a Water Quality Problem for an Air Quality Problem?

Reestablishing perennial vegetation along riparian areas in agroecosystems reduces nutrient and sediment losses from agricultural lands. However, subsurface (tile) drains bypass traditional buffers routing the majority of shallow groundwater straight to surface waters, limiting their nutrient removal capabilities. Saturated riparian buffers (SRBs) reconnect subsurface drainage water with the soil profile to remove NO in tile water through microbial denitrification. One concern of enhancing denitrification on agricultural landscapes is the potential increase in NO emissions from incomplete denitrification. Our objective was to compare NO emissions from SRBs to traditional buffers and bordering crop fields at two sites, Bear Creek Site 1 and Iowa Site 1, in Central Iowa. We measured NO emissions directly from the soil surface and dissolved in shallow groundwater and estimated indirect emissions from downstream denitrification from 2015 through 2017. Nitrous oxide emissions from soil surfaces were greatest from fertilized corn ( L.). Saturated riparian buffers were only significantly greater ( < 0.05) than traditional buffers in one out of six site-years. Dissolved NO in shallow groundwater seeping from SRBs was not significantly greater ( < 0.05) than dissolved NO from the tile outlet among site years. Indirect NO emissions from rivers and estuaries were significantly reduced from NO removal in both SRBs. Overall, total NO emissions from SRBs were similar to those from traditional buffers and less than those from fertilized corn-soybean [ (L.) Merr.] agriculture. Replacing cultivated land in riparian areas with a SRB has shown potential to subsequently remove NO from surface waters and reduce NO emissions from agricultural landscapes.

In Situ Denitrification in Saturated Riparian Buffers.

Excess NO leaching from the agricultural Midwest via tile drainage water has contributed to both local drinking water and national Gulf of Mexico benthic hypoxia concerns. Both in-field and edge-of-field practices have been designed to help mitigate NO flux to surface waters. Edge-of-field practices focus on maximizing microbial denitrification, the conversion of NO to N gas. This study assessed denitrification rates from two saturated riparian buffers (SRBs) for 2 yr and a third SRB for 1 yr, for a total of five sample years. These SRBs were created by diverting NO-rich tile drainage water into riparian buffers soils. The SRBs in this study removed between 27 and 96% of the total diverted NO load. Measured cumulative average denitrification rate for each SRB sample year accounted for between 3.7 and 77.3% of the total NO removed. Both the cumulative maximum and 90% confidence interval denitrification rates accounted for all of the NO removed by the SRBs in three of the five sample years, indicating that denitrification can be a dominant NO removal mechanism in this edge-of-field practice. When adding the top 20 cm of each core to the cumulative denitrification rates for each SRB, denitrification accounted for between 33 and over 100% of the total NO removed. Buffer age (time since establishment) was speculated to enhance denitrification rates, and there was a trend of the soil closer to the surface making up the majority of the total denitrification rate. Finally, both NO and C could limit denitrification in these SRBs.

Simulating Woodchip Bioreactor Performance Using a Dual-Porosity Model.

There is a general understanding in the scientific community as to how denitrifying bioreactors operate, but we lack a quantitative understanding of the details of the denitrification process acting within them and comprehensive models for simulating their performance. We hypothesized that nitrate transport through woodchip bioreactors would be best described by a dual-porosity transport model where the bioreactor water is divided into a mobile domain (i.e., the water between the woodchips where it is free to flow and solute movement is by advection and dispersion) and an immobile domain of water (i.e., the water mostly within the woodchips that is stagnant and where solute movement is by diffusion alone). We calibrated the dual-porosity model contained in the HYDRUS model for a woodchip bioreactor using the results of a Br breakthrough experiment where we treated Br as a conservative nonadsorbing tracer. We then used the resulting model parameters to describe 2 yr of NO transport and denitrification within a bioreactor supplied by tile drainage. The only model parameters fitted to the NO data were either the zero- or first-order denitrification rate and its temperature dependence. The bioreactor denitrified 2.23 kg N (38%) of the NO entering it in 2013 and 3.73 kg N (49%) of the NO that entered it in 2014. The dual-porosity model fit the NO data very well, with fitted zero-order reaction rates of 8.7 and 6.8 mg N L d in 2013 and 2014, respectively, and corresponding first-order reaction rates of 0.99 and 1.02 d. For the 2-yr data set, both reaction rate models fit the data equally well. Consistent model parameters fitted for the 2 yr indicated that the model used was robust and a promising approach for modeling fate and transport of NO in woodchip bioreactors.

Nitrogen fertilizer effects on soil carbon balances in midwestern U.S. agricultural systems.

A single ecosystem dominates the Midwestern United States, occupying 26 million hectares in five states alone: the corn-soybean agroecosystem [Zea mays L.-Glycine max (L.) Merr.]. Nitrogen (N) fertilization could influence the soil carbon (C) balance in this system because the corn phase is fertilized in 97-100% of farms, at an average rate of 135 kg N x ha(-1) x yr(-1). We evaluated the impacts on two major processes that determine the soil C balance, the rates of organic-carbon (OC) inputs and decay, at four levels of N fertilization, 0, 90, 180, and 270 kg/ha, in two long-term experimental sites in Mollisols in Iowa, USA. We compared the corn-soybean system with other experimental cropping systems fertilized with N in the corn phases only: continuous corn for grain; corn-corn-oats (Avena sativa L.)-alfalfa (Medicago sativa L.; corn-oats-alfalfa-alfalfa; and continuous soybean. In all systems, we estimated long-term OC inputs and decay rates over all phases of the rotations, based on long-term yield data, harvest indices (HI), and root:shoot data. For corn, we measured these two ratios in the four N treatments in a single year in each site; for other crops we used published ratios. Total OC inputs were calculated as aboveground plus belowground net primary production (NPP) minus harvested yield. For corn, measured total OC inputs increased with N fertilization (P < 0.05, both sites). Belowground NPP, comprising only 6-22% of total corn NPP, was not significantly influenced by N fertilization. When all phases of the crop rotations were evaluated over the long term, OC decay rates increased concomitantly with OC input rates in several systems. Increases in decay rates with N fertilization apparently offset gains in carbon inputs to the soil in such a way that soil C sequestration was virtually nil in 78% of the systems studied, despite up to 48 years of N additions. The quantity of belowground OC inputs was the best predictor of long-term soil C storage. This indicates that, in these systems, in comparison with increased N-fertilizer additions, selection of crops with high belowground NPP is a more effective management practice for increasing soil C sequestration.

Denitrification in wood chip bioreactors at different water flows.

Subsurface drainage in agricultural watersheds exports a large quantity of nitrate-nitrogen (NO(3)-N) and concentrations frequently exceed 10 mg L(-1). A laboratory column study was conducted to investigate the ability of a wood chip bioreactor to promote denitrification under mean water flow rates of 2.9, 6.6, 8.7 and 13.6 cm d(-1) which are representative of flows entering subsurface drainage tiles. Columns were packed with wood chips and inoculated with a small amount of oxidized till and incubated at 10 degrees C. Silicone sampling cells at the effluent ports were used for N(2)O sampling. (15)Nitrate was added to dosing water at 50 mg L(-1) and effluent was collected and analyzed for NO(3)-N, NH(4)-N, and dissolved organic carbon. Mean NO(3)-N concentrations in the effluent were 0.0, 18.5, 24.2, and 35.3 mg L(-1) for the flow rates 2.9, 6.6, 8.7, and 13.6 cm d(-1), respectively, which correspond to 100, 64, 52, and 30% efficiency of removal. The NO(3)-N removal rates per gram of wood increased with increasing flow rates. Denitrification was found to be the dominant NO(3)-N removal mechanism as immobilization of (15)NO(3)-N was negligible compared with the quantity of (15)NO(3)-N removed. Nitrous oxide production from the columns ranged from 0.003 to 0.028% of the N denitrified, indicating that complete denitrification generally occurred. Based on these observations, wood chip bioreactors may be successful at removing significant quantities of NO(3)-N, and reducing NO(3)-N concentration from water moving to subsurface drainage at flow rates observed in central Iowa subsoil.

In situ bioreactors and deep drain-pipe installation to reduce nitrate losses in artificially drained fields.

Nitrate in water removed from fields by subsurface drain ('tile') systems is often at concentrations exceeding the 10 mg N L(-1) maximum contaminant level (MCL) set by the USEPA for drinking water and has been implicated in contributing to the hypoxia problem within the northern Gulf of Mexico. Because previous research shows that N fertilizer management alone is not sufficient for reducing NO(3) concentrations in subsurface drainage below the MCL, additional approaches are needed. In this field study, we compared the NO(3) losses in tile drainage from a conventional drainage system (CN) consisting of a free-flowing pipe installed 1.2 m below the soil surface to losses in tile drainage from two alternative drainage designs. The alternative treatments were a deep tile (DT), where the tile drain was installed 0.6 m deeper than the conventional tile depth, but with the outlet maintained at 1.2 m, and a denitrification wall (DW), where trenches excavated parallel to the tile and filled with woodchips serve as additional carbon sources to increase denitrification. Four replicate 30.5- by 42.7-m field plots were installed for each treatment in 1999 and a corn-soybean rotation initiated in 2000. Over 5 yr (2001-2005) the tile flow from the DW treatment had annual average NO(3) concentrations significantly lower than the CN treatment (8.8 vs. 22.1 mg N L(-1)). This represented an annual reduction in NO(3) mass loss of 29 kg N ha(-1) or a 55% reduction in nitrate mass lost in tile drainage for the DW treatment. The DT treatment did not consistently lower NO(3) concentrations, nor reduce the annual NO(3) mass loss in drainage. The DT treatment did exhibit lower NO(3) concentrations in tile drainage than the CN treatment during late summer when tile flow rates were minimal. There was no difference in crop yields for any of the treatments. Thus, denitrification walls are able to substantially reduce NO(3) concentrations in tile drainage for at least 5 yr.

Comparing carbon substrates for denitrification of subsurface drainage water.

Nitrate in water from tile drained corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] fields in the U.S. Midwest contributes to nitrate contamination of surface waters. Denitrification-based biofilters are a promising strategy for reducing nitrate concentrations, but these systems require an external carbon supply to sustain denitrification. The ability of four organic materials to serve as carbon substrates for denitrification biofilters was evaluated in this laboratory study. Wood chips, wood chips amended with soybean oil, cornstalks, and cardboard fibers were mixed with subsoil (oxidized till) and incubated anaerobically for 180 d. Periodically, 15NO3-N was added to maintain nitrate N concentrations between 10 and 100 mg L-1. All of the materials stimulated NO3-N removal and the degree of removal from highest to lowest was: cornstalks, cardboard fibers, wood chips with oil, and wood chips alone. Analysis of 15N showed that immobilization and dissimilatory nitrate reduction to ammonium accounted for <4% of NO3-N removal in all treatments, therefore denitrification was the dominant NO3-N removal process. Cardboard fibers, wood chips and oil, and wood chips alone did not support as much denitrification as cornstalks, but their rates of NO3-N removal were steady and would probably continue longer than cornstalks. The addition of soybean oil to wood chips significantly increased denitrification over wood chips alone.