Increases in agricultural nitrogen (N) inputs driven by synthetic N fertilizer application over the past century have led to higher crop yields but have also intensified riverine nitrate (NO3-) loading, contributing to environmental degradation. Drainage water management (DWM) is a best management practice (BMP) implemented on agricultural ditches to reduce downstream NO3- loading by slowing ditch discharge with drainage control structures that raise the in-field water table, creating anaerobic conditions. More anaerobic conditions stimulate denitrification and possibly methanogenesis. Denitrification consumes NO3-, thereby reducing the downstream N loading, but also increases production of N gases nitric oxide (NO), nitrous oxide (N2O), and dinitrogen (N2). This research examined the potential consequence of greenhouse gas (GHG) emissions, specifically methane (CH4) and N2O, as a result of DWM-induced low oxygen conditions in a replicated experimental design. Using multiple methods such as soil gas flux measurements, N isotope analyses, gases dissolved in groundwater, and N budgets, this project examined the potential pollution trade-offs between dissolved NO3- and soil GHG fluxes.

In chapter 2, I quantified soil N2O and CH4 fluxes using static chambers over three years in a corn/soybean rotation system. I also measured soil environmental variables to assess controls on gas production. Results indicated that the DWM treatment raised the groundwater level near the ditch edge but did not increase the surface soil moisture, which likely led to the observation that DWM did not significantly increase soil N2O and CH4 emissions. Variation in N2O fluxes were heavily influenced by N fertilizer application events. A N budget indicated that this farm site had a lower than average N use efficiency in the U.S. and higher than average soil N2O emissions.

In chapter 3, I qualitatively and quantitatively examined the role of denitrification in this DWM system by using natural abundance NO3- isotopes measured across a leaching continuum (topsoil to deep soil to groundwater to ditch water). Results demonstrated that isotopic values of δ15N and δ18O increased in residual NO3- along the leaching continuum, providing evidence of denitrification. However, the net effects of nitrification and denitrification resulted in NO3- less enriched in 15N than expected by denitrification alone. These isotopic values were then applied to a mass balance of total N and δ15N to quantitively calculate the magnitude of total gaseous N export and to constrain that estimate using a net N isotopic discrimination factor. The calculated gaseous N export and denitrification rates fell within but toward the high end of previously reported literature ranges. The N budget indicated lower hydrologic N export in the DWM treatment, suggesting increased denitrification, but uncertainty of the corresponding estimates of increased gaseous N export was greater than the difference between treatments, rendering inconclusive the hypothesis that DWM treatment causes more total gaseous N production and denitrification. Inclusion of isotopes in the N mass balance established a lower bound of total gaseous N export, which was still large relative to other budget terms.

In chapter 4, I synthesized results from the previous two chapters to explore the components of total gaseous N export. I also estimated annual export via dissolved N2O and N2 in groundwater entering the drainage ditches. Soil N2 emissions were estimated by subtracting annual estimates of soil N2O and groundwater dissolved N2 and N2O from the total gaseous N export. Results showed that soil N2 emissions dominated the gaseous N export. The N2O/(N2 + N2O) ratios of soil emissions were within but on the lower side of the literature range.

This study demonstrated that, at least for this farm, the decrease in hydrologic N loading due to implementation of DWM outweighed the small and statistically non-significant observed increase in GHG production. This result lends support for policies to further incentivize adoption of DWM in ditched agricultural settings. This study also provides a novel, multi-methodological approach for quantitatively assessing and constraining denitrification rates and N2 emissions. It also is the first study to incorporate measurement of multiple fractions of total gaseous N export on the farm scale as part of annualized agricultural N budget.