Nitrous oxide (N2O) is a potent greenhouse gas and stratospheric ozone-depleting substance. More than half of anthropogenic N2O emissions result from agricultural activities. A broad objective of this on-farm research in eastern Maryland was to investigate whether drainage water management, which reduces nitrate export, would increase greenhouse gas emissions, but here we focus upon comparing chamber and tower measurements of N2O fluxes from a single field. Chamber methods usually suffer from poor spatial and temporal resolution. Automating chambers using in situ fast response analyzers improves temporal but not spatial resolution. Tower-based micrometeorological methods improve both temporal and spatial resolution, but require a high-frequency, high-sensitivity laser instrument. We compared auto-chamber and micrometeorological gradient methods for N2O flux measurement during a period early in the 2019 corn-growing season. A 3 m tall tower was deployed to allow for near-continuous gradient flux measurements using an Aerodyne Quantum Cascade Laser. Four Eosense closed dynamic automated chambers (eocAC) and a multiplexer (eosMX) were installed near the tower and connected to a Picarro G2308 gas analyzer. Both methods captured strong pulses of N2O fluxes after rainfall and fertilization events, demonstrating these major drivers of large emissions. Fluxes from the two methods were linearly correlated (R2 = 0.54), but the slope (1.29 ± 0.08) and y-intercept (48.3 ± 19.2) indicate that the chambers generally estimated higher fluxes. Aggregating over the measurement period, the automated chamber estimate was 2.5 kg N2O-N/ha in 19 days, whereas the tower-based gradient estimate was 1.3 kg N2O-N/ha in 19 days. The tower footprint includes some area (4%) covered by ditches and could extend beyond the field at times, but this is unlikely the only explanation. The small number of chambers may have sampled an area of above average flux, or there could be unknown measurement bias or interpolation error in one or both methods. To our knowledge, this is the first such methodological comparison of N2O fluxes since these sensitive, fast response instruments have become available, and our results demonstrate that additional work is needed to gain more confidence in reported fluxes by either method.

Excess nutrient loading to downstream waters has been a persistent environmental concern, especially in agricultural settings. Drainage water management (DWM) is a best management practice intended to reduce nitrogen export from fertilized lands by increasing groundwater levels, slowing the loss of nutrient-rich water and increasing its time in contact with the soil, thus creating greater opportunity for denitrification. This BMP has shown to be effective at reducing dissolved nitrate (TDN) export, but a question remains about potential unintended pollution swapping. The concern is that denitrification could result in nitrous oxide (N2O) emissions and that higher soil moisture could also create suitable conditions for methanogenesis and methane (CH4) emissions. Here we report on two years of monthly static soil gas chamber fluxes and hydrologic nutrient fluxes during a full corn/soybean rotation cycle on the Eastern Shore of Maryland. For N2O, there were significant interactions between season, crop type, and treatment, such as higher fluxes during the fertilization period in the corn year in the DWM treatment, which was consistent with our concern about pollution swapping. However, this brief additional pulse of N2O did not result in a statistically significant increase at an annual scale, nor was there an increase in annual CH4 emissions. At the same time, annual TDN load was significantly lower in the DWM ditches compared to the control. With no significant treatment effect on soil gas fluxes and a significant treatment effect on TDN export, we conclude that pollution swapping of nitrate reduction for greenhouse gases did not occur significantly in this application of DWM to a corn/soybean system. We did, however, find evidence of pollution swapping of phosphorus and nitrogen, as total phosphorus load was higher in the DWM. With more water in the field, the reduced conditions appear to cause a release of soil bound phosphorus. While greenhouse gas production may not be as much of a concern, increased phosphorus export represents a form of pollution swapping that must be accounted for in determining the value of this BMP.