Direct and Indirect Nitrous Oxide Emissions from Saturated Riparian Buffers and Woodchip Bioreactors: Are We Trading a Water Quality Problem for and Air Quality Problem?.

Edge-of-field conservation practices have the potential to remove large quantities of nitrate (NO₃) from tile drained water. Practices including saturated riparian buffers (SRBs) and woodchip bioreactors utilize microbial denitrification to convert NO₃ to nitrogen gas. Unfortunately, incomplete denitrification can result in nitrous oxide (N₂O) production. The global warming potential of N₂O is 298 times that of carbon dioxide and N₂O is considered a major contributor to global climate change. The objective of this study was to quantify both direct and indirect N₂O emissions from SRBs and woodchip bioreactors in Central Iowa. Direct emissions were measured from the soil surface of each conservation practice. Indirect emissions were measured as dissolved N₂O concentrations from water flowing through SRBs and bioreactors. For comparison, N₂O emissions were measured from conventional unsaturated riparian buffers and agricultural fields adjacent to the SRBs. Direct emissions from SRBs were only significantly greater than the unsaturated counterparts at one site, in one year. Direct emissions from both SRBs and bioreactors were significantly less than field emissions. Furthermore, N₂O production was only a small fraction of the total NO₃ removed, suggesting the majority of NO₃ was taken up by plants or completely denitrified. Dissolved N₂O concentrations in the SRBs were greatest closest to the distribution tile and negligible at the stream edge. Comparatively, dissolved N₂O concentrations in the bioreactors increased from the inlet to the outlet. The increase of dissolved N₂O from bioreactors increases the potential for indirect emissions after the treated water enters surface waters. Significantly reducing nitrate loss to surface waters is going to take a suite of conservation practices and it is important to evaluate potential pitfalls before mass implementation. The inclusion of both direct and indirect N₂O emissions is essential for evaluating the global warming potential of edge-of field conservation practices.