The Influence Of Winter Field Cover On Spring Nitrous Oxide Emissions

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Agriculture is responsible for 58% of the anthropogenic emissions of nitrous oxide (N2O), a source of stratospheric ozone depletion and a greenhouse gas that contributes to global climate change. In temperate regions, a majority of this N2O is emitted during freeze-thaw cycles (FTC) in the spring. Climate change models predict that warming trends in the northeast will result in less snow cover, potentially leading to colder soils that may in turn lead to higher N2O emissions. A winter soil management strategy is needed to mitigate spring N2O emissions. In this study, I examined the influence of two winter field covers, snow and winter rye, on soil temperature and nitrogen (N) content and subsequent spring N2O emissions from a NY corn field over two years. A 2 x 2 factorial of rye (+/-) by snow cover (+/- ) was established in a randomized complete block design. Nitrous oxide emissions were measured bi-weekly using a static chamber method. The first season (2006-07) was a cold winter (2309 h below 0 degrees C at 8 cm soil depth), historically typical for the region. The snow removal treatment resulted in colder soils and higher N2O fluxes (73.3 vs. 57.9 ng N2O-N cm-2 h-1). The rye cover had no effect on N2O emissions. The second season (2007-08) was a much milder winter (1271 h below freezing at 8 cm soil depth), with lower N2O fluxes overall. Winter rye cover resulted in lower N2O fluxes (5.9 vs. 33.7 ng N2O-N cm-2 h-1), but snow removal had no effect. These results suggest that if winters remain typically cold in the Northeastern U.S., but snowfall is reduced, we may expect higher N2O emissions, with winter rye cover unlikely to mitigate this. If, however, less snow cover is due to warmer temperatures as predicted, we may be trending towards lower spring N2O emissions where winter rye cover cropping may be a useful mitigation tool. The field experiment showed that temperature buffering created by an insulating soil cover during the winter may lead to lower N2O emissions in the spring. Insulation may result in higher minimum soil temperatures, shorter freeze duration, fewer FTC, and slower rates of freezing and thawing. One of these temperature variables, slower thawing, was examined by measuring N2O fluxes in a laboratory-simulated FTC. Slower thawing led to higher N2O emissions (1200 vs. 750 ng N2O-N cm-2 h-1). This suggests that slower thawing is not the mechanism responsible for lower N2O emissions observed in agricultural fields with soil cover. Rather, one of the other variables mentioned may be more important. It is unclear if high spring emissions result from a decrease in the efficiency of N2O reduction to N2. A laboratory-simulated FTC was also used to investigate the ratio of N2O to total gaseous N emitted (rN2O) during periods of high N2O emissions. Results showed that rN2O decreased (0.64 0.0) over time after thawing. This suggests that a lack of reduction of N2O to N2 may contribute to high N2O emissions measured during soil thawing. Gaining an understanding of why N2O emissions are high during spring thawing and how these emissions are affected by snow cover, rye cover cropping and the rate of soil thawing will aid researchers and land owners in designing useful N2O emission mitigation strategies.
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