Agricultural nitrous oxide emissions across twenty years of micrometeorological observations show trends associated with soil freezing and fertilizer application

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Emissions of nitrous oxide (N 2 O) from agricultural soils can be substantial during the non-growing season (NGS; defined in Canada as November 1 to April 30) but this conclusion is supported by only limited observations. Year-round studies utilizing multi-year observations are needed to confirm NGS emission trends and link these emissions to drivers, thus enabling recommendations for improved management practices. This study addresses these needs by providing twenty years of quasi-continuous, micrometeorological observations of N 2 O from an agricultural field near Ottawa, Ontario, marginally gap-filled using the DeNitrification and DeComposition model (DNDCv.CAN). With these observations, we confirm two distinct N 2 O emission periods frequently observed at the site, each with its own environmental driver. The first occurs immediately following snowmelt (DOY 68 to 114), while the second follows N-fertilizer application (DOY 131 to 172). Nitrogen fertilization rate was the main driver of the second sustained emission event. The main driver of the spring thaw emissions was confirmed to be the number of soil cumulative freezing degree days (CFD), consistent with a previous study from other Canadian experimental sites. Although our data supported the previously published non-linear relationship of increasing NGS N 2 O emissions with CFD, we found that higher N 2 O emissions occurred under fewer CFD than previously reported. Our results show that nitrogen fertilizer application results in large, routine N 2 O fluxes post-application, while NGS emissions, particularly at spring snowmelt, are strongly related to CFD. • Two decades of flux gradient observations of N 2 O emissions are reported. • Key emission periods occurred following snowmelt and post-fertilization. • 44% of emissions occurred in the non-growing season, mostly after snowmelt. • Soil freezing dynamics was a key driver of non-growing season N 2 O emissions.

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