Overwinter and Spring Thaw Nitrous Oxide Fluxes in a Northern Prairie Cropland Are Limited but a Significant Proportion of Annual Emissions

Abstract

Croplands that experience seasonal soil freezing and thawing have been shown to be significant sources of N₂O emissions. Yet, there is a paucity of year-round N₂O emission data for one of the most significant crop production regions that seasonally freeze, the Prairies. Here, we present micrometeorological N₂O fluxes measured over 4 years in Saskatchewan, Canada, to evaluate the magnitude of freeze-thaw N₂O emissions and investigate its driving factors. Significant thaw related emissions occurred in 2 of the 4 years and were associated with relatively higher fall nitrate levels and a more gradual soil thawing period. Overall, fall soil nitrate levels were a strong explanatory variable for the differences in non-growing season (NGS) N₂O emission (r² = 0.485). Measured cumulative N₂O emissions for the NGS were 123–938 g N ha⁻¹ and were much smaller than those obtained at other cold climate sites but amounted to 52% of annual totals on average. The November to April period contributed 30% of the annual total emissions in years without major thaw events, but 70% in years with significant thaws. NGS N₂O emissions were not explained by cumulative freezing degree days unlike most other cold climate sites. We propose that NGS N₂O emissions are more strongly influenced by thaw dynamics during freezing-thawing conditions in dry regions, whereas freezing intensity is the dominant factor for wetter regions. Our results indicate that even for a semi-arid region freeze-thaw is an important source of N₂O emissions and must be considered for more accurate reporting and development of mitigation strategies.