Weather and Management Interactions Determine the Importance of Winter and Spring Thaw Nitrous Oxide Emissions in Cold Climates.

The freezing and thawing of soil in cold climates often produces large emissions of nitrous oxide (N₂O), a trace gas that contributes to the enhanced greenhouse effect and stratospheric ozone destruction.  Soil moisture, availability of carbon and nitrogen substrates, and freeze temperature and duration are the major factors that have been identified as controlling the magnitude of N₂O emissions induced by freeze-thaw cycles.  We have conducted several studies deploying a micrometeorological technique to measure surface N₂O fluxes at a long-term Ontario site established in 2000 with the objective of improving our understanding of how management affects N₂O emissions.  We have observed significant inter-annual variability in fluxes with non-growing season (November–April) emissions, comprising between 30% and 90% of the annual emissions.  The variable magnitude of N₂O fluxes during thawing has been associated with accumulated degree-hours below 0° C (at 5 cm soil depth), a measure of duration and intensity of soil freezing.  Here we discuss our past findings and present data from a recent study comparing N₂O flux from annual (corn) and perennial (hay) crops receiving dairy manure that was initiated in November 2011.  Emission rates from November 2011 to end of February 2012 on corn-harvested plots after fall manure application were twice as large as the plot that did not received any manure (13 vs. 6 g N ha^-1 d^-1 on average).  In contrast, N₂O flux averaged 1.5 g N ha^-1 d^-1 for the hayfield.  During the spring thaw which occurred mid-March in 2012, rates approximately doubled for all plots.  About 30% of the winter (Nov-Apr) emissions occurred during the 15 day thaw period, and Nov-Apr emissions comprised ~54% of annual total emission.  Data for 2012/2013 will also be presented and discussed within the context of our long-term N₂O flux time series.