In cold climates, large emissions of N2O have been found to occur from agricultural soils at spring thaw. Of fundamental importance is determining whether the majority of N2O emitted is produced before spring thaw and then released, or if it is produced at the onset of spring thaw. It has been suggested that physical release comprises the often observed first N2O emission event at thaw, while the subsequent event is likely due to enhanced denitrification. To test this, the objective of this study was to compare changes in soil profile N2O content over time to N 2O surface fluxes at spring thaw measured using the micrometeorological flux-gradient technique, which allowed for capturing of the dynamic nature of N2O flux during thawing. Measurements were conducted from October 2010 to May 2011, in a 1.5-ha plot (conventional till) in Ontario, Canada. Soil profile measurements included water content, temperature, and N2O gas concentration weekly overwinter and daily during the main spring-thaw. Soil N2O mass in gaseous and aqueous forms was estimated. Soil N2O mass in the 0- to 70-cm profile reached a total 23 mg N2O m-2 and decreased rapidly during spring thaw when two events of heightened flux occurred. During the first event, surface N2O fluxes corresponded to 24% of the change in mass, however during the second event N2O fluxes were five times larger. Our results provide the first quantitative evidence for the suggestion that N2O spring thaw emission is comprised by a first event mostly due to physical release, followed by a second event with fluxes predominantly due to de novo production. The physical release event was simultaneous with large decreases in N2O trapped in the top soil layer and not due to N2O trapped at depth. Exact apportioning of surface flux into N2O sources (i.e., accumulated in profile vs. newly produced) is not possible, but assuming that most of the first emission event was due to physical release gives a rough estimate of 22% for the contribution of physical release.
