Linking Nitrous Oxide Flux During Spring Thaw to Nitrate Denitrification in the Soil Profile

The importance of spring thaw nitrous oxide (N 2 O) fluxes to the total N 2 O emission budget in cold climates has been recognized recently. Two mechanisms have been proposed to explain the burst in N 2 O fluxes due to soil freezing and thawing: enhanced microbial activity due to increased nutrient availability at spring thaw, and release of N 2 O trapped at depth during winter. The objective of this study was to determine whether increased surface N 2 O fluxes were due to physical release at spring thaw of N 2 O accumulated all winter at depth in the soil profile, or whether fluxes were due to rapid N 2 O production in the surface layer during the thaw process. Micrometeorological flux measurements and a chamber method applied to in situ soil columns receiving 15 N tracer were used in Ontario, Canada during winters of 2003 and 2004. Labeled K 15 NO 3 fertilizer (60% excess 15 N) at the rate of 100 kg N ha −1 was applied to two layers, that is, surface layer 0 to 5 cm (SL) and deep layer 12 to 17 cm (DL) in nondisturbed soil columns placed in the field during the winter. The burst in N 2 O fluxes from the soil surface measured by both methods occurred within the same period of soil thawing. Denitrification was the main mechanism responsible for N 2 O production, and conditions conducive to N 2 O and N 2 production occurred both in the SL and DL during thawing. Despite high 15 N 2 O concentrations at depth, the burst in N 2 O fluxes from DL soil columns were 1.5 to 5 times lower than that from SL soil columns as more N 2 O from DL was converted to N 2 before diffusing out of the soil profile. Comparison of N 2 O fluxes originating from SL and DL soil columns indicates that the source of N 2 O burst at spring thaw is mostly ‘newly’ produced N 2 O in the surface layer, and not the release of N 2 O trapped in the unfrozen soil beneath the frozen layers.