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SOIL PHYSICS

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

^a Dep. of Land Resource Science Univ. of Guelph Guelph, ON Canada N1G 2W1
^b Institute of Soil Science and Fertilizer Shanxi Academy of Agricultural Science Taiyuan, Shanxi P.R. China
^c Soils and Crops Research and Development Center Agriculture and Agri-food Canada, 2560 Hochelaga Blvd., Sainte-Foy, QC, Canada, G1V 2J3

^* Corresponding author (cwagnerr{at}uoguelph.ca).

The importance of spring thaw nitrous oxide (N₂O) fluxes to the total N₂O emission budget in cold climates has been recognized recently. Two mechanisms have been proposed to explain the burst in N₂O fluxes due to soil freezing and thawing: enhanced microbial activity due to increased nutrient availability at spring thaw, and release of N₂O trapped at depth during winter. The objective of this study was to determine whether increased surface N₂O fluxes were due to physical release at spring thaw of N₂O accumulated all winter at depth in the soil profile, or whether fluxes were due to rapid N₂O production in the surface layer during the thaw process. Micrometeorological flux measurements and a chamber method applied to in situ soil columns receiving ¹⁵N tracer were used in Ontario, Canada during winters of 2003 and 2004. Labeled K¹⁵NO₃ fertilizer (60% excess ¹⁵N) 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 N₂O fluxes from the soil surface measured by both methods occurred within the same period of soil thawing. Denitrification was the main mechanism responsible for N₂O production, and conditions conducive to N₂O and N₂ production occurred both in the SL and DL during thawing. Despite high ¹⁵N₂O concentrations at depth, the burst in N₂O fluxes from DL soil columns were 1.5 to 5 times lower than that from SL soil columns as more N₂O from DL was converted to N₂ before diffusing out of the soil profile. Comparison of N₂O fluxes originating from SL and DL soil columns indicates that the source of N₂O burst at spring thaw is mostly ‘newly’ produced N₂O in the surface layer, and not the release of N₂O trapped in the unfrozen soil beneath the frozen layers.

Abbreviations: DL, deep layer 12 to 17 cm • SL, surface layer 0 to 5 cm