Soil Conditions Affecting Emissions of Nitrous Oxide From Thawing Soil.

Nitrous oxide emissions from thawing soil can comprise a considerable portion of annual emissions from agricultural soils in temperate environments. This presentation will review findings of laboratory and field studies on the soil conditions affecting thaw emissions. Nitrate addition to surface soil (0-10 cm) enhanced thaw emission. Thaw emissions, averaged for deeper collected soil (10-30 and 30-60 cm), was 0.3% with NO₃^- treatment and 1.2% without NO₃^- treatment of that for surface soil treated similarly. Higher thaw emission for surface soil was related to higher organic matter and microbial biomass C contents and denitrifying enzyme activity than deeper collections of soil. Increasing the bulk density of soil from 1.1, 1.2, and 1.25 Mg m^-3 decreased thaw emission. A second freeze-thaw cycle of the highest compaction treatment resulted in an emission of 2.3% of the first freeze-thaw cycle. Acetylene increased thaw emission of N₂O and more so for NO₃^- untreated than treated soil. Using the acetylene inhibition method, the N₂O:N₂ ratio of gas produced was higher for frozen (0.17) than cold (0.07) treated soil, respectively, without the addition of NO₃^-.  The addition of NO₃^- increased the N₂O:N₂ ratio of gas produced with the ratio being 2.45 and 0.53 and for frozen and cold treated soil. The results are consistent with biological denitrification being a source of N₂O with conditions promoting N₂O production rather than consumption enhancing thaw emissions.