Processes leading to N2O emissions in grassland soil during freezing and thawing

Nitrous oxide (N2O) emissions during periods of freezing–thawing can amount to more than 70% of the total annual N2O loss from soil in temperate climates. In this study, 15N-labelled grassland soil was subjected to freezing–thawing conditions to characterise the nitrogen transformation processes operating in soils under these conditions. Nitrogen transformations during a freezing–thawing event were separated into three stages: freezing, thawing and post-thawing. Ammonium (NH4+) and nitrate (NO3−) concentrations increased during the freezing period and the 15N excess in the NH4+ and NO3− pools increased by approximately 4 and 2.5 at.% in treatments where either the NH4+ or NO3− pool had been labelled. This suggests that previously unavailable N (characterised by high 15N excess values) must have been either fixed on soil colloids or immobilised by the microbial biomass shortly after fertilizer application. A tracing model was developed to quantify the N transformation rates. The thawing period was characterised by high gross mineralisation (31.2 μg N g−1 d−1) and low net nitrification (0.1 μg N g−1 d−1) rates. During the post-thawing period the mineralisation rate decreased and the nitrification rate increased substantially to values of 3.8 and 11.4 μg N g−1 d−1, respectively. The enrichment of the nitrous oxide (N2O) increased during the thawing period to peak values of 9.3, 5.6 and 17.3 at.% 15N in the three treatments where either NO3−, NH4+ or both NH4+ and NO3− had been labelled. The enrichment of the N2O corresponded to the enrichment of the NO3− that appeared during the freezing. This indicated that the burst of N2O is associated with reduction of NO3− which became available during the freeze–thaw episode.