N 2O emissions and product ratios of nitrification and denitrification as affected by freezing and thawing

Abstract

Agricultural soils contribute significantly to atmospheric nitrous oxide (N 2O). A considerable part of the annual N 2O emission may occur during the cold season, possibly supported by high product ratios in denitrification (N 2O/(N 2+N 2O)) and nitrification (N 2O-N/(NO 3 −-N+NO 2 −-N)) at low temperatures and/or in response to freeze–thaw perturbation. Water-soluble organic materials released from frost-sensitive catch crops and green manure may further increase winter emissions. We conducted short-term laboratory incubations under standardized moisture and oxygen (O 2) conditions, using nitrogen (N) tracers ( 15N) to determine process rates and sources of emitted N 2O after freeze–thaw treatment of soil or after addition of freeze–thaw extract from clover. Soil respiration and N 2O production was stimulated by freeze–thaw or addition of plant extract. The N 2O emission response was inversely related to O 2 concentration, indicating denitrification as the quantitatively prevailing process. Denitrification product ratios in the two studied soils (pH 4.5 and 7.0) remained largely unaltered by freeze–thaw or freeze–thaw-released plant material, refuting the hypothesis that high winter emissions are due to frost damage of N 2O reductase activity. Nitrification rates estimated by nitrate (NO 3 −) pool enrichment were 1.5–1.8 μg NO 3-N g −1 dw soil d −1 in freeze–thaw-treated soil when incubated at O 2 concentrations above 2.3 vol% and one order of magnitude lower at 0.8 vol% O 2. Thus, the experiments captured a situation with severely O 2-limited nitrification. As expected, the O 2 stress at 0.8 vol% resulted in a high nitrification product ratio (0.3 g g −1). Despite this high product ratio, only 4.4% of the measured N 2O accumulation originated from nitrification, reaffirming that denitrification was the main N 2O source at the various tested O 2 concentrations in freeze–thaw-affected soil. N 2O emission response to both freeze–thaw and plant extract addition appeared strongly linked to stimulation of carbon (C) respiration, suggesting that freeze–thaw-induced release of decomposable organic C was the major driving force for N 2O emissions in our soils, both by fuelling denitrifiers and by depleting O 2. The soluble C (applied as plant extract) necessary to induce a CO 2 and N 2O production rate comparable with that of freeze–thaw was 20–30 μg C g −1 soil dw. This is in the range of estimates for over-winter soluble C loss from catch crops and green manure plots reported in the literature. Thus, freeze–thaw-released organic C from plants may play a significant role in freeze–thaw-related N 2O emissions.