Soil nitrate concentrations increase the nitrous oxide to dinitrogen emissions ratio at high soil moisture conditions in intensive dairy pastures

Abstract High nitrogen (N) input is typical of irrigated intensively managed dairy pasture systems, but the plant uptake of the applied N is often low. This leads to high nitrate (NO 3 − ) accumulation in the soil which, under wet conditions, can trigger significant loss of N as nitrous oxide (N 2 O) and dinitrogen (N 2 ). We investigated the effect of soil moisture [60, 80 and 100% water filled pore space (WFPS)] and NO 3 − concentrations (45 mg and 80 mg NO 3 − -N kg − 1 soil) on emissions of N 2 O and N 2 by applying 15 NO 3 − -N as a tracer in laboratory conditions using soil from an intensive dairy pasture. The flux of N 2 dominated N loss at 80% and 100% WFPS and N 2 O emissions were more strongly affected by NO 3 − concentrations compared to N 2 emissions. N 2 O emissions responded more strongly to NO 3 − concentrations at 100% WFPS than at 80% WFPS. The flux of N 2 O was significantly lower at 60% WFPS compared to the other WFPS levels, with no detectable N 2 emission. Denitrification dominated N 2 O emissions at 100% WFPS, whereas at 80% WFPS, contributions from denitrification and other sources were similar. These results demonstrate that while managing soil moisture and NO 3 − availability can reduce N 2 O emissions in intensive dairy pastures, substantial fertiliser N losses as N 2 are likely to persist, limiting improvements in overall N use efficiency.