Nitrogen application rate and nitrous oxide flux from a pastoral soil

Nitrogen (N) can be lost to the atmosphere by nitrous oxide (N2O) flux from soils. For pastoral soils, N is applied in N fertiliser and excreta deposited by farmed, grazing animals. We postulated that soilN2O flux would be linearly related to the N application rate (Nrate). As a test, we applied eight treatments as rates of urea (0–1500 kg N ha−1) to samples of a pastoral soil and measured the N2O flux, pH and ammonium (NH4 +) and nitrate (NO3 −) concentrations at intervals for 67 days. When fluxes were accumulated, the variability or variance of treated replicates increased with time and increasing Nrate. For 67-day, cumulative N2O fluxes, a linear regression with the N application rate, weighted by the inverse of the replicates’ variance, yielded a statistically significant relationship (P < 0.05), accounting for 92% of the variability. A second-order term was too small to be statistically significant for another regression using a polynomial function. Thus, there was no evidence to reject the linear hypothesis. The fluxes were also related to the pH and (NH4 + + NO3 −) concentration. These relationships warrant further study in different soils under field conditions.