Cattle Slurry Applied Before Fertilizer Nitrate Lowers Nitrous Oxide and Dinitrogen Emissions

Nitrous oxide emissions increase because of denitrification in the first few days after cattle (Bos taurus) slurry (CS) is applied to grassland soils fertilized with NO3 Denitrifying conditions are created when the readily decomposable C in the CS is oxidized by the soil microbial biomass when NO3 is present and O2 is deficient. Half of the readily decomposable C in CS can be volatile fatty acids (VFAs) that take up to 4 d to degrade. The timing of CS application relative to fertilizer‐NO3 application could therefore affect the losses of N2O and N2 We used the 15N gas‐flux method to measure N2O and N2 fluxes from grassland when CS containing 60 kg NH4‐N ha−1 was applied 4, 3, 2, 1, and 0 d before the application of 60 kg N ha−1 of K15NO3 For a field experiment repeated in April, May, August, and October 1998, CS applied 3 or 4 d before KNO3 had no significant effect in any month on the flux of N2O in the 124 h after KNO3 application. On average over all months, the extra emission of N2O‐N over the control was equivalent to 0.8, 1.1, and 2.9% of KNO3‐N for prior applications of CS at 2, 1, and 0 d, respectively. When CS was applied 4 d prior to KNO3 there was no significant effect on the flux of N2 in any month. The maximum loss of N2O + N2 was 8.3% of the KNO3 applied (5 kg N ha−1) when CS and KNO3 were applied at the same time in April.