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 NO 3 Denitrifying conditions are created when the readily decomposable C in the CS is oxidized by the soil microbial biomass when NO 3 is present and O 2 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‐NO 3 application could therefore affect the losses of N 2 O and N 2 We used the 15 N gas‐flux method to measure N 2 O and N 2 fluxes from grassland when CS containing 60 kg NH 4 ‐N ha −1 was applied 4, 3, 2, 1, and 0 d before the application of 60 kg N ha −1 of K 15 NO 3 For a field experiment repeated in April, May, August, and October 1998, CS applied 3 or 4 d before KNO 3 had no significant effect in any month on the flux of N 2 O in the 124 h after KNO 3 application. On average over all months, the extra emission of N 2 O‐N over the control was equivalent to 0.8, 1.1, and 2.9% of KNO 3 ‐N for prior applications of CS at 2, 1, and 0 d, respectively. When CS was applied 4 d prior to KNO 3 there was no significant effect on the flux of N 2 in any month. The maximum loss of N 2 O + N 2 was 8.3% of the KNO 3 applied (5 kg N ha −1 ) when CS and KNO 3 were applied at the same time in April.