Cattle Slurry Affects Nitrous Oxide and Dinitrogen Emissions from Fertilizer Nitrate

Readily decomposable C in organic manures could enhance denitrification of NO 3 existing in soil. The worst scenario occurs when farmers apply manures and NO 3 ‐containing fertilizers at the same time to meet the nutrient requirements of the next crop. If the denitrification produced N 2 O rather than N 2 , the emission factor of 1.25% of fertilizer N used to calculate national inventories for N 2 O would be an underestimate for this farming practice. We used the 15 N gas–flux method to measure N 2 O and N 2 fluxes from grassland when cattle slurry (CS) containing 60 kg NH 4 –N ha −1 and KNO 3 (60 kg N ha −1 ) were applied at the same time. By labeling the KNO 3 and the NH 4 in CS, we quantified the processes producing N 2 O and checked for N 2 production by microbial processes other than denitrification. On average over field experiments replicated in March, May, August, and September 1997, CS increased the flux of N 2 O by 0.63% of the applied NO 3 –N in the 104 h after application, but had no significant effect on the flux of N 2 The maximum flux of N 2 O was always observed in the first measurement period (5–7 h) after CS application. All of the N 2 O was formed by reduction from NO 3 apart from in August when 10% was formed by nitrification in the CS treatment. There was no evidence for production of N 2 by other processes such as heterotrophic nitrifier denitrification or anaerobic NH 4 oxidation.