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DIVISION S-8—NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

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

Department of Agriculture and Rural Development, Agricultural and Environmental Science Division, Newforge Lane, Belfast BT9 5PX, UK

* Corresponding author (jim.stevens{at}dardni.gov.uk)

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.

Abbreviations: CS, cattle slurry • VFA, volatile fatty acid • WFPS, water-filled pore space