Denitrification Loss from Managed Grassland

Abstract

Denitrification has been measured in the field using the acetylene-inhibition technique. At most times during the year denitrification in soils under cut swards continued at a background rate of 0.01 to 0.02 kg N ha⁻¹ d⁻¹. Higher rates (0.2 to 1 kg N ha⁻¹ d⁻¹) were only observed following the application of ammonium nitrate fertilizer to wet soil in the spring and summer. These higher rates arose from the combined effects of high nitrate contents and reduced air-filled porosities in the soil surface. Limiting factors on conditions conducive to the higher rates of loss were soil water content >20% (equivalent to air-filled porosities <25 to 42%), soil nitrate >5 to 10 μg Ng⁻¹ and soil temperature 8^°C. Annual loss of 5 to 8% of the N applied to cut swards was indicated. High rates of NO emission were invariably associated with high rates of denitrification. At certain times the soil acted as a sink for atmospheric N₂O. Only 1.6% of the fertilizer applied was lost as N₂O, this gas comprising 21% of the total denitrification loss. Up to 55% of the annual denitrification loss occured following application of N in late winter/early spring. This loss was eliminated by applying N in an ammonium form. Rates of denitrification in a grazed sward were more than twice those in a comparable cut sward. This could be attributed to the return of N in_-excreta resulting in the maintenance of soil nitrate contents >5 pg g in the grazed sward. Additional loss through denitrification is expected to be associated with the return of slurry and other wastes from winter-housed cattle.