LONG-TERM MEASUREMENT OF DENITRIFICATION IN 3 CONTRASTING PASTURES GRAZED BY SHEEP

Three pastures-white clover and perennial ryegrass, a herbal ley comprising grasses and broadleaf species including legumes, and a perennial ryegrass sward fertilised with split applications of urea supplying 400 kg N ha(-1) yr(-1)-were established in a randomised block experiment on a freely drained fine sandy loam (mixed mesic Dystric Eutrochrept) at Palmerston North, New Zealand (ca latitude 40 degrees S). Soil denitrification rates were measured periodically during 21 months when the pastures were rotationally grazed by sheep. N2O production was measured using the acetylene-block technique adapted to soil cores incubated for 24 h in gas-tight chambers in the field. The influence of soil moisture, temperature, the availability of soil nitrate and the frequency of grazing on the denitrification rate was assessed. Soil temperatures (at 10 cm depth) remained above 4-6 degrees C, the lower limit for denitrification. The highest losses by denitrification occurred in winter when soil moisture was at or above field capacity for extended periods. The critical air-filled porosity below which denitrification was accelerated was found to be 17% v/v. When conditions were suitable, the rate of denitrification was controlled by the availability of NO3- and NO2-. In the Grass + N400 system, these were supplied principally by the hydrolysis and nitrification of urea fertiliser, but in the legume-based pastures the input of readily decomposable excretal N from the grazing animals had a marked effect on the supply of these substrates. Total denitrification loss was 3.4, 4.4 and 19.3 kg N ha(-1) yr(-1) for Grass-clover, Herbal ley and Grass + N400, respectively. The ratio of N2O:N-2 lost from these pastures averaged 0.38, 0.30 and 0.27, respectively. Irrespective of whether N was supplied to the pastures by symbiotic fixation or in fertiliser, N2O emissions were 1-1.3% of the annual N input.