Response of soil mineral nitrogen to total nitrogen and land-use in a long-term study of wheat pasture rotations

Abstract

All 6 treatments of a long-term wheat-pasture rotation experiment were soil sampled after a cultivated fallow close to sowing at Wagga Wagga. Analyses of the nitrogen (N) data show that total N (0-1 5 cm) could be used as a predictor of mineral N in the profile at sowing. Mineral N levels were expressed as a percentage of total N and reported as a mineralisation index. This index increased after a period of clover and was considerably higher for the first crop after pasture than for subsequent crops. The mineralisation index also increased as the pasture intensity (years pasture/total years) increased from 0.33 to 0.67. Total N alone accounted for no more than 20% of the variation in mineral N at sowing. The variance accounted for increased to 49.9 and 53.7% respectively when pasture frequency and crop number after pasture were included in the models.