Key processes of the nitrogen cycle in an irrigated and a non-irrigated grazed pasture

The paper presents integrated measurements of N fixation, net mineralisatio n, pasture yield and change in soil mineral N over a 12 month period for da iry pastures on a sandy loam soil in the South East of South Australia. The two adjacent pastures studied were an irrigated perennial white clover-rye grass and an annual non-irrigated subterranean clover with mixed annual gra sses. This produced the most comprehensive mineral N balance reported for g razed pastures, to the authors' knowledge, allowing calculation of gaseous and leaching losses of N (210 kg ha(-1) in the irrigated and paddock and 81 kg ha(-1) in the non irrigated paddock) primarily from urine patches. In b oth paddocks these losses were about three times the N yield in milk (61 an d 28 kg N ha(-1) respectively) and were replenished by biological N fixatio n (294 and 100 kg N ha(-1)). However, mineralisation of soil organic N, exc retal N and pasture residues (687 and 438 kg N ha(-1)) was the major source of mineral N for cycling and losses. The results demonstrate the enormous impact of pasture management on N fluxes and reinforce the importance of li vestock urine on the magnitude of N fluxes including gaseous and leaching l osses.