TRANSFORMATION IN SOIL AND TURNOVER TO WHEAT OF NITROGEN FROM COMPONENTS OF GRAZED PASTURE IN THE SOUTH OF WESTERN-AUSTRALIA

Nitrogen (N) mineralisation from mature subterranean clover (Trifolium subterraneum L.) shoots and roots and from sheep urine and faeces, an d N uptake by wheat from the shoots, urine, and faeces, were determine d with N-15 in a field study in the Western Australian wheatbelt. Trea tments were applied to the soil surface of confined micro-plots in aut umn and incorporated into soil immediately before wheat was sown in wi nter. Mature subterranean clover shoots containing 18 kg N/ha were app lied to the soil surface, and root material containing 17 kg N/ha was mixed into soil. N-15-labelled urine and faeces were obtained from hou sed sheep fed N-15-labelled wheat straw and grain. Urine was applied a t the rates of 151 and 301 kg N/ha, and faeces was added at the rate o f 47 kg N/ha. There was a loss of 14% of shoot N-15 in the 2 months th is residue was on the soil surface, although very little mineralisatio n occurred. On the assumption that wind-blow caused the initial loss o f N-15; 28% of shoot N mineralised in 6 months following incorporation of shoot residues into soil, and crop recovery was 11% of the N-15 ap plied. N mineralisation from the mature roots was 26% in 6 months. NH3 volatilisation from urine, estimated by difference, was 25% for high urine (0.517 mL/cm(2)) and 33% for low urine (0.258 mL/cm(2)) applicat ion rates, the loss occurring in the first 2 weeks. Wheat uptake was 2 3% of the high urine N-15 and 22% of the low urine N-15. Leaching loss es from unplanted micro-plots were approximately 25-30% of urine N-15. In contrast, leaching losses from planted micro-plots were estimated to be approximately 10% of urine N-15. Approximately 30% of faecal N w as mineralised and recovery of faeces N by wheat was 1% of applied N-1 5. The relative contributions of these components to N turnover in the ley pasture wheat rotation are discussed. It is concluded that assess ments of the potential turnover of N in pastures to cropping phases ne ed to consider the low rates of N mineralisation of above-ground herba ge, the potential for supply of N from the total root system, the effe ct of grazing on NH3 volatilisation, and consequent loss of N fixed by legumes.