Transformation in soil and turnover to wheat of nitrogen from components of grazed pasture in the south of Western Australia

Abstract

Nitrogen (N) mineralisation from mature subterranean clover (Trifolium subterraneum L.) shoots and roots and from sheep urine and faeces, and N uptake by wheat from the shoots, urine, and faeces, were determined with 15 N in a field study in the Western Australian wheatbelt. Treatments were applied to the soil surface of confined micro-plots in autumn and incorporated into soil immediately before wheat was sown in winter. Mature subterranean clover shoots containing 18 kg N/ha were applied to the soil surface, and root material containing 17 kg N/ha was mixed into soil. ¹⁵N-labelled urine and faeces were obtained from housed sheep fed ¹⁵N-labelled wheat straw and grain. Urine was applied at the rates of 151 and 301 kg N/ha, and faeces was added at the rate of 47 kg N/ha. There was a loss of 14% of shoot ¹⁵N in the 2 months this residue was on the soil surface, although very little mineralisation occurred. On the assumption that wind-blow caused the initial loss of ¹⁵N, 28% of shoot N mineralised in 6 months following incorporation of shoot residues into soil, and crop recovery was 11% of the ¹⁵N applied. 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/cm²) and 33% for low urine (0·258 mL/cm²) application rates, the loss occurring in the first 2 weeks. Wheat uptake was 23% of the high urine ¹⁵N and 22% of the low urine ¹⁵N. Leaching losses from unplanted micro-plots were approximately 25-30% of urine ¹⁵N. In contrast, leaching losses from planted micro-plots were estimated to be approximately 10% of urine ¹⁵N. Approximately 30% of faecal N was mineralised and recovery of faeces N by wheat was 1% of applied ¹⁵N. The relative contributions of these components to N turnover in the ley pasture wheat rotation are discussed. It is concluded that assessments of the potential turnover of N in pastures to cropping phases need to consider the low rates of N mineralisation of above-ground herbage, the potential for supply of N from the total root system, the effect of grazing on NH₃volatilisation, and consequent loss of N fixed by legumes.