Isotope-aided studies of nitrogen requirements of systems containing a grazed pasture component

The legume ley cropping system, in which a non-legume grain crop is grown in rotation with a grazed legume based pasture ley, is frequently practiced in Australia. Simulation models are used to compare the dynamics of N cycling through such a system to that of a continuous pasture system. The role of 15N in determining the N transformations in these models is discussed. The uptake of N by Digitaria and Stylosanthes from 15N-labelled plant litter of each species was measured in glasshouse growth studies. The N from the legume, Stylosanthes, was initially more available and a larger amount of N was taken up (26%) over the two seasons than from the grass (15%). Similar results were obtained in field studies using 15N in enclosed microplots. The proportion of the N taken up from various 15N-labelled types of plant material, and that remaining in the litter and in the soil were measured. The results were used to provide estimates of N loss from the system. (author)