Transfer of biologically fixed nitrogen to the non-legume component of mixed pastures

Pasture ecosystems are extremely diverse, as are the management procedures imposed upon them by the pastoralist. In low input pasture enterprises in marginal areas, legume nitrogen fixation is frequently (but not invariably) crucial to continued productivity. Legumes usually do not dominate a pasture and their role in transferring fixed nitrogen to a non-legume, frequently graminaceous, species is important. Methods for measuring this transfer are critically assessed in terms of their usefulness in realistic pasture environments. Existing techniques all have serious disadvantages in this respect. Isotopic studies of individual processes within the transfer system are described and some new lines of investigation are proposed. The value of isotopic studies in improving pasture management is discussed. (author)