A SURVEY OF PROPORTIONAL DEPENDENCE OF SUBTERRANEAN CLOVER AND OTHER PASTURE LEGUMES ON N-2 FIXATION IN SOUTH-WEST AUSTRALIA UTILIZING N-15 NATURAL-ABUNDANCE

In an attempt to understand why pasture production in southern Australia has declined markedly in recent years a survey of the symbiotic performance of the legume component of annual pastures on 81 farms (243 sites) was undertaken in the southern coastal region of Western Australia. The N-15 natural abundance technique was used to determine the percentage of plant nitrogen derived from the atmosphere (degrees Ndfa) using capeweed (Arctotheca calendula) as principal non-fixing reference species. %Ndfa values were then related to edaphic and management information, e.g. soil total nitrogen, soil pH, stocking rates and cropping history of the sites. The principal legume species encountered exhibited similar mean %Ndfa values but substantial variation in symbiotic performance was evident across the sites, viz, Trifolium subterraneum 72%Ndfa (n = 184, range of values encountered 0-100%), Medicago spp.71 degrees Ndfa (n = 24, range 7-100%), Lotus spp. 81%Ndfa (n = 15, range 1-100%) Ornithopus compressus 76%Ndfa (n = 15, range 25-100%) and Trifolium balansae 69%Ndfa (n = 7, range 0-100 degrees). In the case of subterranean clover, the most widely occurring species, almost one third (29 degrees) of sites surveyed recorded %Ndfa values within the range 0-65%, suggesting that symbiotic performance might well be quite widely limiting to herbage production in the study region. Of the 24 edaphic and management factors evaluated, only one, %Al in shoot, DM showed a significant relationship with degrees Ndfa, with 40 degrees of the pastures surveyed deemed at risk in terms of acidity related aluminium toxicity. Correlations of %Ndfa with soil pH and soil total N produced examples of high values %Ndfa for sub-clover being associated with very low soil pH or high soil N, suggesting possible adaptation of symbiotic partnerships to acidity or high mineral N.