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

In an attempt to understand why pasture production in southern Austral ia has declined markedly in recent years a survey of the symbiotic per formance of the legume component of annual pastures on 81 farms (243 s ites) was undertaken in the southern coastal region of Western Austral ia. The N-15 natural abundance technique was used to determine the per centage of plant nitrogen derived from the atmosphere (degrees Ndfa) u sing capeweed (Arctotheca calendula) as principal non-fixing reference species. %Ndfa values were then related to edaphic and management inf ormation, e.g. soil total nitrogen, soil pH, stocking rates and croppi ng history of the sites. The principal legume species encountered exhi bited 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.7 1 degrees Ndfa (n = 24, range 7-100%), Lotus spp. 81%Ndfa (n = 15, ran ge 1-100%) Ornithopus compressus 76%Ndfa (n = 15, range 25-100%) and T rifolium balansae 69%Ndfa (n = 7, range 0-100 degrees). In the case of subterranean clover, the most widely occurring species, almost one th ird (29 degrees) of sites surveyed recorded %Ndfa values within the ra nge 0-65%, suggesting that symbiotic performance might well be quite w idely limiting to herbage production in the study region. Of the 24 ed aphic and management factors evaluated, only one, %Al in shoot, DM sho wed a significant relationship with degrees Ndfa, with 40 degrees of t he pastures surveyed deemed at risk in terms of acidity related alumin ium toxicity. Correlations of %Ndfa with soil pH and soil total N prod uced examples of high values %Ndfa for sub-clover being associated wit h very low soil pH or high soil N, suggesting possible adaptation of s ymbiotic partnerships to acidity or high mineral N.