ESTIMATES OF SEASONAL NITROGEN-FIXATION OF ANNUAL SUBTERRANEAN CLOVER-BASED PASTURES USING THE N-15 NATURAL-ABUNDANCE TECHNIQUE

Annual pasture legumes play a key role in ley farming systems of southern Australia, providing biologically fixed nitrogen (N) to drive the production of the pastures as well as subsequent crops grown in rotation. Seasonal inputs of biologically fixed N in shoot biomass of the subterranean clover (Trifolium subterraneum) component of grazed annual pastures were assessed using the N-15 natural abundance technique and appropriately timed sampling of herbage dry matter (DM) for N accumulation. AL three study sites spanning a gradient across the Western Australian wheatbelt from 300 to 600 mm annual rainfall the performance of the clover and non-legume herbs and grasses was examined as paired comparisons involving two management treatments expected to give contrasting effects on pasture productivity, botanical composition and N-2 fixation. The proportion of clover N derived from atmospheric N-2 fixation (%Ndfa) ranged from 65 to 95% across sites, treatments and sampling times. Amounts of fixed N accumulated in clover shoot biomass ranged from 50 to 125 kg ha(-1), and paralleled trends in clover production. Substantial increases in pasture production in high yielding treatments generally occurred without decrease in %Ndfa, suggesting that N-2 fixation was essentially non-limiting to performance of the clover component. Seasonal profiles for accumulation of fixed N were skewed towards the late winter and spring period, particularly in low plant density pastures following a cereal crop. There were seasonal, site and treatment-specific effects on the proportion of clover and non-legume pasture components and consequently clover yield and N-2 fixation were variably affected by competition from non-legume species.