NITROGEN-FIXATION IN GRAZED AND UNGRAZED SUBTERRANEAN CLOVER PASTURE IN SOUTH-WEST AUSTRALIA ASSESSED BY THE N-15 NATURAL-ABUNDANCE TECHNIQUE

The progress of N-2 fixation by subterranean clover (Trifolium subterr aneum L.) was followed throughout a growing season in adjacent grazed and ungrazed portions of a pasture at Mount Barker, W.A. Proportions o f plant nitrogen derived from the atmosphere (%Ndfa) were determined a t a sequence of sampling times using the N-15 natural abundance techni que with capeweed (Arctotheca calendula L.) as non-fixing reference sp ecies. Cumulative yields of fixed N by above ground biomass of clover were determined from %Ndfa values, concurrent estimates of dry matter (DM) yields, and percentage nitrogen in clover shoot DM. Seasonal DM y ields of clover, capeweed and mixed grasses were in the approximate ra tio 60:20:20. Total herbage yields were 11.8 and 7.8 t ha(-1) for the grazed and ungrazed swards respectively. Poorer performance of the lat ter was attributed to shading by taller grasses late in the season. St arting from a low value of 58%, Ndfa of the ungrazed sward became unif ormly high (73-88%) for the rest of the season. Clover of the more pro ductive grazed sward behaved similarly except for a significant mid wi nter depression to 55%Ndfa, probably caused by excessive defoliation t hrough overgrazing. Fixed N recovered from clover shoot biomass was 10 3 and 188 kg N ha(-1) for ungrazed and grazed pasture respectively. Mi neral N under the grazed sward first consisted mostly of nitrate, and then predominantly of ammonium. Soil-derived N was utilized roughly eq ually by clover, grasses and capeweed and a field study of %Ndfa of su bterranean clover grown in varying proportion with either the main pas ture grass (Lolium rigidum Gaudin) or capeweed indicated the grass to be the more effective competitor for soil N against the clover. The da ta suggested that reliable estimates of seasonal accumulation of fixed N by pastures would be obtained from assessments of cumulative biomas s yield of clover N with a single determination of %Ndfa at peak produ ctivity in mid to late spring.