Symbiotic nitrogen fixation by Trifolium arvense in semi-arid short tussock grasslands

We have measured symbiotic N fixation from the widespread annual Trifolium arvense (haresfoot clover) in two studies in the semi-arid zone of the Mackenzie Basin using a N-15 dilution technique over two growing seasons. Apart from one low value of 69%, the percentage of plant nitrogen derived from the atmosphere (%Ndfa) was consistently high in one study at Tomahawk Gully (82-91%). Calculated inputs of symbiotic N fixation from T arvense was highly variable over the landscape, and between seasons. Calculated symbiotic N fixation inputs on different landscape units in Tomahawk Gully ranged from <0.1 to 3.4 kg N ha(-1) in 2000-01, and from <0.1 to 11.4 kg N ha(-1) in 2001-02 when there was more favourable summer rainfall. Weighted mean rates of N fixation over the whole Tomahawk Gully catchment were 0.5 and 1.6 kg N ha(-1), in the respective seasons. In a study at St Cuthbert, the results were variable in the absence of sulphur (S) fertiliser (56-76%). However, the application of 25 kg ha(-1) of fertiliser S was associated with an increase in the %Ndfa to 89-92% and a 51/2-fold increase in the symbiotically fixed N. Application of small amounts of S fertiliser to areas with naturalised legumes within semi-arid grassland offers potential for improving the sustainability of grazed tussock grasslands.