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

Abstract 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 15N 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 5½‐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.