Nitrogen fixation by subterranean clover (Trifolium subterraneum L.) growing in pure culture and in mixtures with varying densities of lucerne (Medicago sativa L.) or phalaris (Phalaris aquatica L.)

The proportions of biologically fixed (P-fix) plant nitrogen (N) and the total amounts of N-2 fixed by subterranean clover (Trifolium subterraneum L.) growing in pure culture and in mixtures with different densities (5, 10, 20, or 40 plants/m(2)) of newly sown phalaris (Phalaris aquatica L.) or lucerne (Medicago sativa L.) were followed over 3 years in a field study using the N-15 natural abundance technique. The amount of fixed N in subterranean clover was linearly related to shoot biomass. Over the 3-year period, subterranean clover fixed 23-34 kg N/t shoot biomass compared with 17-29 kg N/t shoot biomass in lucerne. Based on above-ground biomass, pure subterranean clover fixed 314 kg N/ha over the 3 years compared with 420-510 kg N/ha by lucerne-clover mixtures and 143-177 kg N/ha by phalaris-clover mixtures. The superior N-2 fixation by the lucerne-subterranean clover mixtures was due to the N fixed by the lucerne and the presence of a higher subterranean clover biomass relative to that occurring in the adjacent phalaris plots. In the first year, 92% of subterranean clover shoot N was derived from fixation compared with only 59% of lucerne. The reliance of clover upon fixed N-2 remained high (73-95%) throughout the 3 years in all swards, except in pure subterranean clover and lucerne in August 1996 (56 and 64%, respectively). Subterranean clover usually fixed a higher proportion of its N when grown in mixtures with phalaris than with lucerne. The calculated Pfix values for lucerne (47-61% in 1995 and 39-52% in 1996) were consistently lower than in subterranean clover and tended to increase with lucerne density. Although lucerne derived a lower proportion of its N from fixation than subterranean clover, its tissue N concentration was consistently higher, indicating it was effective at scavenging soil mineral N. It was concluded that including lucerne in wheat-belt pastures will increase inputs of fixed N. Although lucerne decreased subterranean clover biomass, it maintained or raised Pfix values compared with pure subterranean clover swards. The presence of phalaris maintained a high dependence on N-2 fixation by subterranean clover, but overall these swards fixed less N due to the lower clover herbage yields. Perennial and annual legumes appear compatible if sown in a mix and can contribute more N-2 to the system than where the annual is sown alone or with a perennial grass. These findings suggest that increases in the amount of N-2 fixed can be achieved through different legume combinations without interfering greatly with the N fixation process. Different combinations may also result in more efficient use of fixed N-2 through reduced leaching. Further work looking at combinations of annuals possibly with different maturity times, different annual and perennial legume combinations, and pure combinations of perennial (e.g. lucerne) could be investigated with the aim of maximising N-2 fixation and use. Grazing management to encourage clover production in mixtures with phalaris will be necessary before the potential of subterranean clover to contribute fixed N-2 in these swards is fully realised.