Factors affecting the potential contributions of N2 fixation by legumes in Australian pasture systems

The amounts of foliage nitrogen (N) fixed by various annual and perennial legumes growing in Australian pastures range from <10 to >250 kg N/ha. year. Differences in N-2 fixation result from variations in the proportion of the legume-N derived from atmospheric N-2 (%Ndfa) and/or the amount of legume-N accumulated during growth. On-farm surveys of %Ndfa achieved by legumes growing in farmers' paddocks in Australia indicated that N-2 fixation contributed >65% of the legume's N requirements in three-quarters of the annual legumes examined, but this decreased to two-thirds of lucerne (Medicago sativa; also known as alfalfa), and half of white clover (Trifolium repens) samples. Factors such as low numbers or the poor effectiveness of rhizobial strains in the soil, water stress, high soil concentrations of N, and nutrient disorders contribute to poor nodulation and %Ndfa values <65%, but there is also evidence that the observed %Ndfa can be dependent on the legume species present, and whether the legume is grown in a pure stand or in a mixed sward. The accumulation of legume-N relates primarily to the legume content and net productivity of the pasture. For many legume species, similar to 20 kg of shoot-N is fixed on average for every tonne of herbage dry matter produced. Legume productivity can be influenced by (i) sowing and establishment techniques and other strategies that enhance the legume content in pasture swards; (ii) the amelioration of soil constraints; (iii) the use of new legume species (and host-rhizobial strain combinations) that are more tolerant of hostile soil environments than subterranean clover (T. subterraneum) or annual medics (Medicago spp); and (iv) the inclusion of perennials such as lucerne to offset the year-to-year variability in productivity and N-2 fixation that is a common occurrence with annual legumes.