BIOLOGICAL NITROGEN-FIXATION IN MIXED LEGUME GRASS PASTURES

Biological nitrogen fixation (BNF) in mixed legume/grass pastures is reviewed along with the importance of transfer of fixed nitrogen (N) to associated grasses. Estimates of BNF depend on the method of measurement and some of the advantages and limitations of the main methods are outlined. The amounts of N fixed from atmospheric N2 in legume/grass pastures throughout the world is summarised and range from 13 to 682 kg N ha-1 yr-1. The corresponding range for grazed pastures, which have been assessed for white clover pastures only, is 55 to 296 kg N ha-1 yr-1. Biological nitrogen fixation by legumes in mixed pastures is influenced by three primary factors; legume persistence and production, soil N status, and competition with the associated grass(es). These factors and the interactions between them are discussed. Legume persistence, production and BNF is also influenced by many factors and this review centres on the important effects of soil moisture status, soil acidity, nutrition, and pests and disease. Soil N status interacts directly with BNF in the short and long term. In the short-term, increases in soil inorganic N occurs during dry conditions and where N fertiliser is used, and these will reduce BNF. In the long-term, BNF leads to accumulation of soil N, grass dominance, and reduced BNF. However, cyclical patterns of legume and grass dominance can occur due, at least in part, to temporal changes in plant-available N levels in soil. Thus, there is a dynamic relationship between legumes and grasses whereby uptake of soil N by grass reduces the inhibitory effect of soil N on BNF and competition by grasses reduces legume production and BNF. Factors affecting the competition between legumes and grasses are considered including grass species, grazing animals, and grazing or cutting management. Some fixed N is transferred from legumes to associated grasses. The amount of N transferred 'below-ground', predominantly through decomposition of legume roots and nodules, has been estimated at 3 to 102 kg N ha-1 yr-1 or 2 to 26% of BNF. In grazed pasture, N is also transferred 'above-ground' via return in animal excreta and this can be of a similar magnitude to 'below-ground' transfer. Increased BNF in mixed legume/grass pastures is being obtained through selection or breeding of legumes for increased productivity and/or to minimise effects of nutrient limitations, low soil moisture, soil acidity, and pests and disease. Ultimately, this will reduce the need to modify the pasture environment and increase the role of legumes in low-input, sustainable agriculture.