Optimising biological N 2 fixation by legumes in farming systems

Whether grown as pulses for grain, as green manure, as pastures or as the tree components of agro-forestry systems, the value of leguminous crops lies in their ability to fix atmospheric N2, so reducing the use of expensive fertiliser-N and enhancing soil fertility. N2 fixing legumes provide the basis for developing sustainable farming systems that incorporate integrated nutrient management. By exploiting the stable nitrogen isotope 15N, it has been possible to reliably measure rates of N2 fixation in a wide range of agro-ecological field situations involving many leguminous species. The accumulated data demonstrate that there is a wealth of genetic diversity among legumes and their Rhizobium symbionts which can be used to enhance N2 fixation. Practical agronomic and microbiological means to maximise N inputs by legumes have also been identified.