Optimising biological N2 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 N-2, so reducing the use of expensive fertiliser-N and enhancing soil fertility. N-2 fixing legumes provide the basis for developing sustainable farming systems that incorporate integrated nutrient management. By exploiting the stable nitrogen isotope N-15, it has been possible to reliably measure rates of N-2 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 N-2 fixation. Practical agronomic and microbiological means to maximise N inputs by legumes have also been identified.