Biological Nitrogen Fixation: Enhancement Techniques

Legume biological nitrogen fixation (BNF), resulting from the symbiotic relationship between legumes and rhizobia, a soil bacteria, is a key process in agriculture with the crop legumes alone supplying about 24 million tons of nitrogen (N) annually for global grain production. The specific amount of N fixed by a legume crop in a farmer's field essentially depends on the productivity of the crop and the level of soil mineral N in the field. Productivity can be enhanced by optimizing crop use of growing season and space, andnutrient and water resources, with careful attention to species and cultivar choice, planting time, row spacing and plant density, pest, disease and weed control, and crop nutrition. The suppressive effects of soil mineral N on BNF can be reduced through practices that lead to reductions in soil mineral N, such as notillage. Inoculation of legumes at sowing is another practice that can enhance BNF. It is virtually impossible to obtain accurate figures, but it is likely that about 50 million hectares of crop legumes, equivalent to about 25% of the area sown globally, are inoculated each year.