Improvement of legume productivity and role of symbiotic nitrogen fixation in cropping systems: Overcoming the physiological and agronomic limitations

Nitrogen-fixing legumes can meet most of their N-needs through symbiotic nitrogen fixation (SNF). However, in most cases, inclusion of a legume in a cropping system does not ensure the attainment of such levels ofSNF in the field. Several environmental factors including drought, temperature and soil nutrient status dramatically affect the process at molecular/functional level and thus playa part in determining the actual amount of nitrogen fixed by a given legume in the field. lhis chapter reviews the status of SNF in response to most significant environmental constraints, and focuses on specific cases of harnessing SNF by improving its tolerance to stress factors with tI1.e aim of enhancing system productivity. Several examples are discussed, including the selection of legume crops tolerant to drought and salinity and/or allowing high biomass production and solubilization of phosphorus, identifying high nitrogen-fixing and nilrate-N tolerant genotypes and their inclusion in relevant cropping systems, and changes in agronomical management practices for better integration of legumes in cereal cropping systems. Finally, a general framework is discussed for agro-physiological contributions that can help overcome SNF limitation by environmental constraints. The on-farm application of these knowledge-based SNF technologies will strengthen the role of N2-fixing legumes in cropping system