Biological nitrogen fixation and nitrogen fixing trees

Abstract

Biological nitrogen fixation (BNF) refers to the process by which the nitrogen that is present abundantly in the atmosphere as non-reactive molecular dinitrogen (N2) is converted into reactive forms that become available to plants and thereby to all life forms. The key process of BNF is the conversion of N2 to ammonia (NH3) catalyzed by the enzyme nitrogenase, which occurs in a group of microorganisms that exist in symbiotic association with certain plants especially legumes, or as free-living organisms. Frankia, a genus of actinomycetes, also can form a symbiotic association with about 25 genera of non-leguminous plants. Next to photosynthesis, BNF is the second most important biochemical reaction that supports life on Earth. Being a subject of enormous importance and long history, voluminous information is available on different aspects of BNF. This chapter provides an overview of the key principles of BNF and explores its role in agroforestry with special reference to N2-fixing trees (NFTs) that are found mostly in the tropics and subtropics. The major topics included are an explanation of the process of dinitrogen fixation, rhizobia and root nodulation, short accounts of the nitrogen-fixing (legumes and actinorhyzal) plants, measurement of symbiotic nitrogen fixation, transfer of fixed nitrogen and factors affecting it, and the management of BNF in agroforestry. During the early days of agroforestry research in the 1980s, some efforts were initiated for capturing the benefits of BNF, including field-level evaluation of NFT cultivars and provenances and estimation of their nitrogen-fixing potential. The efforts, however, were not pursued vigorously such that the results reported, although used repeatedly, are of limited value, and even questionable at times. The NFTs constitute a valuable natural resource, which when managed properly, can be used in perpetuity in AFS for enhancing plant productivity.