Incorporation of biological nitrogen fixation in agricultural systems

An argument might be made that replacing synthetic fertilizer-N with biologically fixed N2 will lead to lower synthetic N input and possibly lower N2O emissions. Increases in biological fixed N in agro-ecosystems can be implemented by including cover or green manure crops in the rotation or by replacing non-N2-fixing crops such as cereals with legumes. However, at the global scale, an overall increase in biologically fixed N2 will also lead to an increase in N2O emissions (Mosier et al, 1998; Sorai et al, 2007). For example, by including legumes in a pasture, N2O emissions were estimated to increase by a factor of two to three (Duxbury et al, 1982). Like inorganic N fertilizer, N fixed by leguminous crops is converted by the plant into organic N that can be mineralized and nitrified following decomposition of legume residues, and subsequently denitrified. Biologically and synthetically fixed N are indistinguishable once in organic form and equally become potential sources for N2O emissions. Although the actual process of biological N2 fixation probably does not lead to a significant increase in N2O emission, the release of N from root exudates during the growing season and the decomposition of legume residues will lead to an increase in N2O emissions (Rochette and Janzen, 2005).

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