Nodulated legumes contribute significantly to the N economy of soils and, in so doing, increase agricultural yields of food crops (Dakora and Keya, 1997). The amount of N fixed in symbiotic legumes by root-nodule bacteria (species of Rhizobium, Bradyrhizobium, Allorhizobium, Agrorhizobium, Mesorhizobium and Sinorhizobium) has generally been assessed under field conditions using the 15N natural-abundance technique and/or the N difference method. Although the 15N natural-abundance technique has been used successfully for field measurements of N2 fixation in cultivated food grain and pasture legumes, the method is beset by problems associated with the choice of reference plants, especially in natural settings. When using this method, the N derived from atmospheric nitrogen fixation (%Ndfa or Pfix) is usually estimated (Shearer and Kohl, 1986) as:
where δ15Nref is the mean δ15N value of the reference plant, δ15Nleg is the mean δ15N value of the test legume, and the B value is the δ15N of the legume wholly dependent on N2 fixation for its N nutrition. The B value represents the value of atmospheric N2 and usually incorporates the isotopic fractionation associated with N2 reduction in root nodules (Shearer and Kohl, 1986).
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Dakora, F.D., Spriggs, A.C., Ndakidemi, P.A., Belane, A. (2008). Measuring N2 Fixation in Legumes Using 15N Natural Abundance: Some Methodological Problems Associated with Choice of Reference Plants. In: Dakora, F.D., Chimphango, S.B.M., Valentine, A.J., Elmerich, C., Newton, W.E. (eds) Biological Nitrogen Fixation: Towards Poverty Alleviation through Sustainable Agriculture. Current Plant Science and Biotechnology in Agriculture, vol 42. Springer, Dordrecht. https://doi.org/10.1007/978-1-4020-8252-8_13
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