(13)C isotope discrimination correlates with biological nitrogen fixation in soybean (Glycine max (L.) Merrill) - Kumarasinghe, K.S. (International Atomic Energy Agency, Vienna (Austria). Joint FAO/IAEA Div.); Kirda, C; Mohamed, A.R.A.G; Zapata, F; Danso, S.K.A (Plant and soil, 1992)