Fababean (Viciafaba L. var. minor) has high N requirements for optimum yields. There are some indications that this legume can satisfy a greater part of this requirement largely through biological N 2 fiXation. To confirm these observations, more quantitative estimates of N 2 fixed in fababean on different soils and over different seasons are necessary. Also, detailed N nutrition and translocation studies could serve as the basis for any required improvement in N 2 fixation. This study assessed the contribution of soil, fertilizer, and fixed N 2 to the nutrition and growth of fababean at seven growth stages. The soil used is classified as a Typic Eutrocrept. The Avalue approach, involving the application of 20 and 100 kg N/ha of 15 N-labeled ammonium sulphate to fababean and spring wheat (Triticum vulgare) as a reference crop, respectively, was used to assess the relative contributions of N from fixation, soil, and fertilizer to plant growth. At physiological maturity harvest (126 days after planting), fababean had accumulated 209 kg N/ha, 79% of which was derived from fixation, with about 20% coming from soil and I% from the labeled fertilizer. These results confirm that fababean has the ability for high N 2 fixation. Growth and N 2 fixation were, however, low during early growth (0-59 days after planting), but increased rapidly after this period. The maximum N 2 fixation rate was achieved between the early and mid-pod-filling stages, an interval during which N 2 fixation accounted for more than 4 kg N/ha/day. The N 2 fixation activity dropped to almost zero after the mid-podfilling stage. After the mid-pod-filling stage, the N demand of the developing seeds and pods was higher than could be satisfied through the negligible soil uptake of Nand N 2 fixation. Thus, N translocated from the shoot was virtually the only source for pod and seed development at these final stages. With such a high level of N 2 fixation in fababean, it was estimated that after grain removal, fababean growth led to a net positive residual N level in soil.

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