Biological nitrogen fixation (BNF) is of key importance in the N balance of soybean (Glycine max) crops. A number of authors have suggested that a negative balance may occur under high yield conditions. Few studies have measured the contribution of BNF to soil N in the pampas region. The aims of the present study were to compare three BNF determination methods -two isotopic methods using sorghum or a non-nodulating soybean isoline as a reference crop, and one involving the calculation of the

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... ference in N content between the nodulating and non-nodulating soybean isolines -and to estimate the N balance in soybean crops raised under conventional tillage and no tillage practices. The study was performed in 2004-2005; a complete randomised block design was used with three replicates (plot dimensions 3 × 7 m). The different methodologies estimated BNF to account for 45-58% of total plant N, equivalent to 94 to 123 kg N ha -1 . Depending on the methodology for estimating the BNF the soil N balance varied between -7 and 22 kg N ha -1 . With an average grain yield of 1,618 kg ha -1 and a BNF accounting for approximately 50% of total plant N (i.e.,115 kg N ha -1 ), the soil N balance was slightly positive (14 kg ha -1 ) and independent of the tillage practice. The tillage systems had no effect (P < 0.05) on the mass or number of nodules, shoot biomass production at the R1 or R6 growth stages, the N content, BNF, or grain yield. Since the present results were obtained using non-commercial soybean isolines, further research is required to determine the soil N balance when high yielding soybean crops are raised.