Plant and Soil 152: 131-138, 1993. © 1993 Kluwer Academic Publishers. Printed in the Netherlands. PLSO BNF14 Minimizing the effect of mineral nitrogen on biological nitrogen fixation in common bean by increasing nutrient levels S.M. TSAI l, R. BONETTI 1, S.M. AGBALA 2 and R. ROSSETI'O 1 ~Centro de Energia Nuclear na Agricultura, University of S~o Paulo, Caixa P.O. Box 96, Piracicaba CEP 13400-970, S~o Paulo, Brazil; 2Commissariat GOnOral ~ L'Energie Atomique, Kinsasha, Zaire Key words: bean, 15N isotope, nitrogen fixation, N fertilizer, phosphorus, potassium, Rhizobium tropici, sulphur Abstract Although common bean (Phaseolus vulgaris L.) has good potential for N 2 fixation, some additional N provided through fertilizer usually is required for a maximum yield. In this study the suppressive effect of N on nodulation and N 2 fixation was evaluated in an unfertile soil under greenhouse conditions with different levels of soil fertility (low = no P, K and S additions; medium = 50, 63 and 10 mg kg -~ soil and high = 200, 256 and 40 mgkg -1 soil, respectively) and combined with 5, 15, 60 and 120mgN kg -1 soil of ~SN-labelled urea. The overall average nodule number and weight increased under high fertility levels. At low N applications, nitrogen had a synergistic effect on N 2 fixation, by stimulating nodule formation, nitrogenase activity and plant growth. At high fertility and at the highest N rate (120 mg kg -1 soil), the stimulatory effect of N fertilizer on N 2 fixation was still observed, increasing the amounts of N 2 fixed from 88 up to 375 mg N plant -1. These results indicate that a suitable balance of soil nutrients is essential to obtain high N 2 fixation rates and yield in common beans. Introduction Common bean is capable of fixing large amounts of atmospheric N 2 under certain conditions, but usually some mineral N is needed to achieve substantial yields under the prevailing cropping system in Latin America. However, nitrogen fertilization affects nodulation of bean plants and therefore the usually-recommended rates of 40- 60kgNha i suppress N 2 fixation (Ruschel et al., 1979a; Graham, 1981). On the other hand, low rates have been shown to enhance nodule formation and function but are not sufficient to achieve maximum yields (Sistachs, 1970; Rosas and Bliss, 1987). While it is known that bean plants require high levels of P, K, S, Ca, Mg, Mo, Co and B (Bonetti et al., 1984; Franco and Day, 1980; Ruschel and Reuszer, 1973; Saito and Ruschel, 1978), production is restricted mostly to marginal soils where sufficient nutri- ents are not available. In soybean and tropical legumes, nodulation is especially sensitive to non-optimal levels of combined nutrients (Cass- man et al., 1981; Franqa et al., 1973; Gates and Wilson, 1974; Hernandez and Focht, 1985; Lynd and Ansman, 1989, 1990). Gates and Muller (1979) observed that nodu- lation in soybean is affected by unbalanced nutritional conditions of N, P and S, and that N 2 fixation was not affected by mineral N only in presence of high levels of P and S. Lynd and Ansman (1989) found decreases in nodule number of peanut when K was applied alone, but not when P and Ca were added. A stimulat- ory effect of the high rate of 60mgkg -~ of NH4NO 3 on nodulation also has been reported in presence of P (Gates and Wilson, 1974). This study was performed to determine whether applied nitrogen affects nodulation and plant biomass under limited and balanced soil