MINIMIZING THE EFFECT OF MINERAL NITROGEN ON BIOLOGICAL NITROGEN-FIXATION IN COMMON BEAN BY INCREASING NUTRIENT LEVELS

Although common bean (Phaseolus vulgaris L.) has good potential for N2 fixation, some additional N provided through fertilizer usually is re quired for a maximum yield. In this study the suppressive effect of N on nodulation and N2 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 arid 10 mg kg-1 soil and high = 200, 256 and 40 mg kg-1 soil, respectively) and combined with 5, 15 , 60 and 120 mg N kg-1 soil of N-15-labelled urea. The overall average nodule number and weight increased under high fertility levels. At lo w N applications, nitrogen had a synergistic effect on N2 fixation, hy stimulating nodule formation, nitrogenase activity and plant growth. At high fertility and at the highest N rate (120 mg kg-1 soil), the st imulatory effect of N fertilizer on N2 fixation was still observed, in creasing the amounts of N2 fixed from 88 up to 375 mg N plant-1. These results indicate that a suitable balance of soil nutrients is essenti al to obtain high N2 fixation rates and yield in common beans.