EFFECT OF DIFFERENT BIOINOCULANTS ON THE MICROBIAL ACTIVITY IN THE RHIZOSPHERE SOIL OF Phaseolus lunatus var.sieva UNDER FIELD CONDITIONS

Microbial activity was evaluated in Phaseolus lunatus var. sieva rhizosphere soil, during different vegetative growth stages of the crop under field conditions. Seeds were inoculated with Bradyrhizobium sp. strains PCYGIVN3, PSNC4N2 and the interaction of both. Nitrogen fertilizer (N-P-K) and a control without inoculation were also evaluated. Respiration and dehydrogenase activity of microorganisms inhabiting the rhizosphere soil, microbial populations and plant dry weight were determined. The highest CO2 production rate was determined during flowering. Respiration from the rhizosphere soil inoculated with the interaction of both strains was higher than strain PSNC4N2 (0.121 and 0.088 mg CO2.g(-1).h(-1) respectively); the interaction was also significatively higher than the control and the nitrogenous chemical treatment [0.085 mg CO2.g(-1).h(-1)] (p<0.1). Dehydrogenase activity from rhizosphere soil inoculated with Bradyrhizobium strain PSNC4N2 > strain PCYGIVN3 > interaction of both, with 61.04, 56.57 and 51.37 mu g formazan.g(-1).(24h)(-1) respectively, and showed differences with respect to the chemical nitrogen fertilization [40.87 mu g formazan.g(-1).(24h)(-1)]. Higher values of dehydrogenase activity were reported before the harvest, probably due to the influence of the environment and the phytonematode infection of roots. Microbial activity was a more sensitive method than microorganism counting, showing a good correlation with plant dry weight obtained by the interaction of strains treatment and that inoculated with strain PSNC4N2. Both showed the highest dry matter values between flowering and pods maturity stages.