DENITRIFICATION BY RHIZOBIUM-MELILOTI .1. STUDIES OF FREE-LIVING CELLS AND NODULATED PLANTS

The denitrification activity was investigated in thirteen Rhizobium me liloti strains, of which most are proposed inoculant strains and one i s deficient in nitrogen fixation (Fix(-)). One strain each of the spec ies Pseudomonas aeruginosa and Bradyrhizobium japonicum were used as r eferences. We found a great variety regarding the denitrification capa city within R. meliloti strains, most of them with low or no detectabl e activity. Four strains showed a denitrification rate 200 times highe r or more than the strains with low activity, and the highest rate was found in R. meliloti 6963. The average denitrification rate (mu g N2O -N mg(-1) protein h(-1)) by free-living bacteria was 100, 50, and 26 f or P. aeruginosa, B. japonicum, and R. meliloti 6963, respectively. Of nine R. meliloti strains tested, three expressed nitrous oxide reduct ase (Nos) activity, while eight strains gave positive hybridisation si gnals to a gene probe derived from nosZ of Pseudomonas stutzeri. The F ix(-) strain, with a great denitrification capacity, was phenotypicall y Nos(-) and did not hybridise to the nosZ gene. Bacterial growths dur ing nitrate respiration as well as consumption and production of nitro genous oxides were investigated in a closed bioreactor with a limited access for oxygen. The growth rate of R. meliloti was more influenced by decreasing oxygen pressure than P. aeruginosa. Both bacteria accumu lated detectable levels of nitrite and nitrous oxide during the nitrat e respiration, but R. meliloti seems to be much more sensitive for acc umulation of nitrite than the pseudomonad.