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Symbiotic and free living denitrification by bradyrhizobium japonicum



Symbiotic and free living denitrification by bradyrhizobium japonicum



Soil Science Society of America Journal 50(2): 349-354



Denitrification by Bradyrhizobium japonicum (formerly Rhizobium japonicum) was characterized in two strains, USDA-110 and USDA-23, to determine the extent of denitrifying enzyme expression during symbiotic growth with soybean [Glycine max (L.) Merr.]. The rhizobia grew in anaerobic batch cultures, depleting NO3- and producing N2O in the presence of C2H2. Specific denitrifying activities of resting cells of rhizobia were compared with those of the common soil denitrifier Pseudomonas fluorescens, and were found to range from 14 to 18% of the P. fluorescens activity. Bacteroid specific activities were less than activities of free-living rhizobial cells, though the ratio depended on the growth stage of the cells. Denitrification was observed in detached whole nodules containing either of the strains, but not in nodules formed by a NO3- reductase mutant of B. japonicum 110. Denitrification rates of nodules were unaffected by the presence of external O2, though the rates were limited by the external NO3- concentration. Denitrification by intact, nodulated soybean plants under aerobic conditions was observed. In the presence of NO3- denitrification and N2 fixation (C2H2 reduction) occurred simultaneously. The estimated ratio of N denitrified to N fixed ranged from 0 to 0.3, depending on assay conditions. The observations suggest that denitrification in soybean nodules in the field can be a continuous process, independent of soil aeration. However, it is likely to be slow relative to N2 fixation under most conditions.

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Accession: 006555470

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DOI: 10.2136/sssaj1986.03615995005000020019x


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