DENITRIFICATION BY INTACT SOYBEAN NODULES IN RELATION TO NATURAL 15N ENRICHMENT OF NODULES

Publisher Website
Google Scholar
Abstract
The natural 15N abundance of nodules of soybeans (Glycine max (L.) Merrill) which are actively fixing N2 is considerably higher than other tissues. To investigate the question of whether isotopic fractionation associated with denitrification by bacteroids causes this 15N enrichment, we inoculated soybeans with two strains of Rhizobium japonicum. Free-living cultures of one of these (strain USDA 33) were unable to denitrify or respire NO−3, while free-living cultures of the second (strain USDA 138) were capable of denitrification. USDA 138 formed nodules which fixed N2 very efficiently. The N of these nodules was enriched in 15N and the nodules reduced a substantial amount of NO−3 to NO−2 and N2O. Nodules infected with USDA 33 fixed about half as much N2 as those infected with USDA 138. The former nodules were enriched in 15N (although less so than nodules infected with USDA 138), despite the fact that the nodules formed by USDA 33 did not reduce NO−3. Clearly denitrification could not have been the cause of 15N enrichment of nodules infected with strain USDA 33. Alternative causes of 15N enrichment of soybean nodules and their possible metabolic significance are discussed. Key words: δ15N, nitrogen fixation