Denitrification activity of Bradyrhizobium sp. isolated from Argentine soybean cultivated soils

Abstract

Two hundred and fifty strains, all of them representatives of native Bradyrhizobium sp., isolated from soils cultivated with soybean have been characterized by their denitrification activity. In addition, the denitrification potential of those soils was also measured by evaluating the most-probable-number (MPN) of denitrifying bacteria and the denitrification enzyme assay (DEA). Of the 250 isolates tested, 73 were scored as probable denitrifiers by a preliminary screening method. Only 41 were considered denitrifiers because they produced gas bubbles in Durham tubes, cultures reached an absorbance of more than 0.1 and NO 3-and NO 2-were not present. Ten of these 41 were selected to confirm denitrification and to study denitrification genes. According to N 2 O production and cell protein concentration with NO 3-, the isolates could be differentiated in three categories of denitrifiers. The presence of the napA, nirK, norC and nosZ genes was detected by production of a diagnostic PCR product using specific primers. RFLP from the 16S-23S rDNA spacer region (IGS) revealed that denitrifiers strains could be characterized as Bradyrhizobium japonicum and strains which were nonrespiratory denitrifiers as B. elkanii.

Novel European free-living, non-diazotrophic Bradyrhizobium isolates from contrasting soils that lack nodulation and nitrogen fixation genes – a genome comparison

“…Both genomes encode a nitrate reductase, NapA/B, which catalyses the reduction of nitrate to nitrite; the first stage in denitrification 21 22 23 . This is common among Bradyrhizobium including all seven complete genomes ( Table 2 and Fig.…”

“…In addition, all fully-sequenced isolates including both G22 and BF49 contain nirK , encoding a respiratory copper-containing nitrite reductase which is involved in the second stage of denitrification reducing nitrite to nitric oxide. The third stage of denitrification is the conversion of nitric oxide into nitrous oxide, a potent greenhouse gas, and is catalysed by a nitric oxide reductase encoded by norB/C 22 23 . The presence of a nitric oxide reductase gene has been noted in all previously published and complete Bradyrhizobium genomes in addition to G22 and BF49.…”

“…The ability to convert the greenhouse gas, nitrous oxide into environmentally benign nitrogen gas in the final stage of denitrification is an attribute with global importance 24 . Nitrous oxide reductase encoded by nosZ catalyses this process but it is not ubiquitous across Bradyrhizobium 22 23 25 26 . Of the seven published complete genomes, only B. diazoefficiens USDA 110, Bradyrhizobium sp.…”

Analysis of the denitrification pathway and greenhouse gases emissions inBradyrhizobiumsp. strains used as biofertilizers in South America

“…The denitrification genes nap A, nir K, nor B and nos Z were amplified using the specific primers described by Fernández et al . () for Bradyrhizobium . The reaction conditions were the same for all genes analysed in all the strains of interest.…”

“…isolated from the nodules of soybean grown in Argentina, only 41 reduced NO 3 − to N 2 (Fernández et al . ). Considering the vast extension of soil cultivated with soybean in this country, together with Brazil, and that NO 3 − is usually added during its growth, the cultivation of soybean is one of the main sources for N 2 O released into the atmosphere.…”