H(2) evolved by alfalfa root nodules during the process of N(2) fixation may be an important factor influencing the distribution of soil bacteria. To test this hypothesis under field conditions, over 700 bacterial isolates were obtained from fallow soil or from the 3-mm layer of soil surrounding alfalfa (Medicago sativa L.) root nodules, alfalfa roots, or bindweed (Convolvulus arvensis L.) roots. Bacteria were isolated under either aerobic or microaerophilic conditions and were tested for their capacity to metabolize H(2). Isolates showing net H(2) uptake and (3)H(2) incorporation activity under laboratory conditions were assigned a Hup(+) phenotype, whereas organisms with significant H(2) output capacity were designated as a Hout(+) phenotype. Under aerobic isolation conditions two Hup(+) isolates were obtained, whereas under microaerophilic conditions five Hup(+) and two Hout(+) isolates were found. The nine isolates differed on the basis of 24 standard bacteriological characteristics or fatty acid composition. Five of the nine organisms were isolated from soil around root nodules, whereas the other four were found distributed among the other three soil environments. On the basis of the microaerophilic isolations, 4.8% of the total procaryotic isolates from soil around root nodules were capable of oxidizing H(2), and 1.2% could produce H(2). Two of the Hup(+) isolates were identified as Rhizobium meliloti by root nodulation tests, but the fact that none of the isolates reduced C(2)H(2) under the assay conditions suggested that the H(2) metabolism traits were associated with various hydrogenase systems rather than with nitrogenase activity. Results from this study support the concept that H(2) evolution by alfalfa root nodules has a significant effect on the surrounding microenvironment and influences the number and diversity of bacteria occupying that region
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