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Conservation in Soil of H2 Liberated from N2 Fixation by Hup- Nodules

Pigeon peas (Cajanus cajan) were grown in large soil columns (90-cm length by 30-cm diameter) and inoculated with four different strains of cowpea rhizobia, which varied with respect to hydrogen uptake activity (Hup). Despite the profuse liberation of H2 from Hup- nodules in vitro, H2 gas was not detected in any of the soil columns. When H2 was injected into the columns, the rates of consumption were highest in the treatments (including control) containing Hup- nodules (218 and 177 nmol � h−1 � cm−2) and lowest in the Hup+ treatments (158, 92, and 64 nmoles � h−1 � cm−2). In situ H2 uptake rates in small soil cores at fixed distances from the nodules decreased exponentially with distance from the nodule (R2 = 0.99). This decrease in H2 consumption was associated with a similar decrease in numbers of H2-oxidizing chemolithotrophic bacteria as determined by the most-probable-number method. On the basis of two equations derived separately upon diffusive theory (Fix's Law) and kinetic theory (Michaelis-Menten), the empirically derived rate constants and coefficients indicated that all of the H2 emitted from Hup- nodules would be consumed by H2-oxidizing bacteria within a 3- to 4.5-cm radius of the nodule surface. It is concluded that H2 is not lost from the soil-plant ecosystem during N2 fixation in C. cajan but is conserved by H2-oxidizing bacteria.