H2 oxidation, O2 uptake and CO2 fixation in hydrogen treated soils

Abstract

In many legume nodules, the H2 produced as a byproduct of N2 fixation diffuses out of the nodule and is consumed by the soil. To study the fate of this H2 in soil, a H2 treatment system was developed that provided a 300 cm3 sample of a soil:silica sand (2:1) mixture with a H2 exposure rate (147 nmol H2 cm−3hr−1) similar to that calculated exist in soils located within 1–4 cm of nodules (30–254 nmol H2 cm−3hr−1). After 3 weeks of H2 pretreatment, the treated soils had a Km and Vmax for H2 uptake (1028 ppm and 836 nmol cm−3 hr−1, respectively) much greater than that of control, air-treated soil (40.2 ppm and 4.35 nmol cm−3 hr−1, respectively). In the H2 treated soils, O2, CO2 and H2 exchange rates were measured simultaneously in the presence of various pH2. With increasing pH2, a 5-fold increase was observed in O2 uptake, and CO2 evolution declined such that net CO2 fixation was observed in treatments of 680 ppm H2 or more. At the H2 exposure rate used to pretreat the soil, 60% of the electrons from H2 were passed to O2, and 40% were used to support CO2 fixation. The effect of H2 on the energy and C metabolism of soil may account for the well-known effect of legumes in promoting soil C deposition.