PH shift-mediated dehydrogenation and hydrogen production are responsible for microbial iron(III) reduction in submerged paddy soils

Abstract

pH shift-induced responses of iron(III) [Fe(III)] reduction and associated dehydrogenation and hydrogen (H2) production in submerged paddy soils were investigated to improve our understanding of the contribution of fermentative processes to microbial Fe(III) reduction. A 40-day anaerobic incubation experiment was conducted using two paddy soils from geographically distinct regions in northern (Hapli-Stagnic Anthosol, HSA; pH 8.14) and southern China (Fe-accumili-Stagnic Anthosol, FSA; pH 5.00), respectively. The initial soil pH was adjusted with aluminum sulfate or sodium carbonate to strongly acidic, acidic, neutral, alkaline, and strongly alkaline levels. During anaerobic incubation, as the initial soil pH shifts to acidic or lower levels, dehydrogenase activity (DHA) was greatly inhibited. However, increased acid-soluble Fe(III) level was accompanied by decreased maximum Fe(III) reduction potential (a) and prolonged time to maximum Fe(III) reduction rate (T Vmax ). Decreased pH led to a significantly lower peak partial pressure of H2 (ppH 2max ) in HSA and an increase in FSA. The initial shifts in pH to neutral and higher levels had no significant effect on DHA or acid-soluble Fe(III); however, ppH 2max declined in the neutral treatment of HSA, while T Vmax decreased slightly in the presence of a low ppH2 in the neutral to strongly alkaline FSA. DHA was correlated with Fe(II) accumulation in both paddy soils during the rapid Fe(III) reduction stage (p < 0.05). DHA is related to microbially mediated Fe(III) reduction in anaerobic paddy soils. pH shift-mediated enzymatic dehydrogenation and fermentative H2 production are responsible for microbial Fe(III) reduction in flooded rice fields, with initial pH as the main driving force.