Is microbial reduction of Fe (III) in podzolic soils influencing C release?

Fe(III) oxides stabilize soil organic matter by forming Fe organo-mineral associations (Fe-OMA). Under anoxic conditions, Fe-OMA may be destabilized by microbial dissimilatory iron reduction that releases aqueous Fe(II) and also possibly adsorbed or co-precipitated organic matter. Soil spodic horizons that accumulate Fe-OMA are ideal natural materials to study such impact. Here, we study three spodic horizons from pedons (P) of increasing age: P-270 yrs, P-330 yrs and P-530 yrs. Their contents of total carbon and short range ordered (SRO) Fe oxides increase with soil age from, respectively, 1486 to 3618 and 13 to 249 μmol g−1. The samples were incubated for 96 h under anoxic conditions, with and without Shewanella putrefaciens (a model dissimilatory Fe(III)-reducing bacteria) in a minimal medium devoid of any pH buffer and external electron donor. The concentration of dissolved Fe(II), total Fe and organic C was monitored at 9 time steps. With increasing age, both the rate and extent of microbial Fe(III) reduction increased after S. putrefaciens addition. In P-270 yrs, P-330 yrs and P-530 yrs, respectively, the microbial reduction rates were 0.004, 0.026 and 0.114 fmol Fe(II) h−1 cell−1 while the amounts of released Fe(II) were 0.23, 0.32 and 1.98 μmol Fe(II) g−1, both being strongly correlated with SRO Fe oxides content. Dissolved organic carbon (DOC) was released with or without S. putrefaciens in all samples (up to 73 μmol OC g−1 after 96 h in P-530 yrs). Adding S. putrefaciens significantly increased DOC release, but only in P-270 yrs. Podzol development thus increases the impact of anoxia on Fe(II) release since organic matter accumulation impedes Fe oxide crystallization, thereby amplifying Fe availability for Fe reducing microbes. The evolution of Fe oxide content and crystallinity thus affects the fate of both C and Fe in soils.