The Contribution of Fe(III) Reduction to Soil Carbon Mineralization in Montane Meadows Depends on Soil Chemistry, Not Parent Material or Microbial Community

Abstract

AbstractThe long‐term stability of soil carbon (C) is strongly influenced by organo‐mineral interactions. Iron (Fe)‐oxides can both inhibit microbial decomposition by providing physicochemical protection for organic molecules and enhance rates of C mineralization by serving as a terminal electron acceptor, depending on redox conditions. Restoration of floodplain hydrology in montane meadows has been proposed as a method of sequestering C for climate change mitigation. However, dissimilatory microbial reduction of Fe(III) could lead to C losses under increased reducing conditions. In this study, we explored variations in Fe‐C interactions over a range of redox conditions and in soils derived from two distinct parent materials to elucidate biochemical and microbial controls on soil C cycling in Sierra Nevada montane meadows. Soils derived from basalt showed greater rates of Fe(III)‐reduction at increasing soil moisture levels than granitic soils. Increases in Fe(III) reduction, however, were only associated with elevated rates of C mineralization in one basalt soil. Known Fe(III)‐reducing taxa were present in all samples but neither the relative abundance nor richness of Fe(III)‐reducers corresponded with measured rates of Fe(III) reduction. Under reducing conditions, Fe(III)‐reduction was only coupled to C mineralization in the soil with the greatest amount of Fe‐oxide bound C. However, Fe‐oxide ‐bound C was below theoretical limits for C sorption onto Fe‐oxides and not detectable in all soils. Overall, our results suggest that “what's there” in terms of soil chemistry may be a more important driver of C mineralization coupled to Fe(III) reduction than “who's there” in the microbial community.