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

The 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.