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. Differences in parent material were associated with different rates of Fe(III) reduction at increasing soil moisture levels, but not with differences in soil C mineralization. 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, our results suggest that Fe(III) reduction contributes to C mineralization only when Fe-bound C is present. However, Fe-bound C was not present in all of our soils and was below theoretical limits for C sorption onto Fe-oxides where it was found. Overall, our results suggest that meadow-specific soil chemistry drives Fe-C interactions and that the impact of Fe on C cycling in montane meadows may be smaller than in other ecosystems.