Interactions between Litter and Soil Chemistry Drives Carbon Substrate Preference of Microbial Communities

Understanding what factors lead to stable soil carbon (C) formation and loss is critical to predicting how this important C pool will respond to global change. Recent research suggests that high quality litters promote the formation of physically protected soil C by enhancing the formation of microbial products that can subsequently be adsorbed onto mineral surfaces. We used trees that differ in mycorrhizal association, ectomycorrhizal (ECM) vs. arbuscular mycorrhizal (AM), as a model system to investigate the extent to which litter quality and microbial community composition influence microbial necromass production. We hypothesized that high quality AM litter would fuel greater rates of microbial necromass production than low quality ECM litters. Further, we predicted that this litter effect would outweigh the impacts of microbial specialization. To test these hypotheses, we incubated isotopically labeled ¹³C AM (tulip poplar, Liriodendron tulipifera) and ECM (red oak, Quercus rubra) litter in soils from AM and ECM dominated plots in a 100-year-old forest in West Virginia. We coupled assays of microbial activity with state-of-the-art, DNA stable isotopic probing to quantitatively measure the fate of each litter to specific microbial taxa. In AM soil, both litter types were mineralized ¹³CO₂ at the same rate. In the ECM soil, ¹³C was rapidly mineralized from AM litter, while ECM litter was decomposed more slowly. Differences in carbon use efficiency and the allocation of C to enzyme synthesis appear to drive this pattern. AM soil microbes decomposed more of the litter substrate, had a higher CUE and less C investment allocated to enzyme production than ECM soils, a result that suggest greater microbial biomass formation and turnover in AM than ECM soils. The preference for ECM or AM litters by bacteria also differed by soil type. In contrast to the ECM soils where bacteria had no preference for either litter type, bacteria in AM soils preferentially assimilated AM litters. Collectively, these results suggest that there are distinct differences between ECM and AM soils in the substrate specialization of fungi and bacteria that feedback on their carbon use efficiency and production of microbial necromass. This will provide quantitative data that can incorporate microbial traits into ecosystem models.