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 stable soil C formation. 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 the lab. We coupled assays of microbial activity with state-of-the-art, DNA stable isotopic probing to follow the fate of each litter to specific microbial taxa.
In both soil types, the AM litter was mineralized to CO2 more rapidly than the ECM litter. However, these losses were balanced by greater microbial degradation and growth. AM litter consistently drove higher rates of enzymatic activity than ECM litters as well as more 13C recovered in microbial biomass, particularly that of fast-growing copiotrophic bacteria. In contrast, ECM litter decomposition was more dependent on the microbial community. Specifically, microbes in the ECM soils degraded ECM litter to a greater extent than AM soil microbes, with the majority of 13C recovered in the biomass of saprotrophic fungi. Overall, microbial community specialization controlled the fate of ECM litters to a greater extent than AM litters and microbial product formation was greater for AM litters. Thus, these results suggest that microbial community composition may be important for regulating the formation of microbial precursors to stable soil C from ECM but not AM litters, and AM ecosystems may accumulate more stable soil C.