Impacts of an invasive grass on soil organic matter pools vary across a tree-mycorrhizal gradient

Increases in carbon (C) inputs can augment soil organic matter (SOM), or reduce SOM by accelerating decomposition. Thus, there is a need to understand how and why ecosystems differ in their sensitivity to C inputs. Invasive plants that invade wide-ranging habitats, accumulate biomass rapidly, and contribute copious amounts of C to soil can be ideal for addressing this gap. We quantified the effects of the invasive C4 grass, Microstegium vimineum, on SOM in three temperate forests across plots varying in their relative abundance of arbuscular mycorrhizal (AM) versus ectomycorrhizal (ECM) trees. We hypothesized that invasion would differentially affect SOM along the mycorrhizal gradient owing to recognized patterns in nitrogen availability (AM > ECM) and the proportion of unprotected SOM (ECM > AM). Across all sites, M. vimineum was associated with lower particulate organic matter (POM) in ECM-dominated plots, consistent with our hypothesis that invader-derived C inputs should stimulate decomposers to acquire nitrogen from unprotected SOM in soils with low nitrogen availability. However, the pattern of lower POM in the ECM-dominated soils was offset by greater mineral-associated organic matter (MAOM)—and isotopic data suggest this was largely driven by native- rather than invader-derived SOM—implying an invasion-associated transfer of native-derived POM into MAOM. Our results demonstrate a context-dependent shift in the form of SOM in a system with presumably enhanced C inputs. This finding suggests a need to look beyond changes in total SOM stocks, as intrinsic SOM changes could lead to important long-term feedbacks on invasion or priming effects.