Understanding species effects on ecosystem processes requires quantification of the different ways by which species mediate biogeochemical pathways. These questions are particularly relevant when novel species spread across heterogeneous landscapes. While it is often presumed that aboveground litter inputs to soils are the primary way plant species affect soil properties, recent work indicates that root dynamics may exert an equivalent or stronger We hypothesized that inputs of labile plant litter would be more important in forest plots with fast-cycling nutrient economies, while below-ground inputs would be more important in forest plots with slow-cycling nutrient economies. To test this, we examined the effects of one of the most common invasive understory species in eastern North America, Microstegium vimineum. We conducted an experiment using a common, uninvaded soil cores placed into invaded patches and manipulated both root access and litter inputs at 10 sites that varied in their nutrient economy (as assessed by the % of dominance by ectomycorrhizal vs. arbuscular mycorrhizal trees) and repeated the experiment in Georgia, North Carolina, and Indiana. We measured changes in the organic matter content, N mineralization, and nitrification in the soil cores after 2 growing seasons field incubation.
Results/Conclusions
The three locations at which we conducted the experiment strongly affected all measured variables. After controlling for these site effects, we found consistent patterns in the relative importance of leaf litter and root access on organic matter content, N mineralization, and nitrification. There were no differences in any measured variable across the three litter treatments (litter removal, ambient litter levels, doubled litter levels) despite the fact that Microstegium litter is relatively labile and readily decomposes. In contrast, we found significantly higher organic matter content and rates of N mineralization and nitrification in cores where Microstegium roots were allowed to colonize (relative to root-free soil cores). Notably, the magnitude of the below-ground effects on nutrient cycling were equivalent across the gradient of nutrient economies at all three locations. This contrasts with observational studies from these same sites, which has shown that the effects of Microstegium on nitrogen cycling tend to be stronger in EM dominated stands. Our results suggest that short term belowground inputs drive Microstegium impacts on soil organic mater content and nitrogen fluxes.