Grass Invasions Across a Regional Gradient are Associated with Declines in Belowground Carbon Pools

The composition of plant communities everywhere now likely comprises alien as well as native species, and those aliens that become invasive have wide-ranging impacts on the structure and function of recipient ecosystems. These impacts include perturbations to soil carbon (C) cycling, but the direction and magnitude of impacts are species and climate dependent, making it difficult to generalize whether a specific invader will promote losses or gains in soil C stocks. Generalizations of a specific invader??s impacts are necessary; however, because the range of an invader can encompass thousands of square kilometers, meaning their effects can have broad, regional consequences. To quantify broad-scale and context-dependent impacts of a specific invader, multi-site investigations that capture and measure local and regional environmental heterogeneity are necessary. Using this approach, we show that a widespread grass invader of forest understories is associated with declines in soil C during infilling (spreading within the invaded range). Across the 36 study sites, total soil C stocks declined (P?=?0.113) by approximately 12% (estimated mean?±?SD, uninvaded: 2,429?±?512.9 vs. invaded: 2,140?±?520.7?g?C?m^?2). The decline in total soil C is driven by a significant (P?=?0.047) reduction in the native-derived, mineral-associated soil C fraction. This fraction, whose mass and slow turnover makes it an important C store, is approximately 15% lower in invaded (estimated mean?±?SD: 1,560?±?400.4?g?C?m^?2) than uninvaded plots (1,826?±?398.1?g?C?m^?2). Notably, declines in this C fraction are only apparent at 21 of the sites, reflecting how environmental heterogeneity in other variables (specifically pH, soil moisture, and clay content) are important to quantify to determine invader impacts across a region. The 26% decline in microbial biomass with invasion (P?=?0.011; estimated mean?±?SD, uninvaded: 10.05?±?1.79 vs. invaded: 7.40?±?1.80?g?C?m^?2) is also dependent on site characteristics (pH), and reductions are greater where the invader occurs at higher densities. Reductions in microbial biomass and soil C with invasion suggest that grass invasion will alter soil C cycling and decrease forest-C stores across the study region, although invader effects at a specific-site will be dependent on environmental context.