Introduced plants threaten biodiversity and ecosystem processes, including carbon (C) and nitrogen (N) cycles, but little is known about the threshold at which such effects occur. We examined the impact of the invasive shrub Amur honeysuckle on soil organic carbon (SOC) and N density at study sites that varied in invasion history. In plots with and without honeysuckle, we measured honeysuckle abundance and size (basal area) and extracted soil cores. SOC and N densities were highest at the site with the longest invasion history and highest invasion intensity (i.e., greatest abundance and basal area of honeysuckle). Basal area of honeysuckle positively affected SOC and N densities likely because of increased litter decomposition and altered microbial communities. Because honeysuckle increases forest net primary productivity (NPP) and SOC, it also may play a role in C sequestration. Our results demonstrate the need to consider the influence of invasion history and intensity when evaluating the potential impact of invasive species.
