Microstegium vimineum (Trin.) Camus is a shade-tolerant, annual, C₄ grass. It invades the understory of a variety of forest types across a large portion of the southern and eastern United States. It is commonly referred to as Japanese stilt grass or Nepalese browntop. Its impacts include ecosystem engineering effects, sometimes in concert with other exotic species, which completely alter the structure and composition of the subcanopy within deciduous forests. It has been associated with accentuated declines in guilds of birds that specialize on the subcanopy, and declines in woody species seedling density and diversity, which may negatively impact forest regeneration following disturbance. It appears to out-compete other widespread invaders of forest understories, alter soil communities and nutrient dynamics, and may even decrease the abundance of human disease vectors, such as ticks, through modulation of understory microclimate. That it appears unpalatable to herbivores, and has a minimal root biomass, suggests that its effects on soil carbon dynamics likely proceed through the input of its high-quality foliage to soils. Combing observational approaches with enriched and natural abundance stable isotope techniques, we investigated the impacts on soil carbon, and carbon pathways to soils, from M. vimineum across the 2006-2008 growing seasons.

Results/Conclusions

Following priming theory, M. vimineum was associated with decreases in all of the native-derived, soil carbon pools we resolved. Specifically, M. vimineum was associated with 24, 34, 19, 25, 36 and 72% declines in total organic, particulate organic matter, mineral-associated, dissolved organic, mineralizable, and microbial biomass carbon, respectively. Soil carbon formed from M. vimineum inputs did not fully compensate for these decreases, meaning that carbon pools in invaded plots were smaller than in uninvaded plots. Supporting our inferences that carbon-cycling is accelerated under invasion, the microbial community was more active per unit biomass. The paucity of studies investigating impacts of grass invasion on soil carbon-cycling in forests highlights the need to further study M. vimineum to assess its impacts on soil carbon sinks and fertility. Our working hypothesis that priming arose through aboveground M. vimineum litter inputs was challenged when, in 2008, we demonstrated that rhizodeposition from M. vimineum was a significant source of carbon to the soil microbes and that M. vimineum had acquired a diverse and abundant native herbivore community. The multiple carbon-pathways to soils from M. vimineum highlight the need to consider that plant invaders can be as equally integrated into ecosystems as resident natives.