Perennial forb invasions alter greenhouse gas balance between ecosystem and atmosphere in an annual grassland in China.

Grassland ecosystems are sensitive to invasions by plants from other functional groups which can alter soil greenhouse gas (GHG) fluxes. However, the effects of plant invasion on net GHG exchanges between soils and the atmosphere, plant production, and global warming potential (GWP) of annual grasslands is poorly understood. To evaluate the impacts of perennial forb invasions on GHG budgets of an annual grassland in China, we measured soil carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) fluxes over two years in replicated invaded (dominated by Alternanthera philoxeroides or Solidago canadensis) and non-invaded (dominated by the annual grass Eragrostis pilosa or the annual forb Sesbania cannabina) field sites. On average, soil CO2 and N2O emissions from invaded sites were 30% and 76% higher, respectively, relative to sites dominated by native species. Emissions of N2O and CO2 were especially high in Solidago and Alternanthera dominated sites, respectively. Soil CH4 emissions did not vary with plant species. On average, total biomass C of invaded sites was higher than that of the native dominated sites but this reflected the high C in Solidago dominated sites. Global warming potential (GWP) was increased by Alternanthera invasions and decreased by Solidago invasions. Plant invasions affected GWP of these annual grasslands through higher emissions of some GHGs but also sometimes higher biomass C. Together, this suggests that perennial forb invasions could change the net source or sink role of annual grasslands for GHG budgets, but the effects on GWP vary among species depending on GHG responses and C storage.