Grassland ecosystems invaded by exotic plant species often exhibit substantially higher aboveground productivity and soil nitrogen than the native communities they replace. Stable functioning of these systems depends on underlying processes such as microbial carbon (C) and nitrogen (N) cycling, but we know surprisingly little about how these change with plant invasion. We combined an isotopic incubation approach with Illumina sequencing and examined if aboveground plant biomass, microbial community composition, C respiration and assimilation, and N cycling differed among invaded and native plant communities.
We estimated microbial carbon-use efficiency (CUE) by measuring the amount of 13C allocated to respiration versus growth in soils collected underneath native and invasive plants. We included soil from natural invasions of spotted knapweed (Centaurea stoebe), leafy spurge (Euphorbia esula), sulfur cinquefoil (Potentilla recta), cheatgrass (Bromus tectorum) and adjacent intact native grasslands. We also collected soil from 7-year old experimental plots containing monocultures of these same four invaders or mixtures of native plants.
Results/Conclusions
Invasive plants tended to produce more biomass and harbor distinct soil microbial communities. In our field invasions, soil microbial communities had higher rates of respiration and lower biomass than those associated with the native plant communities, leading to a lower CUE. This corresponded with lower substrate C:N, greater gross N mineralization and aboveground plant biomass. In the experimental plots, however, microbial growth and CUE were low across community types, including the native mixtures. This corresponded with a lower abundance of oligotrophic bacteria in the experimental plots relative to field plots.
Our results suggest that in semi-arid grasslands, successful exotic invaders associate with soil microbial communities that process C with a lower efficiency. This, in turn, increases the rate of N cycling potentially contributing to the sustained gains in productivity often observed with exotic plant invasions.