Soil carbon addition affects plant growth in a species-specific way.
Restoration of ex-arable land to species-rich habitats has become common practice in Europe as a result of Agri-Environment Regulations. The results, however, are highly variable and often disappointing. Competition from weedy species as a result of high soil inorganic nitrogen levels can negatively affect the establishment and growth of desirable grassland plant species. One method that has been put forward to alter competitive interactions among plant species on restoration sites is the addition of carbon (C) to the soil. To make C addition a tool for successful restoration of species rich grassland, it should affect plant growth in a species-specific way. We present results of a greenhouse study to assess the species-specific responses of 29 plant species to a range of C volumes added to the soil. Specifically, we tested whether functional group affiliation (legumes, other forbs and grasses), life form (annual and perennial) or nitrophilic status could explain parts of the variation in the response of the selected plant species to C addition. Amending soil with increasing levels of C resulted in a log-linear reduction in biomass accumulation of all plant species tested, but the responses varied significantly between the plant species. Functional group and life form explained a significant amount of variation observed among the plant species tested. The response of legumes to C addition was less pronounced than that of other forbs and grasses. Grasses showed a decrease in shoot:root ratio in response to C addition, while the shoot:root ratio of legumes and forbs remained constant. A greater shoot biomass reduction in response to C addition was found for annual species than for perennial species. No correlation was found between the slope of the regression of log biomass vs. the amount of C added and the Ellenberg N-values, indicating that there is no simple relationship between the nitrophilic status of a plant species and its response to C addition. Synthesis and applications. The results suggest that adding C to soil is a promising tool in grassland restoration. C addition disproportionately reduced above-ground biomass accumulation by annual plant species and grasses, which often dominate early succession on ex-arable land. This may facilitate the establishment and growth of late-seral species and thus the restoration of species-rich grassland on ex-arable land.