Below-ground carbon dioxide (CO2) emissions occur naturally at CO2 springs, but the risk of occurrence at other sites will increase as geologic CO2 storage is implemented to help mitigate climate change. This investigation examines the effects of elevated soil CO2 concentrations from such emissions on vegetation biomass and microbial community biomass, respiration and carbon utilisation in temperate grassland. Soil CO2 concentrations was increased by release of concentrated CO2 gas from a point source 0.6 m below the surface of the soil as a low-level leak (1 l min?1) for 10 weeks. The gassing resulted in reduced vegetation above- and below-ground biomass over time. No significant changes in microbial biomass or carbon utilisation were observed, but a trend towards reduced microbial respiration was apparent. This research provides a first step towards understanding the potential ecological risks of geologic carbon storage, the development of biological leak detection methods, and improved understanding of the effects of elevated soil CO2 concentrations on biological communities.