Nitrogen limitation of microbial decomposition in a grassland under elevated CO2

Carbon accumulation in the terrestrial biosphere could partially offset the effects of anthropogenic CO2 emissions on atmospheric CO2 (refs 1, 2). The net impact of increased CO2 on the carbon balance of terrestrial ecosystem s is unclear, however, because elevated CO2 effects on carbon input to soil s and plant use of water and nutrients often have contrasting effects on mi crobial processes(3-5). Here we show suppression of microbial decomposition in an annual grassland after continuous exposure to increased CO2 for five growing seasons. The increased CO2 enhanced plant nitrogen uptake, microbi al biomass carbon, and available carbon for microbes. But it reduced availa ble soil nitrogen, exacerbated nitrogen constraints on microbes, and reduce d microbial respiration per unit biomass. These results indicate that incre ased CO2 can alter the interaction between plants and microbes in favour of plant utilization of nitrogen, thereby slowing microbial decomposition and increasing ecosystem carbon accumulation.