Nonlinear grassland responses to past and future atmospheric CO2

Carbon sequestration in soil organic matter may moderate increases in atmospheric CO2 concentrations (Ca) as Ca increases to more than 500?µmol?mol-1 this century from interglacial levels of less than 200?µmol?mol-1 (refs 1–6). However, such carbon storage depends on feedbacks between plant responses to Ca and nutrient availability7,8. Here we present evidence that soil carbon storage and nitrogen cycling in a grassland ecosystem are much more responsive to increases in past Ca than to those forecast for the coming century. Along a continuous gradient of 200 to 550?µmol?mol-1 (refs 9, 10), increased Ca promoted higher photosynthetic rates and altered plant tissue chemistry. Soil carbon was lost at subambient Ca, but was unchanged at elevated Ca where losses of old soil carbon offset increases in new carbon. Along the experimental gradient in Ca there was a nonlinear, threefold decrease in nitrogen availability. The differences in sensitivity of carbon storage to historical and future Ca and increased nutrient limitation suggest that the passive sequestration of carbon in soils may have been important historically, but the ability of soils to continue as sinks is limited.