Carbon sequestration in soil organic matter may moderate increases in atmospheric CO2 concentrations (C-a) as C-a increases to more than 500 mumol mol(-1) this century from interglacial levels of less than 200 mumol mol(-1) (refs 1- 6). However, such carbon storage depends on feedbacks between plant responses to Ca and nutrient availability(7,8). 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 mumol mol(-1) (refs 9, 10), increased C-a promoted higher photosynthetic rates and altered plant tissue chemistry. Soil carbon was lost at subambient C-a, but was unchanged at elevated C-a where losses of old soil carbon offset increases in new carbon. Along the experimental gradient in C-a there was a nonlinear, threefold decrease in nitrogen availability. The differences in sensitivity of carbon storage to historical and future C-a 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.
