Nonlinear grassland responses to past and future atmospheric CO2

Abstract

Carbon sequestration in soil organic matter may moderate increases in atmospheric CO₂ concentrations (C_a) as C_a 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 C_a 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 C_a than to those forecast for the coming century. Along a continuous gradient of 200 to 550 µmol 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.