Global patterns and climatic controls of belowground net carbon fixation

Significance The fraction of fixed carbon allocated belowground in terrestrial ecosystems is the most uncertain component of global carbon cycle assessments. Here we present a novel approach to determining global quantification of belowground productivity, which is estimated at 24.7 Pg y −1 and accounts for 46% of terrestrial carbon fixation. Carbon allocated belowground has a longer residence than its aboveground counterpart, playing a key role in long-term carbon storage. Total belowground productivity increases with precipitation, but the rate of increase decreases from arid to humid ecosystems. The fraction of total fixed carbon entering the soil decreases with precipitation and varies significantly among biomes. These results are indicative of the possible impacts of climate and land use changes on the global carbon cycle.