Implications of a large global root biomass for carbon sink estimates and for soil carbon dynamics

Recent evidence suggests that significantly more plant carbon ( C) is stored belowground than existing estimates indicate. This study explores the implications for biome C pool sizes and global C fluxes. It predicts a root C pool of at least 268 Pg, 68% larger than previously thought. Although still a low-precision estimate ( owing to the uncertainties of biome-scalemeasurements), a global root C pool this large implies stronger land C sinks, particularly in tropical and temperate forests, shrubland and savanna. The land sink predicted from revised C inventories is 2.7 Pg yr(-1). This is 0.1 Pg yr(-1) larger than current estimates, within the uncertainties associated with global C fluxes, but conflicting with a smaller sink ( 2.4 Pg yr(-1)) estimated from C balance. Sink estimates derived from C inventories and C balance match, however, if global soil C is assumed to be declining by 0.4 - 0.7% yr(-1), rates that agree with long-term regional rates of soil C loss. Either possibility, a stronger land C sink or widespread soil C loss, argues that these features of the global C cycle should be reassessed to improve the accuracy and precision of C flux and pool estimates at both global and biome scales.