Rhizodeposition Drives One Third of Belowground Carbon Input in Agroecosystems

Soil carbon (C) accumulation, an important mitigation solution to climate change, critically depends on plant C inputs. However, the allocation of belowground C inputs between root biomass and labile rhizodeposits remains poorly quantified. By synthesizing 746 global observations using ¹³C/¹⁴C tracing, we reveal that 17% of plant assimilated C is allocated belowground, with rhizodeposition accounting for one-third of the net belowground C input. Legumes primarily allocate assimilated C to belowground via rhizodeposition, whereas other crops, including barley, wheat, rice, maize, and grasses, invest more in root biomass. Rhizodeposition increases with soil organic C up to a peak at 16-19 g C kg^{- 1} (reaching 8%-10% of assimilated C) and declines thereafter, reflecting a nonlinear relationship between soil fertility and belowground C investment. Environmental change modifies these dynamics as warming suppresses belowground C input by 36%, while drought increases rhizodeposition by 21%. Globally, wheat, maize, and rice contribute 2.70 ± 1.40, 1.76 ± 1.39, and 2.38 ± 1.78 Tg C year^{- 1} to belowground net C input, respectively. These findings fill critical knowledge gaps in the global C cycle and establish crop-specific belowground C partitioning coefficients, providing updated empirical parameters to improve process-based soil C modeling and precise C accounting for climate-smart agriculture.