Rhizodeposition Drives One Third of Belowground Carbon Input in Agroecosystems

ABSTRACT 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 13 C/ 14 C 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.