Most root-derived carbon inputs do not contribute to long-term global soil carbon storage

Plant root-derived carbon (C) inputs (I_root) are the primary source of C in mineral bulk soil. However, a fraction of I_root may lose quickly (I_loos, e.g., via rhizosphere microbial respiration, leaching and fauna feeding) without contributing to long-term bulk soil C storage, yet this loss has never been quantified, particularly on a global scale. In this study we integrated three observational global data sets including soil radiocarbon content, allocation of photosynthetically assimilated C, and root biomass distribution in 2,034 soil profiles to quantify I_root and its contribution to the bulk soil C pool. We show that global average I_root in the 0–200 cm soil profile is 3.5 Mg ha⁻¹ yr⁻¹, ~80% of which (i.e., I_loos) is lost rather than contributing to long-term bulk soil C storage. I_root decreases exponentially with soil depth, and the top 20 cm soil contains >60% of total I_root. Actual C input contributing to long-term bulk soil storage (i.e., I_root–I_loos) shows a similar depth distribution to I_root. We also map I_loos and its depth distribution across the globe. Our results demonstrate the global significance of direct C losses which limit the contribution of I_root to bulk soil C storage; and provide spatially explicit data to facilitate reliable soil C predictions via separating direct C losses from total root-derived C inputs.