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-loss, 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), similar to 80% of which (i.e., I-loss) 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-loss) shows a similar depth distribution to I-root. We also map I-loss 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.

Automated summary

Plant root-derived carbon inputs are the main source of carbon in mineral bulk soil, but a fraction of these inputs may be quickly lost without contributing to long-term soil carbon storage. This study quantified the loss of root-derived carbon on a global scale and found that about 80% of the carbon inputs are lost rather than stored in the soil. The depth distribution of root-derived carbon inputs and their contribution to soil carbon storage were also determined, and a global map of the lost carbon and its distribution was created.