Younger carbon dominates global soil carbon efflux
Liujun Xiao, Guocheng Wang, Mingming Wang, Shuai Zhang, Carlos A. Sierra, Xiaowei Guo, Jinfeng Chang, Zhou Shi, Zhongkui Luo
Abstract
Abstract Soil carbon (C) is comprised of a continuum of organic compounds with distinct ages (i.e., the time a C atom has experienced in soil since the C atom entered soil). The contribution of different age groups to soil C efflux is critical for understanding soil C stability and persistence, but is poorly understood due to the complexity of soil C pool age structure and potential distinct turnover behaviors of age groups. Here, we build upon the quantification of soil C transit times to infer the age of C atoms in soil C efflux ( a efflux ) from seven sequential soil layer depths down to 2 m at a global scale, and compare this age with radiocarbon‐inferred ages of C retained in corresponding soil layers ( a soil ). In the whole 0–2 m soil profile, the mean a efflux is (mean with 5%–95% quantiles) year and is just about one‐eighth of a soil ( year), demonstrating that younger C dominates soil C efflux. With increasing soil depth, both a efflux and a soil are increased, but their disparities are markedly narrowed. That is, the proportional contribution of relatively younger soil C to efflux is decreased in deeper layers, demonstrating that C inputs (new and young) stay longer in deeper layers. Across the globe, we find large spatial variability of the contribution of soil C age groups to C efflux. Especially, in deep soil layers of cold regions (e.g., boreal forests and tundra), a efflux may be older than a soil , suggesting that older C dominates C efflux only under a limited range of conditions. These results imply that most C inputs may not contribute to long‐term soil C storage, particularly in upper layers that hold the majority of new C inputs.