Faster Soil Carbon Aging With Depth at Higher Elevations in a Subtropical Forest

Abstract Earth system models are increasingly adopting multi‐layer soil frameworks to improve simulations of vertical carbon distribution. A critical parameter in these models is the e ‐folding depth ( z τ ), which quantifies the rate at which soil organic carbon (SOC) ages with depth. Specifically, z τ represents the soil depth at which carbon becomes e ‐times older (≈2.7 times older) than surface carbon. Despite its importance, most models assume constant z τ within biomes, leaving its spatial variability largely unclear. To test this assumption, we collected multi‐layer soil samples across eight forest plots spanning a subtropical montane elevational gradient (427–1,474 m) and employed radiocarbon dating to quantify vertical SOC aging patterns. Our results revealed a robust exponential increase in SOC age with depth at all elevations, alongside a 66% decline in z τ from 78.6 cm at the base to 26.4 cm at the summit. This indicated that a 1‐m increase in soil depth approximately amplified SOC age by 4‐fold at the lowest elevation and 44‐fold at the highest position. Despite significant changes in vegetation along the elevational gradient, vegetation type did not play an essential role in controlling z τ variability. Instead, this elevational dependence of z τ was primarily driven by soil water content (22.2% of variability explained), mean annual temperature (19.7%), and soil carbon‐to‐nitrogen ratio (19.0%). These findings suggest z τ as an elevation‐sensitive sentinel of soil carbon dynamics, urging models to incorporate its variability for projections of soil carbon persistence under climate change.