Faster soil carbon aging with depth at higher elevations in a subtropical forest

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 to 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-meter 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.