Rising Temperature May Trigger Deep Soil Carbon Loss Across Forest Ecosystems

Significantly more carbon (C) is stored in deep soil than in shallow horizons, yet how the decomposition of deep soil organic C (SOC) will respond to rising temperature remains unexplored on large scales, leading to considerable uncertainties to predictions of the magnitude and direction of C-cycle feedbacks to climate change. Herein, short-term temperature sensitivity of SOC decomposition (expressed as Q ₁₀) from six depths within the top 1 m soil from 90 upland forest sites (540 soil samples) across China is reported. Results show that Q ₁₀ significantly increases with soil depth, suggesting that deep SOC is more vulnerable to loss with rising temperature in comparison to shallow SOC. Climate is the primary regulator of shallow soil Q ₁₀ but its relative influence declines with depth; in contrast, soil C quality has a minor influence on Q ₁₀ in shallow soil but increases its influence with depth. When considering the depth-dependent Q ₁₀ variations, results further show that using the thermal response of shallow soil layer for the whole soil profile, as is usually done in model predictions, would significantly underestimate soil C-climate feedbacks. The results highlight that Earth system models need to consider multilayer soil C dynamics and their controls to improve prediction accuracy.