Decoupling of Particulate and Mineral-Associated Organic Carbon (MAOC) with Depth Explains Subsoil MAOC Climate Vulnerability in Water-Limited Forests

Partitioning soil organic carbon (SOC) into particulate (POC) and mineral-associated (MAOC) forms is fundamental to understanding carbon sequestration. Yet, how these pools change with depth and respond to climate in semiarid forests remains a critical uncertainty. Our study investigated 35 forest sites across a climatic gradient in northern China to reveal the vertical distribution (0-10 cm vs 40-60 cm) and climate sensitivity of POC and MAOC. We discovered a fundamental depth-dependent shift in both pool interactions and climate sensitivity. While POC and MAOC were strongly coupled in topsoil, they became decoupled in the subsoil. Crucially, subsoil MAOC exhibited significant negative correlations with increased temperature and aridity, a vulnerability absent in the topsoil. Structural equation modeling identified the mechanism behind this pattern: climate indirectly influences topsoil MAOC via POC and mineral protection, but transitions to a direct driver in the subsoil following carbon pool decoupling. This shift from indirect to direct control explains the heightened climate sensitivity of deep MAOC. This finding underscores that the stability of deep carbon is precarious, revealing a significant vulnerability of carbon stocks in semiarid forests to climate change. Our results provide a mechanistic basis for integrating these depth-specific dynamics into Earth system models, which is essential for accurate climate projection.