Previously Unrecognized Land‐Use Control on Deep Soil CO 2 Sink

ABSTRACT Land‐surface models commonly assume that soil CO 2 production is rapidly transferred out of the soil profile, effectively equating respiration with near‐surface signals. However, persistent subsoil CO 2 enrichment despite limited in situ production reflects bidirectional exchange of topsoil‐respired CO 2 , suggesting that the surface‐centric assumption may not be universally valid. Here we combine year‐round, multi‐depth field observations with process‐based modeling (DeepCO 2 ) to quantify the effects of land use (croplands, forests, and forest‐cropland edges) and climate on soil CO 2 production‐transport dynamics. We reveal pronounced vertical decoupling of soil CO 2 , with deep CO 2 (> 1 m) concentrations up to two orders of magnitude above surface concentrations and 9–17 times greater than can be explained by in situ production alone, revealing profile‐scale production‐transport imbalance. Forest soils promote ~2.3‐fold greater deep CO 2 accumulation than cropland and edge systems, driven by enhanced macroporosity and stronger advective coupling. Seasonal climate forcing regulates these regimes, with winter cold spells suppressing exchange, whereas spring–summer warming amplifies CO 2 source strength originating from otherwise “protected” subsoil organic carbon, particularly in cropland and edge soils. These findings demonstrate that a substantial fraction of topsoil‐respired CO 2 is redistributed below the soil‐atmosphere boundary and retained within the subsurface, forming a land‐use‐sensitive CO 2 ‐C sink. Surface‐based CO 2 assessment therefore reflects a transport‐limited, boundary‐conditioned signal rather than a direct proxy for profile‐scale production, and alone cannot resolve the full partitioning of soil‐respired carbon. Incorporating depth‐dependent production‐transport coupling is therefore essential for accurately representing terrestrial carbon allocation and land‐climate feedbacks under ongoing land‐use change and climate warming.