Previously Unrecognized Land-Use Control on Deep Soil CO₂ Sink

Land-surface models commonly assume that soil CO₂ production is rapidly transferred out of the soil profile, effectively equating respiration with near-surface signals. However, persistent subsoil CO₂ enrichment despite limited in situ production reflects bidirectional exchange of topsoil-respired CO₂, suggesting that the surface-centric assumption may not be universally valid. Here we combine year-round, multi-depth field observations with process-based modeling (DeepCO₂) to quantify the effects of land use (croplands, forests, and forest-cropland edges) and climate on soil CO₂ production-transport dynamics. We reveal pronounced vertical decoupling of soil CO₂, with deep CO₂ (> 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 CO₂ 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 CO₂ 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 CO₂ is redistributed below the soil-atmosphere boundary and retained within the subsurface, forming a land-use-sensitive CO₂-C sink. Surface-based CO₂ 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.