Coupling Atmospheric XCO2 with Deep Soil Carbon in the Black Soil Region: A Multi-Source Assessment in the Liaohe Plain, China

Quantifying carbon exchange between the land and atmosphere is crucial for estimating terrestrial carbon sinks and meeting climate goals. However, current bottom-up methods often ignore atmospheric observations and overlook the contribution of deep soil carbon. We integrated ten years of satellite XCO₂ data, emission inventories, and 5,037 soil organic carbon (SOC) measurements (0–5 m depth) from the Liaohe Plain. We analyzed the spatial relationship between atmospheric XCO₂ and SOC at various depths. Results show that correlations between XCO₂ and SOC are weak at the point scale. However, significant correlations (p < 0.01) appear at 1–5 m depth when analyzed within a 5 km radius. The XCO₂-SOC relationship varies by geological zone. Surface SOC drives short-term CO₂ variations. In alluvial zones, deep SOC affects the carbon cycle through water transport. In contrast, wind erosion limits SOC accumulation in aeolian zones. Additionally, XCO₂ levels correlate with environmental factors like Net Primary Productivity and precipitation. This suggests the regional carbon cycle is driven by combined climate, vegetation, and hydrological processes. This study highlights the importance of deep soil in watershed carbon cycling. It offers a new method for assessing regional carbon sinks and supporting land management strategies.