Precipitation gradient modulates partitioning and interrelationship of deep soil organic and inorganic carbon in the loess plateau

Investigating the dynamics of deep soil organic carbon (SOC) and soil inorganic carbon (SIC) is crucial for accurately assessing the terrestrial carbon cycle and the potential of deep soil carbon storage. Here, using deep borehole soil profiles extending to 1800 cm, we investigated the vertical distribution patterns, dominant predictors, and interrelationships of deep SOC and SIC along a precipitation gradient across the Chinese Loess Plateau. The results showed that SOC content decreased significantly with depth, whereas SIC increased in the upper 0–100 cm and then fluctuated markedly below 500 cm. SIC showed lower spatial variability than SOC across the profile. SOC was significantly negatively correlated with SIC across the profile, but this relationship changed sign along the precipitation gradient: significantly positive in the driest zone and significantly negative in the wetter zones (> 500 mm/y). Simple regressions showed that precipitation was significantly associated with SOC and SIC, although with limited explanatory power. Random forest analysis identified TN and depth as the most important predictors of SOC, whereas soil water content, total nitrogen, and depth were the dominant predictors of SIC. These results improve our understanding of precipitation-associated variation in deep soil carbon pools and suggest that soil water-related processes may contribute to the observed SOC-SIC relationship, although the underlying mechanisms require direct verification in future studies.