Soil Inorganic Carbon Content and Its Environmental Controls in the Weibei Loess Region: A Random Forest-Based Spatial Analysis

Soil carbon constitutes the largest terrestrial carbon reservoir, with inorganic forms (SIC) contributing an estimated 20–40% of the global total. Despite its relevance to arid-region carbon cycling and stabilization, SIC remains less studied than soil organic carbon (SOC). This study quantified surface SIC content (0–20 cm) and its environmental drivers across the Weibei Loess region using 3261 soil samples collected between 2008 and 2010. A combination of Random Forest (RF) modeling and optimal parameter geodetector (OPGD) analysis was employed to assess spatial heterogeneity and identify key environmental controls. SIC content ranged from 0.10 to 3.56 g kg−1 (mean = 1.23 ± 0.41 g kg−1), generally lower than reported values in the Tibetan Plateau and Inner Mongolia. Higher concentrations were observed in central areas, with lower values toward the periphery. While mean annual temperature (MAT) and precipitation (MAP) remained key climatic correlates, shortwave radiation (srad) emerged as the strongest control on SIC across the region, exhibiting a significant positive association with its accumulation. Notably, its interaction with wind speed (vs) further amplified this effect, highlighting the synergistic role of radiation and near-surface turbulence in regulating inorganic carbon retention in surface soils. Collectively, these variables explained ~56% of SIC spatial variation. Favorable conditions for SIC accumulation were identified within specific environmental thresholds: srad (171–172 W/m2), MAP (546–587 mm), MAT (10.2–11.5 °C), and vs (1.90–1.96 m/s). These findings offer a quantitative basis for understanding SIC patterns in loess-derived soils and support the development of region-specific strategies for carbon regulation under changing climatic conditions.