Fig. 7: A schematic diagram of dynamic changes of Fe-OC and Ca-OC dominating ROC within the soil profile at regional scale.

The three schematic soil profiles from right to left correspond to increasing aridity and soil pH. With the increase in aridity, which is accompanied by a gradual decrease in precipitation and a progressive thinning of surface cover, the depth of calcium carbonate accumulation gradually shifts upward. The response of ROC to Ca-OC and Fe-OC exhibited variations both horizontally and vertically across different soil layers.