Factors driving inorganic carbon levels in the soils of the conterminous USA

Abstract

Soil inorganic carbon (SIC) has a significant role in the global carbon cycle. The controlling factors on SIC levels and in particular its relation to the soil pH require further study. Here, we analyzed long-term data (63,486 soil samples from 9,998 pedons) from 1950 to 2018, collected across the conterminous United States, to unravel factors controlling SIC. SIC ranged from 0.1–11.8 % at the soil pH of 4.1–10.4. The highest SIC levels were found when the soil pH was 8. Extremely alkaline (pH > 8.5) conditions are not effective in promoting SIC accumulation. An inverse correlation was found between SIC and soil organic carbon (SOC). Using a structural equation model (SEM), topographic features (aspect, elevation, slope, and hillshade) were key drivers when the soil pH was lower than 6.5. In general, SIC levels increased up to about 80 cm depth. Our findings indicate that soil pH was a key driver on the existence of SIC. These findings revealed the complex relationships that regulate SIC, especially the role of soil pH, and underscored the importance of diverse environmental factors when evaluating SIC dynamics.