Geochemical Versus Climatic Controls on Soil Organic Carbon

Soil texture and climate are considered the major controls on soil organic carbon (SOC) storage globally, and model simulations suggest that cooler regions of the planet will be more sensitive to SOC losses caused by climate warming. To investigate this pattern, we measured SOC and geochemical properties in surface (0–10 cm) mineral soil at 198 forested sites across the Boreal Plains, Boreal Shield, and Taiga Shield Ecozones in central Canada, where mean annual air temperature (MAT) ranged from −6.0 to +0.7°C. Across the five ecoregions, SOC was strongly related to soil organic matter (SOM) with SOC:SOM ratio of 0.47. Despite the substantial temperature gradient, we found that SOC was only weakly correlated with temperature, precipitation, and net primary productivity (NPP). Instead, SOC was strongly related (r > 0.7) to soil geochemical properties with SOC increasing in finer textured soils that had higher concentrations of aluminum (Al) and iron (Fe) and lower silica (Si) content. To extend the climate and soil geochemistry gradient, we expanded the analysis to include soils from temperate forests in the Mixedwood Plains Ecozone of southeastern Canada as well as published data from natural shrublands and grassland sites spanning the Southern Hemisphere and found that the strong correlations between SOC, Al + Fe, and Si persisted. These data suggest that soil texture and geochemical properties provide protection to SOC, and relationships with geochemistry must be incorporated in Earth System Models to improve spatial prediction of SOC stocks and their sensitivity to climate change.