Geological control of soil organic carbon and nitrogen stocks at the landscape scale

While SOC concentration was significantly correlated to soil clay concentration for both cropland (r2 = 0.36, p < 0.001) and forest (r2 = 0.44, p < 0.001), there was no significant relation between SOC stocks and soil clay stocks for forest soil (p = 0.11) and a significant but highly scattered positive correlation between SOC and clay stocks in cropland soils (r2 = 0.20, p = 0.02). No significant relation between pH or CaCO3 and SOC stocks was observed. Our results therefore revealed that soil parent material can significantly influence topsoil (0-30 cm) organic C and N stocks but that more research is needed to understand how soil parent material influences SOM stocks as it cannot be simply explained by basic soil physico-chemical parameters (clay and carbonate concentrations or stocks, pH). Overall, our results suggest that a good knowledge of the geology is needed to better constrain SOC stocks as well as SOC stocks evolution in a changing environment from landscape to global scale.