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

Parent material can deeply influence soil. organic matter (SOM) stocks. In this study we tested the hypotheses that parent material had an effect on SOM concentrations and stocks and that this effect may be explained by the influence of soil parent material on basic soil parameters. However, as the factors known to influence SOM stocks such as land use and climate frequently co-vary with geology, testing the influence on SOM stocks of the factor soil parent material alone is challenging. To properly test our hypotheses, we studied SOM stocks of forest and cropland soils in a small landscape (17 km(2)) of the Paris basin (France), i.e. with an homogeneous climate. We collected topsoil samples (0-30 cm) in 50 forest and cropland plots, located in five geological contexts: loess deposit, mudstone, grainstone, chalk and calcareous clay deposits. Basic soil parameters (texture, pH, CaCO3 concentration) and SOM stocks to 30 cm depth (organic C and total N) were determined on the 50 soil samples. Organic C and total N concentrations and stocks in topsoils (0-30 cm) were much higher in forests than in cultivated soils (38.1 (+/- 12.8) vs. 19.0 (+/- 4.7) g C kg(-1) soil and 83.4 (+/- 19.8) vs 48.9 (+/- 9.9) t C ha(-1) for SOC concentrations and stocks respectively). The influence of land-use on organic C and total N concentrations and stocks was modulated by parent material (significant interactions between land-use and parent material, p < 0.05 for concentrations and stocks). Indeed, the difference in organic C and total N concentrations and stocks in topsoils (0-30 cm) was much lower for soils developed on loess deposits. While SOC concentration was significantly correlated to soil clay concentration for both cropland (r(2) = 0.36, p < 0.001) and forest ( r(2) = 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 (r(2) = 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. (C) 2016 Elsevier B.V. All rights reserved.