A novel soil organic C model using climate, soil type and management data at the national scale in France

This report evidences factors controlling soil organic carbon at the national scale by modelling data of 2,158 soil samples from France. The global soil carbon amount, of about 1,500 Gt C, is approximately twice the amount of atmosphere C. Therefore, soil has major impact on atmospheric CO2, and, in turn, climate change. Soil organic carbon further controls many soil properties such as fertility, water retention and aggregate stability. Nonetheless, precise mechanisms ruling interactions between soil organic carbon and environmental factors are not well known at the large scale. Indeed, most soil investigations have been conducted at the plot scale using a limited number of factors. Therefore, a national soil survey of 2,158 soil samples from France was carried out to measure soil properties such as texture, organic carbon, nitrogen and heavy metal content. Here, we studied factors controlling soil organic carbon at the national scale using a model based on stepwise linear regression. Factors analysed were land use, soil texture, rock fragment content, climate and land management. We used several criteria for model selection, such as the Akaike information criterion (AIC), the corrected AIC rule and the Bayesian information criterion. Results show that organic carbon concentrations in fine earth increase with increasing rock fragment content, depending on land use and texture. Spreading farmyard manure and slurry induces higher carbon concentrations mostly in wet and stony grasslands. Nonetheless, a negative correlation has been found between carbon and direct C input from animal excrements on grasslands. Our findings will therefore help to define better land management practices to sequester soil carbon.