A simple pedotransfer function to estimate fine fraction organic carbon contents of surface horizons in French soils

International audience

إنجليزي. It has been proposed to separate soil organic carbon (SOC) into two physical fractions, i.e., fine fraction organiccarbon (OCfine) and coarse fraction organic carbon, to improve our ability to understand controlling factors andpredict SOC contents and SOC stability. In this study, we aimed at building a simple pedotransfer function toestimate OCfine content measured using a size fractionation protocol in French soils. We define OCfine as SOCcontent measured in the < 50 μm fraction. We used information from the French Soil Quality MonitoringNetwork (RMQS, 706 observations) and from a literature search (245 observations) to build and validate thepedotransfer function. We used the Random Forest algorithm and partial dependence plots in an exploratory dataanalysis, to analyze the variables influencing OCfine contents and to study the shape of their relationship withOCfine. Using the knowledge gained from the exploratory analysis, we constructed a simple pedotransfer function.A slight inflection in the OCfine content was detected with increasing SOC content and a good approximationof OCfine content was reached using a second-degree polynomial relationship, with distinct coefficientsdepending on the land cover (i.e., croplands and permanent crops, grasslands, natural grasslands, and forests andshrubs). A cross-validation of the pedotransfer function led to an RMSE of 2.2 g kg−1 and an independentvalidation led to an RMSE of 2.0 g kg−1. The domain of applicability of the pedotransfer function was defined asbeing restricted to topsoil horizons and a SOC content ≤ 90 g kg−1. The estimated OCfine is deemed equivalent toa measure after a complete soil dispersion and a size fractionation protocol. The proposed pedotransfer functioncould be easily used in future soil carbon modeling frameworks. From a land management perspective, the shapeof the relationship found between SOC and OCfine encourages increasing SOC levels to store more carbon in soil,including in the OCfine fraction.