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

Abstract

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