A Novel Depth-Explicit Model to Map the 3D Distribution of Soil Organic Carbon by Water and Tillage Erosion at the Catchment Scale.

Soil organic carbon (SOC) plays a central role in soil fertility, carbon sequestration, and greenhouse gas flux regulation. In agricultural landscapes, water erosion (Ew) and tillage erosion (Et) processes alter the spatial and vertical distribution of SOC, leading to significant losses in erosional areas and accumulations in depositional areas (Baert et al. 2024) ¹. Understanding and quantifying these erosion-driven controls on SOC distribution is essential for the sustainable management of soils and their carbon stocks. This study aims to quantify and model these processes in an agricultural catchment of 92 ha, located in the central Belgian loess belt, by developing a novel fully explicit 3D model prediction SOC as a function of depth, Ew and Et. The performance of this 3D model has been evaluated by comparing SOC stocks obtained through (i) its integrated application, versus (ii) the more classical approach based on a fixed stock calculations per site, considering a reference depth of 0.3m and 1m. The comparison shows that both approaches have very similar performances, both in terms of random (RMSE) and systematic error (%Bias). However, the 3D model has the advantage, over the more classical approach, that it is depth-explicit, and can therefore predict SOC values at any given depth.

The study combined the use of WaTEM/SEDEM model (Notebaert et al. 2006) ² with the sampling of 45 soil profiles until a depth of 1m and a sampling interval of 0.1 m, across different topographical positions (i.e. plateau, convexity, slope, concavity and footslope) covering a wide range of both Ew and Et values in erosional and depositional contexts. In addition, as the proposed novel DSM approach is based on the general depth distribution of Meersmans et al. (2009) ³ , it provides a set of specific parameters related to the vertical heterogeneity of SOC (i.e. SOC at the surface (SOCsurf), SOC at the bottom of the profile(SOCinf), tillage depth (td), and the rate of the exponential decline underneath the plough layer (α)), which on its turn are all expressed as a function of Ew and/or Et rates .

Our research shows that SOC stocks is two-fold higher in depositional areas as compared to eroded sites. Moreover, the present study highlights that Et and Ew are having a different impact on the redistribution, and hence both the vertical and horizontal heterogeneity, of SOC within agricultural landscapes. In this respect, Et mainly affects surface SOC stocks, whereas Ew has a greater impact on deeper stocks. Given its good performance, the presented novel spatially explicit 3D model can be considered as a promising tool for refining the quantification of SOC stocks and associated Digital Soil Mapping-products at the catchment scale.

¹ Baert et al. (2024). Assessing the 3D distribution of soil organic carbon by integrating predictions of water and tillage erosion into a digital soil mapping-approach: A case study for silt loam cropland (Belgium).

² Notebaert et al. (2006). WaTEM / SEDEM version 2006 Manual.

³ Meersmans et al. (2009). Modelling the three dimensional spatial distribution of soil organic carbon (SOC) at the regional scale (Flanders, Belgium).