Three‐Dimensional Mapping of Organic Carbon using Piecewise Depth Functions in the Red Soil Critical Zone Observatory

Abstract

Core Ideas A piecewise depth function was fitted for three‐dimensional simulation of organic C. Regolith depth was used as a depth constraint. Topsoil organic C maps were utilized as covariates and benefited the three‐dimensional simulation. Organic carbon (OC) plays a pivotal role in earth surface systems. However, current three‐dimensional (3D) mapping studies usually focus on a soil depth of 1 m rather than the depth to the bedrock. A top‐down method using piecewise depth functions was proposed to fit the OC vertical decline patterns in a subtropical catchment in southern China. The vertical variation in OC was greatly affected by the heterogeneous topsoil due to natural processes and anthropogenic disturbances. Thus, topsoil OC maps were produced and utilized as covariates to indicate the OC decline rates and to benefit the 3D OC simulation. A distribution map of the underground critical zone thickness (UCZT) was applied as a lower boundary for the 3D simulation. Six widely used mapping techniques were performed to predict the spatial distribution of topsoil OC and depth function parameters. The overall cross‐validation results showed a root mean squared error (RMSE) of 1.7 g kg–1 and a ratio of performance to deviation (RPD) of 1.82. Given limited boreholes, validation showed that the depth function performed better in the lower part (>1 m) than in the upper part (<1 m). The proposed framework could holistically employ sufficient topsoil samples and limited boreholes and provides a promising solution to simulate the 3D information of subsurface systems. We anticipate that continuous 3D OC maps can be integrated with other critical zone information to elucidate the complex interactions of various processes and to support potential service management decisions.