Spatial variation and temporal decline (1985–2017) of soil organic carbon stocks (SOCS) in relation to land use types in Tombel area, South-West Cameroon

Abstract

In carbon cycle and soil fertility management, SOCS play an important role. A better understanding of spatial variation and temporal decline of SOCS is indispensable for sustainable soil resource management. The purpose of this study was to assess the SOCS spatial distribution and quantify its temporal decline in the Tombel (South-West, Cameroon) area over 32 years (between 1985 and 2017) in relation to land use changes. Quantile Regression Forest (QRF) was applied to map the spatial distribution of SOCS in topsoil (0–30 cm), and its temporal dynamics over 32 years was assessed. A set of 91 and 100 soil samples were included for models calibration for 1985 and 2017, respectively, and models performances were assessed using a 10-fold cross-validation, as well as prediction uncertainties. Over three decades, agricultural and built up area (area such as a town or city with a lot of buildings) experienced a fast expansion, with respective land area increasing rates of 83 % and 81 %. The total SOCS in the study area changed from 22.33 Tg (Tera grams) to 14.68 Tg. Accuracies in the assessment showed good performances for 1985 (R² = 0.62, ME = - 0.06 and RMSE = 11.61 Mg ha−1) and 2017 (R² = 0.58, ME = - 0.03 and RMSE = 13.15 Mg ha−1). Climatic and terrain attributes were the most important predictors of the SOCS distribution. There was on average, about 1.3 Mg ha-1 yr-1 of SOCS decrease between 1985 and 2017, likely due to land use change and controlled by exacerbated agricultural land extension and rapid urbanization. The study provided a sound baseline for land management and decision making for soil quality, carbon accounting and carbon sequestration potential.