Effect of terrain gradient on cadmium accumulation in soils

Abstract

Many studies have confirmed that terrain is an important factor affecting cadmium (Cd) accumulation in soil. However, the literature offers limited exploration on the effect mechanism of terrain on soil Cd accumulation. This study aims to explore the effect of terrain on the characteristics and influencing factors of soil Cd accumulation. A total of 182 soil samples are collected across the terrain gradient from soils in Dongguan City. Gradient boosting decision tree is implemented to identify the influencing factors of Cd accumulation in different terrain gradients. The goodness of fit of the models for low, median, and high terrains are 0.85, 0.80, and 0.84, respectively. Cd accumulation shows a significantly decreasing pattern in soils with an increase in terrain gradient (P < 0.001). Approximately 90.9% of Cd accumulation hot spots are observed in gentle terrain areas with a terrain niche index of <0.45, whereas 60.0% of cool spots are concentrated in low hilly areas with a terrain niche index in the range of 0.45–1.50. The influencing factors of Cd accumulation with the greatest relative contribution in low, median, and high terrains are agricultural practices (33.89% and 27.51%) and industrial production (41.32%), respectively. The relative contribution of influencing factors on Cd accumulation is affected by terrain gradient in the following order: agricultural practices (coefficient of variation (CV), 75.0%) > industrial production (CV, 74.6%) > soil properties (CV, 39.3%) > domestic wastes (CV, 26.1%) > traffic emission (CV, 17.9%) > mining (CV, 15.8%). When the terrain niche index is>1.50, the relative contribution of agricultural practices decreases sharply (24.26%) and that of industrial production shows a rapid increase (25.88%). The main reason for this phenomenon is that agricultural practices are more restricted by terrain gradient than industrial production. These findings imply that the differences of characteristics and influencing factors of Cd accumulation with terrain gradient must be considered in the formulation of targeted polices for controlling Cd contamination in soils.