Threshold and interaction effects of environmental variables affecting the spatial distribution of Pb

Understanding the spatial distribution of soil Pb and its potential influence mechanism is significant for controlling Pb pollution in tea plantations and guaranteeing food safety. The linear and nonlinear relationships between Pb and environmental variables have been widely studied, but the threshold and interaction effects of its environmental variables are often neglected. In this study, 81 tea plantation soils were collected and determined the thresholds and interaction effects of environmental variables on Pb through the GBDT model. The results revealed that the 10 selected environmental variables together explained 71.01 % of the variation in Pb content, with the top three factors being pH, total phosphorus (TP), and distance from roads (dis_road). Most environmental variable had threshold effects on Pb. By visualizing the interaction of pH, TP, and dis_road on Pb concentration, we show that both two-way and three-way interactions are able to further explain its variation. The interaction between TP and dis_road alone accounts for 5.70 %, between pH and dis_road alone accounts for 7.92 %, and between pH and TP alone accounts for 5.83 % of the variation in Pb concentration. However, the three-way interaction between pH, TP, and dis_road alone explains 11.85 % of the variation in Pb concentration. Understanding these interactions is important for gaining insight into the underlying processes leading to Pb pollution and promoting the tea industry's sustainable development in China and abroad.