Spatial variation, sources identification and risk assessment of soil heavy metals in a typical Torreya grandis cv. Merrillii plantation region of southeastern China

Torreya grandis (Torreya grandis cv. Merrillii) is a unique nut tree species in China. Currently, researches on Torreya grandis focus on nuts quality and yield, while few works are related to the soil quality of Torreya grandis plantation. In this study, the typical Torreya grandis production areas of Zhuji, Shengzhou, Keqiao and Dongyang cities along the Kuaiji Mountain were selected. A total of 121 topsoil samples (0-20 cm) were collected based on a grid of 1 km × 1 km. The results indicated that the average concentrations of Cd, Cr, Cu, As, Ni and Pb in soils were 0.12, 49.01, 27.95, 14.28, 26.97 and 40.28 mg kg^-1, respectively. The concentrations of six heavy metals all exceeded the background values, and there were different degrees of pollution levels. The results of Moran's I indicated that the spatial high-high clusters of soil heavy metals were mainly distributed in Zhuji and the junction of Shengzhou and Keqiao. The partial least squares path analysis of structural equation modeling (PLS-SEM) showed that OM and soil nutrients had extremely significant effects on soil heavy metals. Sources identification of principle component analysis (PCA) and positive matrix factorization model (PMF) revealed that agricultural activities, natural factors and mining were the main sources of soil heavy metals. The human health risks caused by soil heavy metals pollution were generally acceptable based on Monte Carlo simulation method. For the heavy-metal polluted area, management measures should be considered in order to protect human health.