High-Resolution Geochemical Characteristics of Agricultural Soils: Implications for Fertility Enhancement and Heavy Metal Risk Management in Eastern China

Establishing the soil geochemical baseline and background values is critical for agricultural soil environmental management. This study collected 5207 topsoil (0–20 cm) and 1311 subsoil (150–180 cm) samples from an intensive agricultural area in Eastern China to quantify the element enrichment and depletion patterns, evaluate the integrated soil fertility, and assess the potential ecological risks, with a focus on disentangling the links between human activities and soil environmental changes. The results showed that most elements had higher baseline/background values than national averages, except for CaO, Mo, MgO, Sr, Na2O, and Br, reflecting the control of homogeneous parent material. Topsoil elements largely inherited subsoil characteristics, while anthropogenic disturbances such as fertilization and industrial activities caused the enrichment of Cd, Se, TN, TP, S, and SOC, and the depletion of I, V, and Mn. Soil fertility presented an obvious vertical heterogeneity, in which the topsoil had moderate-to-rich nutrients with a mean SOC of 10.05 g kg−1 and mean TN of 1.10 g kg−1, whereas the subsoil was severely deficient with a mean SOC of 1.96 g kg−1 and TN of 0.66 g kg−1. The integrated fertility index (IFI) indicated that the topsoil and subsoil in Changfeng and western Feixi exhibited higher fertility levels, while Feidong and Hefei had lower fertility levels. An ecological risk assessment identified western Feidong as a high-risk hotpot, with Cd as the primary contributor to potential ecological risk. The source analysis confirmed Ni, As, and Cr as geogenic, Cd as anthropogenic, and Pb and Cu as mixed natural–industrial–agricultural sources. Our findings highlight the necessity of adopting zoned precision fertilization to improve the nutrient efficiency and applying organic amendments to immobilize Cd and reduce the ecological risk. This study provides targeted strategies for soil fertility improvement, precision fertilization, and Cd risk control, supporting sustainable agricultural development.