A total of 240 topsoil samples were collected from the farmland of the YRD region (~45,800 km2
) in June 2014 (Fig. S1). Briefly, at each sampling site, soils were collected from five different cores (0e15 cm depth) which were mixed and packed into a single kraft bag. Please refer to (Sun et al., 2016) for the sampling details. In the laboratory, the collected soil samples were air-dried, ground, and sieved (2 mm mesh) to remove large debris, stones, and pebbles,and finally well mixed. A portion of each soil sample was used to measure soil pH. The remaining soil was passed through a nylon sieve of 100 meshes (0.154 mm) for analyzing the content of soil organic matter (SOM) and heavy metals