Black soils, Brown soils, and Desert soils, three typical soil types of China, were sampled from three farmland zones along distinct climatic gradients (Table 1 ). Surface soil samples (0–20 cm) were collected from a fallow plot without plants to minizine the influence of crop growing. Three sampling plots were collected from each farmland zone. Five random sites were sampled from each plot, and about 2 kg of soil was homogenized as one soil sample after picking out large stones, plant litter, and animal debris. The collected soil samples were screened (~2 mm) and then divided into two subsamples: One subsample was stored at ~4°C for microbial analysis in the next 2 weeks, and the other one was air-dried for the analysis of soil properties. Soil type and the selected soil physicochemical properties are listed in Table 1 .
Sulfadiazine (AR, ≥ 98.6%) was purchased from Shanghai Yuanye Biological Technology Co. Ltd. (Shanghai, China) and prepared in a stock solution (40 mmol/l) with ultra-pure water after being dissolved by methanol (HPLC grade). A total of 1,000 mg/l of Cu2+ stock solution was prepared with CuSO4 in ultra-pure water. The reagents, except methanol, are all of the analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
Sulfadiazine (AR, ≥ 98.6%) was purchased from Shanghai Yuanye Biological Technology Co. Ltd. (Shanghai, China) and prepared in a stock solution (40 mmol/l) with ultra-pure water after being dissolved by methanol (HPLC grade). A total of 1,000 mg/l of Cu2+ stock solution was prepared with CuSO4 in ultra-pure water. The reagents, except methanol, are all of the analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).