2.3 | Soil sampling and analysis
After maize harvest, twelve soil samples were randomly collected from the topsoil layer (0–20 cm) in each plot in October every year. The fresh soil samples from each plot were mixed as a composite sample, air-dried, finely ground, and sieved to pass through a 0.15 mm mesh. The SOC was determined using K2Cr2O7–H2SO4digestion method (Bao, 2000), and the measurement of SOC stocks was based on a constant soil mass (Xu et al., 2019).