2.1 Study site
The study site is located at Jingzhou Hill, Maoxian County, in the arid valley of the Minjiang River, Sichuan, China (31°70′N, 103°87′E, altitude 1637 m). The RFC ranges from 1% to 65% (g g-1), and most rock fragments have a particle size larger than 10 mm (Bao et al., 2012). These characteristics served as the basis for the design of the RFC gradient and selection of particle size. The mean annual precipitation at the nearest climate station (Maoxian County Meteorological Station, 2 km from the study site) was 495 mm, with 83% of the precipitation occurring during the growing season from May to October. The mean annual potential evaporation is 1332 mm, and the annual air temperature is 15.6 ℃. The site has historically been cultivated for agricultural crops, with potatoes and celery planted 2 years before the start of the present study. Typically, cinnamon soil has a clay loam with coarse texture and low fertility (Bao et al., 2012). The soil depth is typically 50–70 cm.