2 MARERIALS AND METHODS
2.1 Soil samples
Soil samples were collected from Meixian County (107°45’36”E, 34°17’24”N), Shaanxi Province, which belongs to a warm and subhumid continental climate. The average annual precipitation is 610 mm, occurring mainly from July to September, with an average annual temperature of 12.9℃. The study area was a traditional agricultural cropland region located in the southern section of the Loess Plateau in China. The soil was formed from loess parent material (according to the USDA particle size classification criteria) (Fu et al., 2017). The diagonal method was used to collect 30 undisturbed soil samples from the surface (0-5 cm depth) with a ring knife (10 cm diameter × 5 cm height). Bulk density (1.29 ± 0.07 g cm-3) was determined with oven drying at 105°C until a constant mass was obtained. The soil organic matter content (1.33% ± 0.04) was measured by the combustion method. The pipette method was used to obtain the particle size distribution (sand (0.02-2 mm) 45.96% ± 0.12%, silt (0.002-0.02 mm) 41.06% ± 0.03%, clay (<0.002 mm) 12.98% ± 0.05%).