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%).