2.1 Study site and soil sampling
Soil samples were taken from sloped cropland in Baiquan and Keshan Counties, Qiqihar, Heilongjiang province (125°30′E–126°31′E, 47°20′N–47°55′N) (Fig. 1), which are both located in the typical black soil region (Luvic Phaeozem, FAO; Liu et al., 2021). The region has a temperate continental monsoon climate with an annual range from - 4 to 4 ºC. Strong winds are frequent in spring, winters are long and frigid, and summers are short and cool. Annual rainfall averages 500-600mm, concentrated primarily in the summer. We used a Global Positioning System (GPS) to acquire each sampling site’s longitude, latitude, and elevation. We used remotely sensed images to calculate the ridge-slope angle for ridge interpretation and a digital elevation model (DEM; Resolution of 30 m) for generating the slope raster image layer of the sampling site using the ArcGIS 10.2 raster calculator. We interviewed farmers to obtain crop yield and management at each soil sampling site. Therefore, data and information on farmers’ knowledge about the management practices of their fields are composed in the area. A total of 108 soil samples were collected randomly from study sites with similar soil types (black soil), vegetation types (soybean), and cultivation systems (Crop Rotation). This study selected four slope gradients (0º-1.5º, 1.5º-3º, 3º-5º, >5º) with the slope ridge angle between 10º-80º. In each slope gradient, soil samples were collected at three positions (top, middle, and bottom) from the surface layer (0-10 cm). To analyze microbial community characteristics, we consider three slope gradient levels: low (0º-1.5º), medium (3º-5º), and high (>5º). Totally 54 disturbed (mixed of five samples collected in an “S” shape) soil samples (for soil physicochemical and biological properties) and 54 undisturbed soil samples (for soil bulk density and water stable aggregate) were collected. The soil samples were transferred to the laboratory for air-drying and preparation for measurements of soil physicochemical and biological properties.