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.