2.2. Experimental design and sampling method
We selected three predominant land use patterns discussed above (alfalfa
artificial grassland: AG, wheat-maize rotation field: WM, and native
grassland: PC). Each land use pattern was located in three
representative sample sites with relative flat surface and had the same
physiographical units and slope gradients, and established nine 20 × 20
m2 plots in each land use type in spring 2011. In
total, we determined 3 representative sample sites and 9 plots as true
replicated in each land use. The native grassland with more than 30-45 m
width was located beside the reclamation field, respectively, and the
distance between each plot exceeded the spatial correlation of most soil
physicochemical and microbial characteristics (< 13.5 m) (Jiao
et al., 2019). The cultivation history of three land use types and soil
basic physiochemical properties in 0-20 cm depth when plots were
established in spring 2011 were shown in Table 1 and Table 2,
respectively.
A field survey was conducted in mid-June 2020 (after the winter wheat
was harvested). We selected 5 quadrats of 1m×1m in each plot for soil
sample collection according to the “S” shape sampling method, and five
soil sampling drills were located at the center and the four diagonal
corners of each 1m×1m quadrat at three depths (0-20cm, 20-40cm, 40-60cm)
by using a drill (8 cm-diameter). All the soil samples at the same layer
of each quadrat were mixed to create a representative soil sample.
Overall, 81 composite soil samples were collected, representing three
land uses, three depths, and nine replicates. The mixed soil samples
were taken back to the laboratory. After removing roots and stones, the
samples were divided into two parts. One part was stored in the
refrigerator at -80℃ for the enzymatic activity, ammonium nitrogen
(NH4+-N), and nitrate nitrogen
(NO3--N). The other part of soil
samples was screened with 0.15 mm and 2 mm sieves respectively after
dried and crushed for soil properties determined. Soil bulk density (BD)
was collected at the intermediate position of 0-20, 20-40, and 40-60cm
soil layer of each quadrat using a cutting ring with 5.0 cm inner
diameter and 100 cm3 volume. The undisturbed soil was
collected in 0-20, 20-40, and 40-60cm soil layer of each quadrat with
aluminum box for the content of water stable aggregate (WSA).