2.3.1 Soil sampling and analysis
Soils were sampled in two different years. Soil nutrient data (0-20 cm)
in 1980 were obtained from the Second National Soil Survey. The number
of sampling data points for total N, available P, and available K are
350, 367, and 112 in Wugong County and 412, 407, and 210 in Meixian
County, respectively. The soil nutrient data for Wugong County was
currently obtained from research studies with others (Li et al., 2016;
Qi et al., 2014). These soil samples were collected in 2018 and 2019 in
Meixian County, consisting of 0-2 m profiles from 10 cereal fields and
38 kiwi orchards. In addition, 22 0-5 m profiles (20 cm intervals) from
orchards were collected in October 2018. For soil sampling from each
field, three sites were randomly selected with relatively flat terraces,
whereas for sloping land, the three sites were selected from the upper,
central, and bottom parts of the field (Lu et al., 2016). Their average
value of three sites for each field was used to represent the sampling
site. The triplicate samples were mixed to obtain a composite sample
from each soil depth and placed in labeled plastic bags, sealed, and
stored immediately at 4 ℃. In addition, 79 soil samples in Meixian
County were collected at 0-20 cm to determine available P, available K,
and total N.
The soil moisture was determined with the oven-drying method at
105◦C. Soil samples were extracted with 1M KCl (1:10
soil: solution), shaken for 1 h, and then filtered. The nitrate-N
concentrations in the KCl extract samples were determined by a
continuous flow N analyzer (Bran and Luebbe AA3, Norderstedt, Germany).
Soil was air-dried, ground, and passed through 1 mm and 0.15 mm sieves
to measure soil organic matter (SOM), total N (TN), available P and
available K. SOM was measured with the
K2Cr2O7 oxidation
volumetric method, TN by the Kjeldahl method, available P (Olsen-P) was
extracted with 0.5 M NaHCO3, and available K was
extracted with 1M NH4OAC.