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 105C. 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.