Sampling and analytical procedures

The experiment began on March 2015 after preparing the experimental device. The soil samples were collected, and the salt ion parameters were measured in June 2015. Based on the simulation experiment and the pertinent literature [16,23], the soil profile sampling interval was designed as follows: each soil layer was 10 cm if the groundwater level was 30–60 cm; each soil layer was 20 cm, including a 0–10 cm surface soil layer, if the groundwater level was 90–120 cm; and if the groundwater level exceeded 120 cm, the soil layer was equal to 30 cm including a 0–10 cm surface soil layer. Three replications were performed for each soil layer.

The soil samples were transported to the laboratory, air-dried for two weeks, evenly mixed and crushed, and then sieved through a 2.0 mm screen. The salinity, pH and electrical conductivity (EC) of the groundwater were measured in situ by a multi-parameter water quality analyser (Horiba U52, Japan), and the water-soluble salt in the soil was extracted according to the forestry industry standards of the People's Republic of China (LY/T 1251–1999). Soil salt anions (Cl^-, SO₄^2-, and NO₃^-) were analysed by ionic chromatography (Dionex IC 2000, America), and cations (K⁺, Na⁺, Ca²⁺, and Mg²⁺) were analysed by flame atomic absorption spectrometry (Shimadzu AA 6800, Japan). During the process of determination, 1% CsNO₃ was added in solution in the process of determination in order to prevent the K⁺ and Na⁺ from ionizing, and 5% LaCl₂ was added to prevent the Ca²⁺ or Mg²⁺ and phosphate producing precipitate. CO₃^2- and HCO₃^- were determined by standard titration with sulfuric acid.