E20 fiveeasy ph meter

The environmental parameters were geographic variables (latitude, longitude, and elevation) and meteorological data [mean annual precipitation (MAP) and mean annual temperature (MAT)] at each sampling site and soil properties [pH, electrical conductivity (EC), total carbon (TC), total nitrogen (TN), and the TC/TN ratio] for each sample. The meteorological data (MAP and MAT) were collected from the Chinese Meteorological Agency Database¹. Soil pH was measured by using an E20-FiveEasy pH meter (Mettler Toledo, Giessen, Germany), and the soil EC was calculated by using an electric conductometer. Both soil measurements used a soil water suspension (a 5:1 vol/vol mixture of deionized water and soil) after shaking for 30 min. Soil TC and TN were determined with a carbon–hydrogen–nitrogen elemental analyzer (2400 II CHN Elemental Analyzer; PerkinElmer, Boston, MA, United States). Meteorological data and soil properties are shown in Supplementary Tables 1, 2, respectively.

Soil pH was measured by using an E20-FiveEasy pH meter (Mettler Toledo, Germany), and soil electrical conductivity, the indicator of soil soluble salt, was determined by using an electric conductometer. Both soil measurements were made using a soil-water suspension (5:1 mixture of deionized water-fresh soil) after shaking for 30 min. Soil moisture was determined gravimetrically at ∼105°C for 6 h. Dissolved total nitrogen (DTN), nitrate (NO₃⁻-N), and ammonium (NH₄⁺-N) were extracted by adding 5 g fresh soil to 50 ml of a 2 M KCl solution, and dissolved organic carbon (DOC) was extracted with 50 ml of deionized water. After shaking for 1 h and standing for 1 h, the supernatant was filtered through glass fiber filters (Fisher G4, 1.2-μm pore space). The concentrations of NO₃⁻-N, NH₄⁺-N, and DTN were determined using a continuous-flow analytical system (San⁺⁺ system; Skalar, Holland). DOC was determined by using a carbon nitrogen analyzer (Multi N/C 3000; Analytik Jena, Germany). Available phosphorus (P) was extracted with a 0.5 M NaHCO₃ solution and measured by the Mo-Sb colorimetric method. Available potassium (K) was extracted with 1 M ammonium acetate (NH₄OAc) and measured by the flame spectrophotometry method. Soil organic matter (SOM) was measured by the K₂Cr₂O₇-H₂SO₄ oxidation method, and total nitrogen (TN) was measured by the Kjeldahl method.