Vario max cn elemental analyzer

The mean annual temperature (MAT) and mean annual precipitation (MAP) at these sampling sites in this study were taken from a global database with a spatial resolution of approximately 1 km² (http://www.worldclim.org) (Wang et al., 2020 (link)). Aridity index data (AI, precipitation/evapotranspiration) were downloaded from Global raster data (CGIAR-CSI, https://cgiarcsi.community) at 30 arcsec resolution. Low value of AI indicates a drier climate.
The 0-10 cm soil layer was sampled using a soil drill with a 6 cm diameter. In each plot, 30 sampling sites were randomly chosen to collect soil and then mixed. Subsequently, after air-drying and removing any pebbles and plant debris, the soil samples were ground to pass a 2-mm sieve. The soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) were measured by elemental analyzer (Vario MAX CN Elemental Analyzer, Elementar, Germany).

All powder samples were oven‐dried at 60°C for 24 hr before analysis of the nutrient concentrations. The total C and N concentrations of the plant samples were determined with an elemental analyzer (Vario MAX CN Elemental Analyzer, Elementar). Before measuring the total P, K, and Ca concentrations, plant samples were digested with HNO₃‐HClO₄. All samples were analyzed for P, K, and Ca with an inductively coupled plasma atomic emission spectrometer (iCAP6300, Thermo Fisher Scientific, Inc.).

Soil samples (150 mg) and plant samples (35 g) were used for total N concentration measurements using an elemental analyzer (Vario MAX CN Elemental Analyzer, Elementar, Germany; Zhao et al., 2016a (link)). In addition, 0.05 g soil samples were soaked in a solution of 6 ml of HNO₃ and 3 ml of HF overnight for P measurements. The solutions were boiled for digestion (Mars X press Microwave Digestion System, CEM Corporation, NC, Matthews, USA), and 0.5 ml of HCLO₄ was added to remove the acid. Finally, the solutions were cooled and the volumes were adjusted to 15 ml for total P measurements (Zhao et al., 2016b (link)). Determinations were conducted using an inductively coupled plasma optical emission spectrometer (Optima 5300 DV, Perkin Elmer, Waltham, MA, USA). After determination of soil total N and P, the ratio of N:P was calculated. Finally, the averages of soil N:P for each site and the average leaf N for each species were calculated.

The total nutrient contents in plant root and leaf samples were determined by an element analyzer (Vario MAX CN Elemental Analyzer, Elementar, Hanau, Germany). The samples were first digested in HNO₃ by using a microwave digestion system (Mars X press Microwave Digestion system, CEM, Matthews, NC, USA). Samples were then analyzed for total nutrient concentrations with an inductively coupled plasma optical emission spectrometer (ICP-OES, Optima 5300 DV, Perkin Elmer, Waltham, WA, USA). The Jaldal Method was used to determine total N content.

The total C and N concentrations of the plant (leaf and fine root), litter, and soil were measured with an elemental analyzer (Vario MAX CN Elemental Analyzer, Elementar, Hanau, Germany). The total P concentrations of plant, litter, and soil were measured using the ammonium molybdate method with a continuous-flow analyzer (AutoAnalyzer 3, Bran Luebbe, Hamburg, Germany), after Se-CuSO₄-K₂SO₄-H₂SO₄, H₂SO₄-H₂O₂, and H₂SO₄-H₂O₂ digestion for soil, litter, and plant samples, respectively.

The area, dry mass, and N and P concentration were measured for each of the sampled leaves. The leaves were scanned using HP Scanjet G3110 (HP), and the pictures were analyzed by using MapInfo12.5 (MapInfo^® software, 2017), and leaf area was recorded. Leaves were oven‐dried to a constant mass at 65°C for 24 hr and then weighed. The average leaf mass and leaf area were calculated as the total mass or area divided by leaf number. Dried leaves were digested by H₂SO₄‐H₂O₂. The total N concentration of leaves was determined by dry combustion using Vario MAX CN Elemental Analyzer (Elementar). Leaf P concentration was measured by the Mo‐Sb colorimetric method using Hitachi U‐3900 (Hitachi).
Leaf N content per unit mass (N_mass, mg/g) was calculated as total N content (mg)/dry weight (g).
Leaf N content per unit area (N_area, mg/ cm²) was calculated as N_mass (mg/g) × dry weight (g)/ area (cm²).
Leaf P content per unit mass (P_mass, mg/g) was calculated as total P content (mg)/dry weight (g).
Leaf P content per unit area (P_area, mg/ cm²) was calculated as P_mass (mg/g) × dry weight (g)/area (cm²).