Vario macro cube element analyzer

Soil organic matter (SOM), pH, total nitrogen (TN), available phosphorus (AP), and available potassium (AK) was measured as described in previous study44 . Soil pH was determined with a compound electrode (PE-10, Sartorious, Germany) in a 1:2.5 soil/water ratio solution. SOM was determined by dichromate oxidation, and TN was measured by a vario MACRO cube element analyzer (Elementar Analysensysteme, Germany). The AP was extracted by sodium bicarbonate and then determined following the molybdenum-blue method while AK was measured using a flame atomic absorption spectrophotometer.

A sediment properties analysis with ~10 g of fresh sediment was conducted at the Institute of Mountain Hazards and Environment, Chinese Academy of Sciences (Chengdu, Sichuan, China), according to Wang et al. (48 (link)). In brief, nitrate and ammonia were detected after 1 M HCl treatment followed by analysis with a colorimetric Auto-analyzer 3 (SEAL Analytical GmbH, Norderstedt, Germany). The concentrations of total carbon (TC) and total nitrogen (TN) were determined by overdrying the sediments at 105°C and then using a Vario Macro cube element analyzer (Elementar Analysensysteme GmbH, Langenselbold, Germany). Total phosphate (TP) was measured after digestion of the sediment with nitric-perchloric acid (49 (link)), using the molybdate colorimetric method with a Shimadzu UV-2450 UV-Visible spectrophotometer (Shimadzu Corp., Kyoto, Japan). Analysis of stable C and N isotopes was conducted in the Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences (Nanjing, Jiangsu, China), using the method described by Liu et al. (50 (link)).

Vario MACRO Cube Element Analyzer (Elementar Analysensysteme GmbH, Hanau, Germany) was used to measure total N content. The NH₄⁺ and NO₃^- contents were measured by extracting the soil with 0.01 mol L^-1 CaCl₂ solution (1:10, w/v) for 30 min and then determining with a flow injection autoanalyzer (FLA star 5000 Analyzer, Foss, Denmark). A chlorate inhibition method was used to determine the PNR according to Kurola et al. (2005) (link). Briefly, 5.0 g of fresh soil was placed in 50-mL centrifuge tubes containing 20 mL of phosphate buffer solution (PBS, NaH₂PO₄, 0.2 g L^-1; Na₂HPO₄, 0.2 g L^-1; KCl, 0.2 g L^-1; NaCl, 8.0 g L^-1; pH 7.4) with 1 mmol L^-1 (NH₄)₂SO₄. Potassium chlorate with a final concentration of 10 mmol L^-1 was added to inhibit nitrite oxidation. The suspension was incubated in an incubator at 25°C for 24 h at dark, and nitrite was extracted with 5 mL of 2 mol L^-1 KCl and determined by a spectrophotometer at 540 nm with N-(1-naphthyl) ethylenediamine dihydrochloride.