Pt 21

The SOC was measured by the Tinsley method (heated dichromate/ titration method). The total-N was determined by the Semimicro-Kjeldahl method. The amount of alkaline-hydrolyzable N (AN) was determined by the alkaline hydrolyzed diffusion method. Total P and K amounts were measured by the NaOH melt method. The available P (AP) was extracted with 0.05 N HCl–0.025 N H₂SO₄ solution and reacted with a solution of l-ascorbic acid and H₂SO₄–(NH₄)₆Mo₇O₂₄ methods. The available K (AK) was determined using flame photometry. All the above-mentioned methods were adapted from Bao⁷¹ and Wang, et al.^{50 (link)}.
The pH of soil solution (1 g soil in 5 mL deionized water) was measured using a pH-meter (Sartorius PT-21; Shanghai, China). The same soil solution was used for determining soil EC using an EC meter (DDS-307; Shanghai Precision Scientific Instruments Co., Ltd., China). Both pH and EC measurement were determined at the same time. Soil gravimetric water was calculated as follows: [(Fresh weight − Air-dried weight) / Dry weight] × 100%. Soil bulk density was calculated as follows: Air-dried soil mass/soil volume of 400 cm^{350 (link)}.

The total nitrogen and carbon contents of the sediment were examined using the Vario Max CNS elemental analyzer (Elementar Analysensysteme GmbH, Hanau, Germany). The pH was determined using a pH meter (Sartorius PT-21, Shanghai, China) and a soil-to-ddH₂O ratio of 1:2.5. Using a flow injection autoanalyzer, nitrate ( ${\text{NO}}_3^{-}$ -N) and ammonium ( ${\text{NH}}_4^{+}$ -N) levels in sediment and water samples were determined (Westco Module 200, Smart Chem, France). Using an auto analyzer, the total phosphorus in sediment and water samples were also analyzed (AA3, Norderstedt, Germany). To evaluate the sediment moisture, 15 g of recent sediment was dehydrated in a 105 °C oven for 48 h. A TOC analyzer analyzed the total soluble organic carbon (TOC) in water samples (vario TOC cube, Elementar, Germany).

Sample preparation details and some of the soil parameters analysis (e.g., bulk density, soil moisture, SOC, total N, available N, total K, available K, total P, available P, and soil texture) have been described previously by Wu et al. (2018 (link)). Soil porosity was calculated by the following formula: soil porosity = (1−bulk density/specific gravity) × 100%. The pH of the soil solution (one-part soil to five-parts water) was measured with an acidity meter (Sartorius PT-21, Shanghai, China). Soil electrical conductivity (EC) was determined with an EC meter (DDS-307, Shanghai Precision Scientific Instruments Co., Ltd., Shanghai, China) (Bao, 2000 ). Soil carbon or nutrient storage were computed as:
Farmland soil carbon or nutrient storage = α_f × ρ_f × 0.2 × (1–V_gravel)
Poplar soil carbon or nutrient storage = α_p × ρ_p × 0.2 × ρ_f/ρ_p × (1–V_gravel)
where, α_f and α_p are the concentrations of farmland and poplar SOC (g kg⁻¹); ρ_f and ρ_p are farmland and poplar soil bulk densities (Mg m⁻³), respectively; 0.2 is the soil thickness (0.2 m); and V_gravel is the proportion of gravel. Details regarding the bulk density correction can be found in Wuest (2009 (link)) and Wu et al. (2018 (link)).