2.3 Field sampling and analysis
Plant and soil samples were collected in August, 2017 after a 17-year period. Aboveground biomass was collected from three 50 × 50 cm subplots by clipping the vegetation. Root biomass was collected from soil cores (8 cm in diameter) in the same subplots at the 0–10, 10–20, 20–30, and 30–40 cm soil layers. Soil bulk density was measured at each soil depth simultaneously with a volumetric steel ring (100 cm3). Soil cores were washed in 2 mm mesh bags to separate roots from the soil. Soil samples were collected from soil cores (5 cm in diameter) of the 0 to 10, 10–20, 20–30, and 30–40 cm soil layers in the same subplot and then mixed into one sample. Each soil sample at 0–10 cm was divided into two sub-samples. One of the sub-samples (approximately 100 g) was brought back to the laboratory and stored at 4 °C to determine dissolved organic carbon (DOC) and labile organic carbon (LOC), and the other sub-samples were air-dried and passed through 10 and 100 mesh sieves to measure pH, soil organic carbon, soil total nitrogen, and phosphorus.
The entire plant biomass was dried for 48 h at 65°C to measure the dry weight. Soil pH was determined for a soil–water ratio of 1:5; SOC was measured using the potassium dichromate oxidation spectrophotometric method; TN concentration was determined using the Kjeldahl method; TP concentration was quantified using the molybdenum antimony anticolorimetric method; LOC content was measured via the potassium permanganate oxidation method; DOC was determined for a soil–water ratio of 1:2.
Soil organic carbon and nitrogen stocks were calculated using the following equations (Gai et al., 2018):
\begin{equation} S_{1}=C\times B\times S\times 10\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (1)\nonumber \\ \end{equation}\begin{equation} S_{2}=\sum_{i=1}^{k}S_{1}\left(k=1,2,3,4\right)\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (2)\nonumber \\ \end{equation}
where S1 is the soil organic carbon and nitrogen storage in the corresponding soil layer (g·m-2), C is the thickness of the soil layer (cm), B is the soil bulk density (g·m-3), S corresponds to the soil organic carbon and nitrogen content (g·kg-1),S 2 is the sum of soil organic carbon and nitrogen storage (g·m-2), and i represents the number of soil layers (i =1, 2, 3, 4).