2.3. C, N, and P concentrations, ratios, and N and P stocks
The C, N, and P concentrations were estimated for the different aboveground and belowground components (i.e., branch, bole, twigs, foliage, fine root, coarse root, and stump root) of the A. nepalensis , Q. leucotrichophora , R. arboreum , and associated species trees (P. pashia , L. ovalifolia ,M. esculenta , Q. floribunda , L. umbrosa , A. indica , J. regia and B. capitata ). In each forest stand, five representative trees of A. nepalensis , Q. leucotrichophora , R. arboreum , and associated species were chosen for the estimation of the tree aboveground and belowground biomass, and nutrient concentrations. From each marked represented tree, mature twigs and fully expanded well illuminated leaves from the middle of the canopy were collected. Five tree ring cores (one from each selected tree) were used to extract bole sampling across the diameter. Tree roots (stump root, coarse root, and fine root) were collected from five soil profiles ranging from 0 to 100 cm and categorized as fine roots (less than 2 mm), coarse roots (more than 2 to 10 mm), and stump roots (greater than 10 mm) (Rawat & Singh, 1988). All samples were brought to the laboratory for chemical analysis. Tree, shrub, and herbaceous components were washed by deionized water and air-dried to constant mass, ground, and sieved through a 0.5 mm screen mesh and kept for chemical analysis.
The vegetation biomass (tree, shrubs, and herbaceous components) and soil C concentrations were determined using the K2Cr2O7 oxidation method (Nelson & Sommers, 1982). Total N was analyzed with the micro-Kjeldahl digestion method (Parkinson & Allen, 1975). Plant and soil samples were digested with H2SO4-H2O2and H2SO4-HClO4separately for total P analysis, and P concentrations were measured using molybdenum antimony colorimetry assays (Parkinson & Allen, 1975). The soil microbial biomass (i.e., MBC, MBN, and MBP) was analyzed by ethanol-free chloroform fumigation-extraction methods (Brookes et al., 1985; Wu et al., 1990). The C, N, and P concentrations of the tree, shrubs, herbaceous components, and soil were defined as g kg-1 in dry mass. The stoichiometric ratios (C:N, C:P, and N:P) in the tree, shrub, herbaceous components, and soil were measured using the following formula:
\begin{equation} C:\ N=\frac{C_{\text{concentration}}}{P_{\text{concentration}}}\nonumber \\ \end{equation}\begin{equation} N:\ P=\frac{N_{\text{concentration}}}{P_{\text{concentration}}}\nonumber \\ \end{equation}\begin{equation} C:\ P=\frac{C_{\text{concentration}}}{P_{\text{concentration}}}\nonumber \\ \end{equation}\begin{equation} \ \nonumber \\ \end{equation}
To evaluate the microbial efficiency of SOC, TN, and TP concentrations, the soil microbial quotients MBC:SOC, MBN:TN, and MBP:TP were calculated (Fan et al., 2010). In each plot, N and P stocks in different vegetation components were obtained by multiplying the N and P concentrations of vegetation components by the total biomass of that component. The vegetation components N and P stock were calculated following Tang et al. (2018).
The soil TN and TP stocks for each soil profile were estimated using the following formula:
\begin{equation} \text{Soil\ stock\ }\left(\text{kg\ ha}^{1}\right)=\ \ \frac{\ TN\ or\ TP\ concentrations\ (g\ kg\ )\ \ \ \times\ soil\ depth\ (cm)\ \ \times bulk\ density\ (g\ cm)\ \times\ 1000}{10}\text{\ \ }\nonumber \\ \end{equation}
Soil microbial biomass (MBC, MBP, and MBP) stock were calculated using the equation below:
\begin{equation} \text{Soil\ microbial\ biomass\ stock\ }\left(\text{kg\ ha}^{1}\right)=\ \frac{\ soil\ microbial\ biomass\ (MBC\ or\ MBN\ or\ MBP)\ \ concentrations\ (\mu\text{g\ g}\ )\ \ \times\ soil\ depth\ (cm)\times\ \ bulk\ density\ (g\ cm)\ \ }{10}\text{\ \ \ \ \ \ \ \ \ }\nonumber \\ \end{equation}
The total ecosystem N and P stock of forest stand were calculated by aggregation of the N and P stock of various components of the ecosystem.