3.1 | Physicochemical analysis of the soil samples
Soil physicochemical properties that could be directly influenced by the soil depth and forest type are shown in Figure 1 and Tables 1 and 2. There were no significant differences in the five physical indicators evaluated, namely soil moisture content [SMC], pH, electric conductivity [EC], total dissolved solids [TDS], and specific soil weight [GS] in the soil samples from different depths (P> 0.05; Figure 1). In contrast, the slope position and forest type did influence the physical indicators (Figure 1). The EC decreased from the -base to the top at different slope positions, while intercropping caused fluctuations in the soil EC and the TDS.
Preliminary analysis results for the soil chemical parameters are presented in Table 1. The TOC, SOM, TN, TC, NH4–N, and AP content were highest in the 0–20 cm depth layer. The increase in the soil chemical parameter values of the surface soil was likely due to the decomposition and transformation of forest tree litter and biological residues, which are major sources of organic matter accumulation. The slope position did not significantly affect the soil physical and chemical indicators. The TOC, HC, SOM, TN, TC NH4–N, AN, and AP contents in the GLF were significantly higher than for the other stands, indicating that G. jasminoides plays an important role in modulating the fertility of the soil in the C. oleiferalow-yielding forest.
A comparison of the soil nutrient content at different soil depths determined that the NH4–N and AN levels were the lowest in the 40–60 cm depth layer and that the AP levels were highest in the 0–20 cm depth layer (Table 2). The NO3–N and AK levels did not significantly vary by soil layer. No significant differences in NO2–N levels were recorded among the different plantations, but the highest NO3–N levels were observed in LF. Comparing the soil nutrient content from the different forest types revealed that the NH4–N level for the intercropping was significantly lower than that of the monoculture(p < 0.05). It could be due to theG. jasminoides and newly planted C. oleifera substantially increasing plant uptake of tNH4–N. Therefore, the nitrogen supplying capacity of the forest soil became insufficient.