3.2. Soil and microbial biomass C, N, and P concentrations and stoichiometry
Results of the one-way ANOVA analysis revealed that forest chronosequence significantly (P < 0.05) influenced the soil SOC, TN, and TP concentrations and varied significantly with soil depth (P < 0.05). Generally, the SOC, TN, and TP concentrations in the various soil layers increased significantly along with forest chronosequence. Across the forest stands, the highest soil SOC, TN, and TP concentrations were found in the 0-10 cm soil depth, and the lowest was found in the 30-50 cm soil depth (Figure 2). Forest stand, soil depth, and their interaction significantly influenced soil microbial biomass concentrations (MBC, MBN, and MBP) (Figure 3). In the 0-10 cm soil depth, the AOOM stand soil microbial biomass concentrations were significantly greater than the other forest stands (Figure 3). The soil microbial biomass concentrations decreased with an increase in the soil depths in all the forest stands. The ANOVA values for the microbial stoichiometric ratios showed that all variables exhibited a highly significant interaction between forest types and soil depth (Figure 3). The estimated soil C:N:P ratios ranged from 71:12:1 to 269:25:1 and were significantly higher than microbial biomass C:N:P ratios which range from 10:2:1 to 22:2:1 (Table 3). The MBC:MBP and MBC:MBN ratios increased with soil depths and reached the maximum at 20-30 cm or 30-50 cm soil depths. Meanwhile, MBN:MBP ratio did not vary with forest types and soil depth (Figure 3). The microbial quotient, i.e., MBC:SOC ratio, MBN:TN ratio, and MBP:TP ratio decreased with forest chronosequence and increased with soil depths. The overall microbial quotient was higher in early young-aged stands (AER, ALR, and AYM stand) compared to older stands (AMOM, AMR, and AOOM stand).
Along the A. nepalensis chronosequence, SOC, TN, and TP concentrations and stoichiometry showed significant relationships with microbial biomass C, N, and P concentrations and stoichiometry. SOC, TN and TP were significantly related MBC (R2 = 0.70, P < 0.0001), MBN (R2 = 0.81, P < 0.0001), and MBP (R2 = 0.53, P < 0.0001), respectively, and soil SOC:TN ratios were positively correlated with the MBC:MBN ratios (R2 = 0.11, P < 0.001) while TN:TP and SOC:TP ratios were not related with MBN:MBP and MBC:MBP ratios, respectively (Figure 4). As shown in Figure 5, soil and microbial C, N, and P stocks varied considerably across forest chronosequence and soil depths. Across forest types, soil and microbial C, N, and P stocks showed decreasing trends with soil depths. Across all three soil depths, the highest soil and microbial C, N, and P stocks were noticed at the AOOM stand and the lowest at the AER stand.