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.