4.2 Soil and microbial biomass C, N, and P and C: N: P
stoichiometry
Similar to several previous studies, present soil C, N, and P
concentrations increased with forest chronosequence (Hooker & Compton,
2003; Guo et al., 2021). These changes could be related to the
continuous supply of litter and root exudates that are significantly
influenced by the forest chronosequence. Furthermore, along with forest
chronosequence, soil microbial activity increases, and soil nutrient
concentration and stocks are further increased. Our results revealed
that the biomass of vegetation components and litter increased along
with the forest chronosequence (Table 1), suggesting that the massive
increase in the soil SOC, TN, TP, MBC, MBN, and MBP concentrations
because of greater litter accumulation (Deng et al., 2016). SOC, TN, and
microbial biomass are mainly driven by the higher quantity of plant
residue (rhizodeposition and litter). Nitrogen-rich and fast-decomposingA. nepalensis litter seem to have contributed to soil carbon and
nutrient enrichment (Binkley et al., 1992; Joshi & Garkoti, 2020). Our
results revealed that the improvement in soil nutrient concentrations
and microbial biomass corresponded to increases in above and belowground
biomass following forest age, particularly in the stands with N-fixingA. nepalensis . This suggests that forest chronosequence alters
vegetation biomass, improves the soil microenvironment, and promotes
soil biological activity, especially MBC, MBN, and MBP (Figure 5). Plant
residue (rhizodeposition and litter) accumulation on the soil increases
along the forest chronosequence, which in turn, positively enhances the
soil microbial activity (Prietzel & Bachmann, 2012; Lucas-Borja et al.,
2016). Furthermore, tree biomass and litter of the A. nepalensisgradually increased along with forest chronosequence (Table 1). This
would cause an increased soil and microbial biomass C, N, and P
concentrations because A. nepalensis litter and roots are
decomposed at a faster rate than other species and provide more raw
material for microbial growth (Joshi & Garkoti 2020; Joshi & Garkoti
2021a). Thus our study indicates that A. nepalensis forest
chronosequence enhances the soil microbial biomass and, as a result,
alters soil physicochemical properties (Joshi & Garkoti 2021a).
The significant variations in soil C/P, C/N, and N/P ratios can be
believed to be due to shifts in vegetation structure or function
impacting the amount and nature of litter production and the degree of
degradation of organic matter (McGroddy et al., 2004; Yang and Luo,
2011; Zechmeister-Boltenstern et al., 2015). The significant variations
in soil C: N: P stoichiometry (TN/ TP, SOC/TN, and SOC/TP ratios) can be
believed to be due to shifts in vegetation structure or function
impacting the amount and nature of litter production and the degree of
degradation of organic matter (Zechmeister-Boltenstern et al., 2015).
The SOC/TN ratio is considered to be an indicator of N mineralization
and soil quality. Low SOC/TN ratios show enhanced microbial activity and
increased organic N decomposition, whereas high SOC/TN ratios indicate
the inverse (Manzoni et al., 2008; Manzoni et al., 2010; Cotrufo et al.,
2019). There is a large difference in the SOC/TN ratios among different
forest stands and soil layers. The average SOC/TN ratio of the top layer
(0-30 cm) in all forest stands is the same as the range of previous
findings (8.95 to 10.28) in other studies in the central Himalaya (Kumar
et al., 2021). In this study, deeper soil layers (30-50 cm) of old age
forest stands had a higher SOC/TN ratio due to low input of N and low
mineralization rates of C and N (Bengtsson et al., 2003). The SOC/ TN
ratios revealed substantial variations between forest stands, which may
be attributed to the significant effect of the SOC and TN concentrations
and their transformation. High SOC/TN, SOC/TP, and TN/TP ratios were
found in AYM, AMOM, AMR, and AMOO and varied significantly (P
< 0.05) along with soil depths and forest age in agreement
with the previous reports (McGroddy et al., 2004; Tipping et al., 2016).
Present SOC/TP and TN/TP ratios for 0-30 cm soil depths ranged from
49.25 to 172.48 and 1.16 to 20.40, respectively. These estimated values
of SOC/TP and SOC/TN are greater than the mean value of 41.94 and 4.30,
respectively, reported by Kumar et al. (2021), and are similar to the
range 184-299 and 12.9-19.4, respectively reported by Qi et al., (2020)
in Chinese mountainous ecosystem. SOC, TN, and TP availability were
assumed to be primary driving factors for microbial biomass C, N, and P
content dynamics. Higher microbial biomass (MBC, MBN, and MBP
concentrations) is generally related to higher litter input, vegetation
cover, soil moisture, and soil nutrient concentrations in older forest
stands (Joshi & Garkoti 2021b). Moreover, microbial biomass content
gradually decreases with increased soil depth across all forest stands
(Figure 4 a, b, & c), probably due to a significant reduction in soil
SOC, TN, and TP concentrations availability with increasing soil depth.
The microbial biomass content was positively correlated with soil SOC,
TN, and TP content, indicating that microbial biomass (MBC, MBN, and
MBP) concentrations were limited by soil SOC, TN, and TP concentrations
across the forest chronosequence. The MBC/MBN ratio in the present study
varied from 7.81 to 11.62 across the forest, which is close to the
global average for MBC/MBP ratio (Cleveland & Liptzin, 2007; Aponte et
al., 2010; Hartman & Richardson, 2013; Xu et al., 2013). We found that
the MBC/MBN and MBC/MBP ratios in different forest stands remained
relatively homoeostatic compared to MBN/MBP ratios. Previous research
has illustrated that soil microorganisms have constant C: N: P
stoichiometry homeostasis along with forest chronosequence (Yu et al.,
2010; Xu et al., 2013).
The microbial quotient, i.e., MBC/SOC, MBN/TN, and MBP/TP ratios were
lower in older forest stands (AMR, AMOM, and AMOO stand) compared to the
young stands (AER, ALR, and AYM). With the exception of the AER stand,
the present MBC/SOC ratios (ranging from 1-7 %) are greater than the
values reported for the temperate forests (1.8-2.9 % Vance et al.,
1987b). Moreover, MBN/TN and MBP/TP ratios ranging from 1-5 % are also
greater than the temperate forest soils (1.6-3%; Zhong & Makeschin
2006), suggesting that microbial biomass contribution was high in soil
which indicates microorganisms were found to play a significant role in
nutrient cycling. Also, the MBC/SOC, MBN/TN, and MBP/TP ratiosĀ are the
key indicators of nutrient availability to soil microorganisms (Dilly et
al., 2003).