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).