4.3. Distribution of N-P stocks in the different ecosystems
Forest stands varied significantly in the distribution of N and P stocks in different vegetation components and soil layers. The forest structure, vegetation biomass, and nutrient concentrations determined the N and P stock in the vegetative components and different soil layers (Frédéric et al., 2010; Phoenix et al., 2012; Joshi and Garkoti, 2021 b). Present vegetation biomass P (19.19 to 233.91 kg ha-1) and N (346.77 to 4662.02 kg ha-1 ) stocks are higher than the values reported (60.5 kg ha-1 for P and 577 kg ha-1for N) by Zhang et al. (2018). Late succession forest stands have higher vegetation biomass and nutrient concentrations and greater litter biomass, which may enhance the forest nutrient dynamics (Joshi & Garkoti 2021b). The high range of C/P and N/P ratio and the low P stock in the soil in young stands (AER, ALR, and AYM) may affect the enzymatic and microbial activities that decompose organic matter, which may, in turn, limit the microbial activities and forest growth during early stages of chronosequence (Richardson et al. 2004). The contribution of soil N and P to the ecosystem N and P stocks ranged from 70.18-48.82 % and 63.49-74.80 %, respectively, which indicate that soil TN and TP stocks (including soil microbial biomass stocks) play a key role in ecosystem N and P stock. Our results reveal the progressive increment in vegetation and soil nitrogen and phosphorus stock along the forest chronosequence. The order of the different N and P stocks in our study sites follows the order: soil > trees > shrubs > herb > litter (Figure 6). Hence, higher litter amount, soil nutrients, and soil microbial biomass (e.g., Deiss et al., 2018) may improve ecosystem N and P stocks. Finally, our findings indicated that there was an interaction between N and P stocks in the plant-soil system.