A B S T R A C T
In the mid and high elevations of the central Himalaya, Nepalese alder (Alnus nepalensis D. Don) occurs in areas affected by landslide/slip and is a nitrogen-fixing species; it quickly improves soil physical and chemical properties and facilitates the restoration of degraded forests. In the present study, we evaluated the effect ofA. nepalensis forest chronosequence on the carbon (C), nitrogen (N), and phosphorus (P) concentrations, N and P stocks, and stoichiometry in the soil, including microbial biomass C (MBC), microbial biomass N (MBN) and microbial biomass P (MBP), and in the plant components. Six naturally occurring forest stands were identified in a chronosequence of A. nepalensis (3–270 years old) forest stands, namely alder-early regenerating (AER), alder-late regenerating (ALR), alder young-mixed (AYM), alder mature-oak (Quercus leucotrichophora ) mixed (AMOM), alder mature-rhododendron (Rhododendron arboreum ) mixed (AMR), and alder old-oak (Q. leucotrichophora ) mixed (AOOM) forests. The biomass of tree components was estimated using species-specific allometric equations developed by previous workers for the region. Soil total N and total P stocks of each species were determined by the N and P concentrations and bulk density. Structural equation modeling (SEM) was performed to quantify the contribution of N and P pools to ecosystem nitrogen and phosphorus stock. The results of this study revealed that the stoichiometry (C/N, N/P, and C/P ratios) of tree components, i.e., leaves, litter, twig, and soil and microbial biomass varied widely, and the presence of nitrogen-fixing A. nepalensis in different succession stages significantly improved the soil and microbial biomass stoichiometry. Total vegetation (tree, herbs, shrubs, and litter) biomass N stock ranged from 346.77 to 4662.06 kg N ha-1, and soil N stock varied from 816.48 to 7334.24 kg N ha-1. Total ecosystem N and P stocks were ranged from 1163.26 to 11996.31 kg N ha-1 and 76.10 to 799.28 kg P ha-1, respectively, and positively increased with A. nepalensis total biomass. The soil P stock accounting 63.49 to 74.80% of the total P stocks of the forest ecosystems. Overall, our findings suggest thatA. nepalensis forest chronosequence enhanced the N and P stock, and introducing this species in degraded forests appears to be an option for enhancing forest conservation and rehabilitation actions in central Himalaya.
Keywords: Succession; Nitrogen-fixing species; Biomass; Plant-soil nutrient concentrations; Microbial biomass.
Introduction
Nitrogen-fixing early successional plant species are typically the first to colonize degraded habitats, which fast restore the soil’s physical and chemical structure and generally facilitate the establishment of other tree species (Batterman et al., 2013; Menge & Chazdon 2016). The facilitative influence of nitrogen-fixing pioneer trees on forest regrowth and biomass regeneration on nitrogen-limited soil is one of the important consistent influences observed in forest ecosystems (Walker and de Moral, 2003; Callaway, 2007; Bonanomi et al., 2011). This cohesion indicates that nitrogen-fixing pioneer trees function as nurse and keystone species (Power et al. 1996). Nitrogen-fixing trees also play a significant role in the C, N, and P cycles and accelerate cycles of Ca and other rock-derived nutrients in the forests  (Menge et al. 2019; Perakis & Pett-Ridge 2019; Joshi and Garkoti 2021b; Pereira et al., 2021). Therefore, changes in the cycles of these elements influence the soil-plant nutrient limitations and may result in altered stoichiometry in the plant-litter-soil system.
The stoichiometry of young ecosystems is controlled by N supply; meanwhile, old ecosystems are governed mainly by the limitation of P supply (Elser et al., 2007; Richardson et al., 2004). Nevertheless, the correlation between soil N and P and the direction of stoichiometry dynamics during successional stages remains controversial (Hooker & Compton, 2003; Yang & Luo, 2011; Yang et al., 2021). It is expected that ecosystem stoichiometry will alter due to changes in biomass production, species composition, and soil properties induced by forest succession (Hooker & Compton, 2003; Yang & Luo, 2011; Ouyang et al., 2017; Yang et al., 2021). Because rapidly growing nitrogen-fixing early successional species and slow-growing late succession species can have substantial variations in C, N, and P concentrations in the above and belowground plant components, stoichiometry and the coupling between C, N, and P in ecosystem components change when species composition changes during forest succession (Hooker, & Compton 2003; McGroddy et al., 2004). Previous studies have analyzed the soil and plant C:N:P stoichiometry in the forest of the central Himalaya (Rawat et al., 2020a; Kumar et al., 2021; Kumar and Garkoti, 2022). Moreover, information on the relationship of C:N:P stoichiometry between soil, plant, and litter and the influence of forest chronosequence on ecosystem C:N:P stoichiometry and nutrient stocks have rarely been evaluated in the central Himalaya forests.
Himalayan forests are subject to many drivers of change, including forest landslide/land slips (due to the natural settings and slopes), forest fire, agricultural expansions, and climate change, which alter both forest and soil health (i.e., structure, productivity, nutrient cycling, and forest successional processes) (Singh 2007; Måren et al., 2014; Verma et al., 2021; Negi 2022). In the central Himalayan mid and high altitudes, forest degradation and soil erosion due to landslide/ slip have been common features (Pandey et al., 2020; Joshi and Garkoti, 2021b). White oak (Q. leucotrichophora ) is a major forest-forming, late-successional, and keystone tree species in the central Himalayas. It covers approximately a 20,000 km2 area, and in many sites, it is subjected to natural and anthropogenic disturbances (Pandey et al., 2020; Dhyani et al., 2020). The natural rehabilitation of Q. leucotrichophoradegraded forest becomes an essential priority for the future forest restoration of the central Himalayan region (Dhyani et al. 2020). Nitrogen-fixing A. nepalensis is a fast-growing early successional tree species that often forms pure stands in areas affected by landslide/ slip sites over 1400 m.a.s.l. but also occurs mixed with other late-successional species in the central Himalaya. In addition, being a nurse species, it enhances the ecosystem C, soil C, soil N, improves the physical and chemical properties of the soil, restores the habitat, and facilitates forest succession (Joshi and Garkoti 2021a; Joshi and Garkoti 2021b).
This study investigated the C:N:P stoichiometry in soil and plant components and ecosystem N and P stocks with respect to a chronosequence of A. nepalensis forest stands in the central Himalaya. The study also evaluated the influence of A. nepalensis chronosequence on soil and microbial biomass stoichiometry. We hypothesized that 1) plant-soil C:N:P stoichiometry and ecosystem N and P stocks change with respect to chronosequence of A. nepalensis forest stand 2) different plant components may show different C:N:P stoichiometry. The most active component, i.e., leaves, fine roots, and twigs, has a higher nutrient content than other components. 3) since A. nepalensis is a nitrogen-fixing tree, it reveals the change of soil and microbial biomass C:N:P stoichiometry along with the soil depth.