In terrestrial ecosystems, forest soils, as the largest carbon (C) reservoir, play a pivotal role in C cycling (Nottingham et al., 2020) and can potentially affect nitrogen (N) dynamics (Ndossi et al., 2020). Many studies have indicated that geography, soil, and litter properties (soil pH, nutrient content, and water content) and aboveground biotic factors (stand composition and diversity) are important predictors of soil C and N stocking (Delgado-Baquerizo et al., 2017; Walker et al., 2018). In addition, altitude has long been considered a potential driver of soil nutrient turnover through its influence on the biotic and abiotic components of forest ecosystems (Gebrewahid et al., 2018). Although altitude may not have a direct impact on soil organic matter (SOM) turnover, it can still influence this process indirectly by shaping soil, climatic, and vegetation conditions (Sierra and Causeret, 2018; Devi and Sherpa, 2019). Climatic conditions (i.e., decreased temperature and humidity) are heavily influenced by altitude, which in turn affects vegetation distribution/composition and alters the quality and quantity of litter/soil characteristics and C loss (Kotas et al., 2018; Quan et al., 2019). Recent studies have documented how changes in climatic conditions and forest canopy composition associated with altitude can affect the litter decomposition rate (Cardelli et al., 2019), soil microbial/enzyme activities (Feng et al., 2019; D’Alò et al., 2021), and nutrient storage in different ways (Bello et al., 2015).

As the main source of SOM, tree species composition and diversity along an altitudinal gradient may have a strong impact on C and N cycling (Gebrewahid et al., 2018; Sierra and Causeret, 2018). Despite the importance of soil nutrient cycling in the characterization of soil biogeochemistry under varying forest ecosystems (Bello et al., 2015; Frouz, 2018), the nature of soil nutrient reservoirs and their turnover in different regions are largely unknown. SOM decomposition involves both chemical and biological processes, including enzymatic catalysis (Sierra and Causeret, 2018; Nottingham et al., 2019), which generally decrease with increasing altitude, resulting in slower decomposition of organic matter (OM) (D’Alò et al., 2021).

Soil extracellular enzyme activities are essential for C and N cycling/stocking and litter decomposition and thus for microbial life (Baldrian and Štursová, 2011; Šnajdr et al., 2013; D’Alò et al., 2021). They also play an essential role in soil fertility as useful indicators for soil management (Baldrian et al., 2010; Cardelli et al., 2019). Changes in tree species composition along an altitudinal gradient alter soil microbial activity and thus the activity of soil enzymes (Feng et al., 2019; Ndossi et al., 2020). In fact, enzyme activity is sensitive to changes in vegetation and is closely related to soil primary quality, and health (Šnajdr et al., 2013; Nottingham et al., 2019). Soil nutrients (Cardelli et al., 2019), forest stands (Šnajdr et al., 2013), and climate (Walker et al., 2018) are factors affecting extracellular enzyme activity along altitudinal gradients. Recent studies have shown that the activity of C- and N-dependent enzymes has different trends with increasing altitude (Ndossi et al., 2020; D’Alò et al., 2021). The effects of altitude on soil extracellular enzyme activities are highly variable across individual studies (Lladó et al., 2017), since they do not follow simple biochemical rules (Baldrian et al., 2013). It is widely assumed that extracellular enzyme activity increases with increase in temperature, but in the forest altitudinal gradient their activity is more related to the quality of available OM, vegetation, and nutrient demands of the microbial biomass (Nottingham et al., 2019; D’Alò et al., 2021), which can have different trends with altitude (Lladó et al., 2017; Ndossi et al., 2020). Meng et al. (2020) reported that the higher C content of the soils at the higher altitude may denote a bigger labile C availability for C-dependent extracellular enzymes.

Moreover, knowledge about the function and activity of soil microorganisms along altitude is essential for understanding forest soil productivity (Massaccesi et al., 2020), as soil microbes play a fundamental role in biogeochemical cycling (Walker et al., 2018). Soil microbes play a pivotal role in the mineralization and decomposition of SOM by producing various hydrolytic enzymes (Cardelli et al., 2019). In particular, basal respiration (BR), substrate-induced respiration (SIR), microbial biomass C (MBC), microbial biomass N (MBN), and metabolic and microbial quotients as microbial indicators change with forest vegetation along the altitudinal gradient, while soil organic C (SOC) is often less affected (Walker et al., 2018; Massaccesi et al., 2020). In most cases, MBC and MBN showed different trends with changes of forest types along the altitudinal gradient (Massaccesi et al., 2020; Ndossi et al., 2020). Climatic conditions, vegetation, and edaphic factors also contribute to changes in substrate availabilities and microbial activities (Walker et al., 2018; Ndossi et al., 2020). Trees, as the primary source of plant biomass, affect microbial processes in ecosystems via specific litter chemistry and rhizodeposition (Šnajdr et al., 2013). Moreover, soil microbial activities are directly linked to C and N turnover/stock (Urbanová et al., 2015; Razanamalala et al., 2018) and mineralization of OM (Walker et al., 2018; Nottingham et al., 2019).

Soil fauna represents another important driver of SOM dynamics (García-Palacios et al., 2013; Frouz, 2018). Soil macro- and meso-organisms play key roles in nutrient cycling and SOM decomposition through their own metabolism, stimulation of microbial activity, and bioturbation (Frouz, 2018). Soil fauna is known as the engineer of the soil ecosystem that interacts with the soil microbiome and influences litter decomposition and nutrient cycling (García-Palacios et al., 2013). Climate directly alters litter decay because of the sensitivity of processes mediated by soil fauna to factors such as temperature or precipitation (García-Palacios et al., 2013). Recent studies have identified environmental factors (i.e., temperature and moisture) (García-Palacios et al., 2013), stand composition (Massaccesi et al., 2020), and soil and litter properties (Bayranvand et al., 2017b) as the main factors influencing the activity of soil fauna and flora in natural forests along altitudinal gradients (Bayranvand et al., 2021b).

Hyrcanian forests are located south of the Caspian Sea in northern Iran and are covered by temperate broadleaf forests (Bayranvand et al., 2017b), which are similar to vegetation in parts of Europe, northern Turkey, and the Caucasus (Talebi et al., 2013). Forest stand composition and climate change with altitude (Talebi et al., 2013), which in turn affect soil microbial and enzyme activities, and nutrient turnover (Gebrewahid et al., 2018; Banday et al., 2019). The aim of the present study was to investigate (i) how a shift in forest stand composition along an altitudinal gradient affects litter and soil chemical properties and soil C and N cycling; (ii) how biological (earthworm, nematode, protozoa, and fine-root biomass), microbial (BR, SIR, and microbial biomass C and N), and enzymatic activity patterns change along an elevation gradient; and (iii) which factors determine C and N stocks across transects. We hypothesized that (i) the C stock increases with altitude due to decreased litter decomposition and quality of beech forests and a low soil temperature (ST); (ii) the N stock is greater at low altitudes where tree diversity, pH, and litter quality are high; and (iii) the soil microbial respiration, MBC, and C-dependent enzymes are associated with higher soil and litter C with increasing altitude where beech forest are dominant, while the opposite is assumed for the N stock.

The tree composition of forests covering the northern part of the Alborz Mountains changes with altitude (Talebi et al., 2013), a condition that is associated with altered soil C and nutrient stocks. The high C stocks at high altitudes can be due to the continuous accumulation of leaf litter and slower decomposition because of the lower temperatures at the higher elevation levels (Banday et al., 2019). The dominance of F. orientalis Lipsky at high elevations may likely contribute to C sequestration, resulting in a higher litter C/N ratio and higher soil C compared with other deciduous forest stands at lower altitudes (Talebi et al., 2013; Kooch and Bayranvand, 2017). Compared with other tree species found at low altitudes, the beech forest floor is more recalcitrant to decomposition, which could be due to its lower nutrient contents (Delgado-Baquerizo et al., 2017), higher C/N ratio (Bayranvand et al., 2017a), and high lignin content (Kooch and Bayranvand, 2017). The differences in soil C stocks in the two forest types thus originate from their different vegetation types, as the quantity, and quality of litter are crucial drivers that affect the soil microbial biomass and soil pH (Banday et al., 2019; Massaccesi et al., 2020). Low temperatures at higher altitudes are another factor contributing to the higher C stock, as in cooler climates, soils have higher OM due to a slow mineralization rate and a high forest-floor thickness (Gebrewahid et al., 2018; Banday et al., 2019). We found that plain mixed forest with species of P. persica, Q. castaneifolia, and C. betulus at low altitudes had higher litter quality (high N and low C/N), higher soil fertilities and N stocks than beech forest stands at higher altitudes. Krashevska et al. (2017) noted that a high initial litter N concentration and low C/N ratio as well as low lignin and high holo-cellulose concentrations are associated with high litter quality and promote rapid decomposition (Šnajdr et al., 2013).

Previous analyses by Šnajdr et al. (2013) and Feng et al. (2019) suggested that tree species determine the quality and quantity of resources available for soil microorganisms and enzyme functions. Our analyses revealed that higher enzyme activities are typical of higher altitudes under beech forests with high litter thickness, litter, and soil C and C/N ratios under appropriate SM. Šnajdr et al. (2008) studied the relationship between enzyme production and litter decomposition by fungal communities under different forest covers and noted that the decomposition of litter components, such as cellulose and lignin, is often attributed to cellulase enzyme activities. In agreement, enzymes, such as laccase, endoglucanase, β-glucosidase, and β-xylosidase, were reported to be produced in Fagus sylvatica forest by Lepista nuda fungi. Mn-peroxidase and laccase contribute to lignin degradation, which results in both depolymerization and partial mineralization (Valášková et al., 2007). In the present study, the activities of β-glucosidase, acid phosphatase, N-acetylglucosaminidase, arylsulfatase, and laccase were higher at higher elevations (1,000 to 1,500 m) in beech forests where litter thickness, C and C/N ratios, fine-root biomass, and SM were significantly higher (Banday et al., 2019). Increased C-dependent enzyme activity is likely due to a higher availability of aboveground litterfall (with higher C) and fine-root necromass to soil microorganisms (Feng et al., 2019). In addition to a lower decomposition rate, litter quality also decreased with altitude (Marian et al., 2019).

Increased soil enzyme activities could be associated with higher C accumulation since soil microbial biomass and heterotrophic respiration increase with plant density (Feng et al., 2019). Kang et al. (2009) also noted that higher activities of acid phosphatase and oxidase were dependent on the soil C content. In previous studies, it has been reported that the activities of β-glucosidase, cellobiohydrolase, acid phosphatase, and N-acetylglucosaminidase positively correlate with soil C (Štursová and Baldrian, 2011; Xu et al., 2020). Our findings are consistent with recent studies highlighting that higher enzyme activities are positively correlated with altitude and C, particularly under beech forests with high C sources and MBC (Štursová and Baldrian, 2011; Šnajdr et al., 2013; Xiao et al., 2019).

Principal component analysis showed that the higher soil C, moisture, SIR, BR, MBC, and litter thickness at the highest altitude (1,500 m; under beech forests) were associated with C stock, β-glucosidase, N-acetylglucosaminidase, arylsulfatase, β-xylosidase, β-galactosidase, endoxylanase, peroxidase, and Mn-peroxidase enzyme activities. Similarly, Cardelli et al. (2019) showed a positive correlation between C and the β-glucosidase, cellulase, xylosidase, and glucuronidase activities under F. sylvatica. These enzymes are involved in the degradation of cellulose, pectin and lignin, which are the main components of forest-floor plant residues (Cardelli et al., 2019). Previous studies have also reported a correlation of arylsulfatase and acid phosphatase enzyme activities with soil C content in deciduous forests (Baldrian et al., 2013; Kooch and Bayranvand, 2017). We also showed the highest enzyme and biological activities (i.e., nematode and protozoa density) under mixed beech–hornbeam–maple forest with high MBC and moderate litter/soil qualities and thickness. This could be due to a high C in beech litter/soil and a high litter decomposition of hornbeam and maple trees (Kooch and Bayranvand, 2019). Dehydrogenases and oxidases facilitate the degradation of lignin and thus improve C cycling through the release of nutrients from plant residues (Massaccesi et al., 2020). Enzymes such as urease and alanine aminopeptidase are involved in N cycling in terrestrial ecosystems (Šnajdr et al., 2013; Feng et al., 2019).

In this study, the activities of alanine and leucine aminopeptidase were significantly higher in the plain mixed forest with high litter/soil N and pH, while their activities declined at high altitudes and at low temperatures. pH and N are both important factors affecting enzyme activity (Šnajdr et al., 2013). A general decline in qCO₂ and N with altitude could be attributed to a decreased ST and pH (Massaccesi et al., 2020). Štursová and Baldrian (2011) showed that the specific activity of leucine aminopeptidase increases with pH and N. A decrease in pH at high altitudes likely results from leaching of bases from the surface due to high precipitation or higher concentrations of H⁺ ions due to higher litter decomposition (Banday et al., 2019). The PCA showed that the soil N stock was associated with higher litter N, pH, qCO₂, MBN, and activities of lipase, alanine and leucine aminopeptidase enzymes. We also noted a clear association of chitinase and leucine-aminopeptidase enzymes with high soil quality (high N and low C/N), consistent with their activities under high fresh OM (Cardelli et al., 2019).

Soil C accumulation is enhanced by low ST and acidic and anaerobic conditions, all of which inhibit the decomposition process (Banday et al., 2019; Devi and Sherpa, 2019). As an important driver of the microbial community and microbial C limitation (Walker et al., 2018), ST was significantly lower at high altitudes than at low altitudes (Cui et al., 2019). Increased soil C and MBC are known to positively correlate with high SM and low ST (Baldrian et al., 2010; Quan et al., 2019).

The C stock was positively correlated with SM, MBC, BR, and SIR but negatively correlated with higher levels of ST. Microbial entropy and MBC were enhanced at middle altitudes (1,000 m), where the dominant tree species were beech, hornbeam, and maple. The activities of soil organisms, including earthworms, protozoa, and nematodes, depend on soil conditions, such as pH, N, and available nutrients, which are affected by the rate of OM decomposition (Kooch and Bayranvand, 2017). Soil N and pH are the main factors affecting litter decomposition, which is the major source of the soil microbial qCO₂ under high temperatures (Quan et al., 2019). Hornbeam forest had the highest litter quality, soil fertility, and soil organism activities compared with beech forest. In fact, soils under hornbeam had higher nutrient concentrations and pH that make conditions more favorable for microbial activities, whereas poor beech litter quality was due to a lower N concentration, higher C/N ratio, and higher levels of recalcitrant compounds associated with lower MBN and N sequestration (Kooch and Bayranvand, 2017). Among chemical variables, N had the strongest effect on the microbial biomass in litter and soil (Urbanová et al., 2015). Thus, the higher rate of N mineralization under hornbeam can be linked to the lower C/N ratio of its topsoil (Kooch and Bayranvand, 2019).

Microbial and enzyme activities can be used as sensitive indicators of soil function in response to different products of forest aboveground along altitudinal gradient, which helps to better understand the dynamics of both C and N cycling. Our findings indicate that N stock increases under plain mixed forest, which likely results from increased ST, tree diversity, litter N, MBN, pH, and leucine/alanine aminopeptidase enzymes. At higher altitudes, higher litter thickness, litter/soil C and C/N ratios, BR, SIR, and MBC and β-glucosidase, acid phosphatase, arylsulfatase, and laccase activities were associated with the lowest ST, highest SM, and thus greater C accumulation. In addition, our results showed that decreased litter/soil N and pH were associated with lower soil microbial and enzyme functions, which can be seen well at the 500-m altitude. Plain forests with high tree diversity, which produce high litter quality, had the highest N-dependent enzyme activity and soil fertilities, while mountainous forests with dominant beech trees had the potential to store more C and thus help to increase MBC. This study provides novel insights into different enzyme activities along altitude and improves our understanding of soil C and N cycling in forests.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding authors.

MA, GSJ, and JG are supervisors and advisors of the Ph.D. thesis of MB, respectively, and actively contributed to the design of the study. MB and PB performed the material preparation, data collection, and analysis. MB, PB, and MA analyzed and interpreted the soil properties, and enzyme activity data. MA contributed to identifying and classifying the vegetation cover. MB and JG analyzed the data. MB, PB, JG, and GSJ wrote the manuscript. All the authors contributed to the study conception and design, and read and approved the final manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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