Background and Objectives: Tree species influence soil health, which encompasses a set of physical, chemical, and biological properties affecting ecosystem functioning, through differences in litter quality, nutrient cycling, and changes in soil physicochemical properties. Despite the importance of this issue, the combined effects of tree species and seasonal variation on various soil health indicators in the Hyrcanian forests, particularly through an integrated assessment of litter quality, physicochemical properties, fine-root biomass, soil fauna, and microbial activity, have received limited attention. Therefore, in the present study, four native broadleaved tree species of the Hyrcanian forests, including Quercus castaneifolia C.A.Mey., Pterocarya fraxinifolia (Lam.) Spach., Acer velutinum Bioss., and Carpinus betulus L., were selected as representative species to investigate their differences in litter quality and fine-root biomass and their effects on soil physicochemical properties, soil fauna (earthworms and nematodes), and microbial activity.
Materials and Methods: This study was conducted in the Khanikan Forest, located in the southern part of Chalous and Nowshahr counties, northern Iran. For each tree species, eight individual trees with a similar diameter at breast height (approximately 50 cm) were selected as replicates. Litter and soil samples were collected during the summer (August) from beneath the canopy of individual trees using 30 × 30 cm and 30 × 30 × 15 cm sampling plots, respectively. Litter carbon and nitrogen, soil physicochemical properties, fine root biomass, nematode population, the density and biomass of ecological groups of earthworms (epigeic, anecic, and endogeic), soil microbial respiration, and carbon mineralization were determined. Soil biological indicators were evaluated in four seasons (spring, summer, autumn, and winter). Data were analyzed using analysis of variance (ANOVA), Duncan's multiple range test, and principal component analysis (PCA).
Results: The results showed that litter quality differed significantly among the tree species; Quercus castaneifolia had the highest carbon and C/N ratio and the lowest nitrogen content, whereas Carpinus betulus had the highest nitrogen content and the lowest C/N ratio. Most soil physicochemical properties also differed significantly among species; soil pH, electrical conductivity, nitrogen, and available nutrients were highest under Carpinus betulus, whereas soil organic carbon and C/N ratio were highest under Quercus castaneifolia. The highest fine-root biomass was observed under Pterocarya fraxinifolia, while the lowest occurred under Quercus castaneifolia. All soil faunal and microbial indicators were significantly (p < 0.05) affected by tree species, and the effect of season was also significant (p < 0.05) for most variables. The highest abundance and biomass of earthworms and the highest nematode populations occurred in autumn, whereas the highest microbial respiration and carbon mineralization were recorded in summer; the lowest values for most indicators were observed in winter. Across all seasons, Carpinus betulus had the highest and Quercus castaneifolia the lowest values of soil biological indicators. Principal component analysis also supported this pattern; the first principal component, explaining 66.93% of the variation, separated Carpinus betulus in association with nutrient availability, microbial activity, earthworms, and nematodes from Quercus castaneifolia in association with litter carbon, litter C/N ratio, and soil organic carbon.
Conclusion: The findings showed that tree species, through differences in litter quality and soil physicochemical properties, create distinct pathways in organic matter cycling, nutrient dynamics, and soil biological activity. Among the studied species, Carpinus betulus, by providing more favorable conditions for soil organisms, microbial respiration, and carbon mineralization, had the greatest contribution to improving soil biological quality, whereas Quercus castaneifolia, through greater organic matter accumulation and a higher C/N ratio, was associated with slower decomposition and nutrient cycling processes. Seasonal variation, particularly the favorable conditions in autumn and summer, also played an important role in regulating soil organism activity and microbial processes. These findings highlight the importance of tree species effects in maintaining soil health and supporting sustainable management of the Hyrcanian forests.