اثر پوشش‌های جنگلی، مرتع مشجر و غیرمشجر برویژگی‌های لایه آلی و معدنی خاک

Background and Objective :Vegetation cover type, particularly various tree species, plays a pivotal role in enhancing the physical and chemical structure of soil through the production of litter and organic residues. Differences in vegetation types, such as forests, wooded pastures, and non-wooded grasslands, lead to variations in organic matter content, nutrient availability, and soil biological activities. Soil microorganisms and enzyme activities, influenced by plant species, serve as critical indicators for assessing soil quality, fertility, and sustainability. In this context, the present study investigates the characteristics of organic and mineral soil layers in areas with different vegetation covers in the Gil-Kala summer pasture region of Nowshahr, Mazandaran Province, Iran. The study focuses on four vegetation types: a forest dominated by Oriental beech (Fagus orientalis), a mixed shrub land of hawthorn and barberry (Crataegus microphylla and Berberis integerrima), a shrubland dominated by blackthorn (Prunus spinosa), and a grassland dominated by wheatgrass (Agropyron longiaristatum).
Materials and Methods :To evaluate the effects of different land covers on the properties of organic and mineral soil layers, preliminary surveys and field visits were conducted to select contiguous areas with minimal variations in elevation (1600–1610 meters above sea level), slope gradient (5–8%), and slope aspect. In each of the four vegetation types under study, two one-hectare plots (100 m × 100 m) were selected, with a minimum distance of 500 meters between them. Within each plot, five samples of the organic layer (litter or fine debris) and the mineral layer (30 cm × 30 cm to a depth of 10 cm) were collected, resulting in a total of 10 litter samples and 10 soil samples per vegetation type for laboratory analysis. One portion of the soil samples was air-dried and sieved through a 2-mm mesh for physical and chemical analyses, while another portion was stored at 4°C for biological tests. The nutrient content of the litter—including nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg)—was measured through standard laboratory mineralization procedures. Laboratory incubation methods were used to assess the activities of urease, phosphatase, arylsulfatase, and invertase enzymes. Earthworms were manually separated from the soil, washed in water, and preserved in alcohol-containing containers. They were identified based on morphological characteristics such as size, body length, color, clitellum position and shape, and the location and type of reproductive organs. Soil mites and collembolans were counted using the Berlese funnel method, soil nematodes were extracted using the Baermann funnel and centrifugation technique, and soil protozoa were quantified under a microscope at 50x magnification. Bacterial and fungal populations were recorded using culture-based methods.
Results :The results indicate that the highest nitrogen content in the organic layer (1.96%) was observed in the beech forest, while the lowest (1.03%) was found in the wheatgrass grassland. Similarly, the highest values for aggregate stability (72%), clay content (42%), coarse aggregates (52%), and fine aggregates (35%) were recorded in the beech forest. In contrast, the lowest values for these properties were 56%, 28%, 30%, and 19%, respectively, in the wheatgrass grassland. The highest pH (7.11) and soil fertility parameters were also associated with the beech forest, while the lowest values were observed in the wheatgrass grassland. Furthermore, the beech forest exhibited the highest abundance (4.1 individuals/m²) and biomass (42.48 mg/m²) of epigeic earthworms, abundance (1.2 individuals/m²) and biomass (4.66 mg/m²) of anecic earthworms, total earthworm abundance (9.5 individuals/m²), soil mite abundance (67,343.3 individuals/m²), soil nematode abundance (811 individuals/100 g soil), and soil protozoan abundance (613 individuals/100 g soil). Additionally, the highest populations of bacteria (4.47 × 10⁷/g soil) and fungi (1.72 × 10⁷/g soil), basal respiration (0.46 mg CO₂/g/day), microbial biomass carbon (362.8 mg/kg), microbial biomass nitrogen (58.28 mg/kg), and the microbial biomass nitrogen-to-phosphorus ratio (2.2) were observed in the beech forest.
Conclusion :Overall, the findings of this study demonstrate that the conversion of herbaceous vegetation to woody (tree and shrub) vegetation in the mountainous Gil-Kala region of Nowshahr, Mazandaran Province, significantly alters the properties of both organic and mineral soil layers. Notable changes include variations in organic layer nitrogen content, aggregate stability, clay percentage, and the proportions of coarse and fine aggregates, all of which showed significant differences among vegetation types. Other parameters, such as ammonium, nitrate, geometric mean of enzyme activities, soil pH, electrical conductivity, total carbon, carbon in fine aggregates, organic matter, total nitrogen, fixed nitrogen, nitrogen in fine aggregates, dissolved organic nitrogen, nutrient elements (phosphorus, potassium, calcium, and magnesium), fine root biomass, and the activities of enzymes such as urease, acid phosphatase, arylsulfatase, and invertase, also exhibited significant changes. Furthermore, attributes such as the abundance and biomass of epigeic, anecic, and endogeic earthworms, as well as the populations of total earthworms, soil mites, nematodes, and protozoa, displayed marked differences across the land cover types. These findings suggest that tree and shrub covers, due to their longer persistence, ability to provide shade, reduce evaporation, and increase soil moisture, play a significant role in improving soil structure and supporting the growth of earthworm and microbial populations. These organisms are crucial for organic matter decomposition, nutrient cycling, and the enhancement of soil’s physical, chemical, and biological properties, ultimately leading to improved soil fertility and productivity. Consequently, the results indicate that expanding woody vegetation cover can create more favorable conditions for optimal soil functioning and serve as an effective strategy for restoring degraded lands. Therefore, in regions with similar semi-arid and mountainous climates facing land degradation and land-use changes, the development of woody vegetation, particularly beech, is recommended.