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Soil plays an essential role in providing ecosystem services, especially in mountain ecosystems which are often considered as fragile and sensitive systems and commonly consist of a mosaic of forest and rangeland plant communities. The relationship between above-ground plant cover and the properties of soil organic and mineral layers in mountain areas are rarely studied. This research aimed to assess the effect of different land covers (i.e. forest, forest-rangeland ecotone, and rangeland) on soil functional indicators, i.e. fertility and biological activities, in the Hyrcanian region of northern Iran. We hypothesized that (i) the presence of tree cover enhances soil fertility and biological activities and creates hot spots (islands) of soil functional indicators especially in the topsoil, (ii) litter quality and organic matter fractions are the drivers for activities of soil organisms, nutrient cycles and transformation processes in mountain ecosystems. Litter (O-horizon including L, F and H layers) and mineral soil samples (in two separate depths of 0–10 cm and 10–20 cm) were taken using iron frames (30 × 30 cm). In total, 45 litter and 90 soil samples were transferred to the laboratory. Soil characteristic especially in the 0–10 cm depth, litter carbon (C), nitrogen (N) and C/N ratio were significantly affected by different land covers showing the maximum of soil organic C and microbial activity under forest. Our findings showed that the studied land covers, as well as litter and soil properties can be separated by PCA output. The first and second axes, accounted more than 50% of the explained variance in each of the studied soil depths. Soils with a better quality of litter (i.e. lower C/N ratio), higher values for organic matter fractions, soil fertility indicators and soil biological activities can be attributed to the forest. In contrast, positions of low soil fertility indicator values and biota abundance were imposed by forest-rangeland ecotone and rangeland. Although each land cover plays a prominent ecological role and takes its place in the evolutionary process, forests are essential because of their capacity to store and transform carbon and nutrients and to create hotspots identified by functional soil indicators. Based on our findings, soil functions decreased ranked in the order forest > forest-rangeland ecotone > rangeland, which can be assigned to the lower density of trees, and the amount of litter mass and litter quality. It can be concluded that tree covers have a prominent role in increasing soil functions, which should be given special consideration in the restoration of degraded mountain ecosystems.

The type of vegetation cover affects the contents of organic matter and other soil features that can lead to variability of soil microbial processes, which play a key role in the nutrient cycle. This issue is especially important in mountainous semi-arid ecosystems, which have been introduced as fragile and vulnerable habitats. In the present study, labile contents of soil organic matter (SOM), microbial and enzymatic indices under the influence of forest and rangeland cover [i.e. natural forest with dominant species of Zelkova carpinifolia (Pall.) K. Koch. And three rangeland covers, converted form forest site almost 30 years ago, with dominant species of Festuca ovina L. Dactylis glomerata L. and Stachys byzantina K. Koch.] in Northern Iran have been considered including seasonal changes. The study aimed to investigate i) the effect of forest conversion to rangelands on SOM fractions, (ii) temporal dynamics of soil microbial processes and enzyme functions in different vegetation types, and (iii) determining the relationship between indices related to the content of organic matter and the size of microbial population in mountain ecosystems. Results showed that Zelkova tree type enhanced the levels of SOM contents, carbon and nutrient (nitrogen, phosphorous, potassium, calcium and magnesium) pools, and also with higher values in the summer season, the size of microbial population and processes. Additionally, the SOM turnover and microbial efficiency were almost suppressed in order to Zelkova > Festuca > Dactylis > Stachys vegetation types, respectively. Based on the obtained results, it can be concluded that the presence of tree cover in mountain ecosystems can strengthen soil function and create fertile islands. Therefore, the protection of natural forests is emphasized for the proper management and sustainability of habitats in semi-arid regions.

The effect of vegetation types at mountainous sites on the relationships between soil biota and environmental variables applicable for the evaluation of soil quality and functions are barely studied and remain almost unknown. In the present study, the effect of forest degradation and vegetation types changes [i.e. natural forest with dominant species of Zelkova carpinifolia (Pall.) K. Koch. and three rangelands with dominant species of Festuca ovina L. Dactylis glomerata L. and Stachys byzantina K. Koch.] on the dynamics of soil properties and density of soil organisms in northern Iran has been considered. For each studied vegetation types, three different sites (at a distance of 4–6 km) with 1 ha area were selected. In summer (August), litter or O-horizon (i.e. L, F, and H) and soil (0–10 cm) layers were collected in an area of 30 × 30 cm. Also, in order to study the dynamics of soil properties, in addition to summer, soil samples were collected in the spring (May) and autumn (November) in each of the habitats. The study area is commonly covered with snow from late December to early April every year and therefore soil sampling was not considered in the winter season. A total of 48 litters (12 samples for each of those studied vegetation types), in the summer, and a total of 144 soil [(4 vegetation types × 3 seasons × 12 samples)] samples were collected and transferred to the laboratory. According to our findings, with higher values in the autumn season, the highest density of the soil earthworm (epigeic, anecic and endogeic), microarthropods (Acarina and collembola), nematode, and protozoa were detected in the Zelkova vegetation soils. Also, the highest density of bacterial and soil fungal communities were observed, especially in summer, under the tree cover of forest habitat. Based on our data, forest degradation and vegetation type changes suppressed the density of soil biota ranked in Zelkova > Festuca > Dactylis > Stachys due to low quality of litter, soil climate, and fertility condition. Although each vegetation type plays a prominent ecological role, forest covers are worthwhile because of their capacity to create hotspots identified by soil fauna and flora densities, which play a key role in the nutrient cycle and ecosystem services, in mountain habitats.