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The properties of litter and soil, which are probably influenced by vegetation traits, have an effect on the fractions of C and N and ultimately global warming. In this study, litter and soil samples were collected from representative forestlands, including a mixed natural forest dominated by hornbeam‑ironwood and tree plantations of ash, maple, cypress and pine which are situated in a temperate area of Northern Iran. Main litter characteristics, soil aggregation and fractions of C and N were studied. Pine had the highest values of litter thickness (14.46 cm), C (60.31%) and C/N ratio (81.77 and 20.28 for litter and soil, respectively). However, litter N (2.20%), micro aggregates of total soil (39.75%), aggregate stability (73.86%) and macro aggregates of total soil (54.87%) were found significantly higher under the natural stands. Total C (2.58%), C in macro and micro aggregates (0.31 and 0.33 respectively), POM-C (4. 68 g/kg), DOC (125.54 mg/kg) and C sequestration (45.34 Mg/ha) were considerably higher under pine (for POM-C, pine ≈ cypress) and the greater values of total N (0.34%), N in macro and micro aggregates (0.10 and 0.08 respectively), POM-N (0.47 g/kg), DON (33.80 mg/kg) and N sequestration (6.86 Mg/ha) were observed under natural stands. Under our conditions, conversion of natural forest is due to reduction of soil N fractions and C increased under pine tree plantations.

Our findings show the differential impacts of tree types (hornbeam–ironwood natural forest and plantations of ash, maple, cypress and pine) on soil physical–chemical characteristics and how dominant tree species regulate the size and activity of soil microbial and enzymes. Soil microbial and enzyme activities have an important influence on nutrient cycling. The variation in soil carbon (C) and nitrogen (N) microbial indices as well as enzyme activities were determined under hornbeam–ironwood natural forest and 25-years old plantations of ash, maple, cypress and pine in northern Iran. Four soil profiles were dug along the four parallel transects in the central part of each forest stand, resulting in 16 soil samples for each stand at a depth of 0–15 cm. The results showed significant differences in most of studied soil physico-chemical properties among different land covers. Soil microbial basal respiration (MR) and substrate induced respiration (SIR) were found to be significantly higher under natural forest than in the plantation stands. Microbial biomass C (MBC) did not differ for the forest stands, whereas natural forest had significantly higher qCO2 (MR: MBC), microbial entropy (MBC: Corg) and C availability index (MR: SIR) values. The establishment of plantations reduced the values of soil microbial biomass N, mineral nitrogen (NH4–N and NO3–N) and N mineralization with the least under coniferous stands of cypress and pine, while greater amounts were observed under natural forest. With replacement in the type of tree species from natural forest to plantations, acid phosphatase, arylsulphatase and invertase enzyme activities decreased significantly in the topsoil, whereas like natural forest, ash plantation improved urease enzyme. This research recommends conservation of natural forests for increase of soil quality and plantation of suitable native broad-leaved species for rehabilitation of degraded natural forests.

Forest degradation causes soil carbon losses and affects soil C and N cycling. However, it is not clear how reforestation of degraded areas with different species affects soil and ecosystem restoration, particularly in northern Iran. The aim of this study was to evaluate the effect of forest degradation and reforestation with two contrasting species: Alnus and Quercus as N2-fixing and non N2-fixing tree species, respectively, on soil quality and function after 30 years in the northern Iran region. We selected four forest stands: Carpinus betulus-Parrotia persica natural forest (NF), Alnus subcordata (AP) and Quercus castaneifolia (QP) plantations as rehabilitated areas, and a degraded natural forest (DNF). We examined the main litter and soil chemical properties and soil C and N microbial fractions. Litter and soil physico-chemical properties and microbial activity differed among land covers. As expected, deforestation caused a decrease in most soil C and N fractions as well as in soil microbial activity with an increase in metabolic quotient-qCO2. The plantation with Quercus spp. improved substrate induced respiration, as well as microbial biomass carbon and dissolved organic carbon compared to the Alnus plantation, whereas mineral N was similar in the natural site and the Alnus plantation. Although forest plantations had not reached the same values of microbial activity as the natural forest soils, soil C and N fractions were similar to the natural site after 30 years. The findings of this study support the importance of preserving natural forests for soil conservation. In addition, employing of N2 fixing trees such as Alnus spp. or, Quercus spp., suitable native broadleaved species, is proposed for the rehabilitation of degraded natural forests. Although both forest plantations led to soil recovery after 30 years, Quercus plantations were more efficient in restoring microbial communities than Alnus plantations indicating that litter quantity is more important than litter quality for soil recovery.