Regeneration of belowground properties and nutrient pools in soil after compaction: response to the reforestation with native tree species in the Hyrcanian forest

Reforestation with native broad-leaved tree species changes the aboveground functions that regulate the quality of litter inputs and soil-root systems, which ultimately influences on soil properties and nutrients pool. The current study characterizes the regeneration in the floor layer and topsoil at depth pf 0-10 cm in the skid trails in response to the reforestation of four tree species (FE; Fraxinus excelsior, PA; Prunus avium, AC; Acer cappadocicum, and QC; Quercus castaneifolia) after clear-cutting in the degraded forests, comparing to the undisturbed natural forest (CB-PP; Carpinus betulus - Parrotia persica). Results showed significant differences in litter layer properties among tree species, with the highest litter thickness, C, and C/N ratio under the QC and AC and the greatest litter N in CB-PP and FE. FE plantation resulted in an enhance soil bulk density (1.14 g cm⁻³), total porosity (55.85%), macroporosity (37.72%), penetration resistance (1.43 MPa), soil moisture (33.4%), and aggregate stability (51.7%) compared to other tree plantations, whereas these values under the FE plantation were still lower than those of the CB-PP stand over a 30-year period after logging operation. The lowest amount of soil C (3.34%), soil C/N ratio (14.58), and C storage (38.13 Mg ha⁻¹) were found under the FE plantation, whereas the greatest amount of soil pH (6.61), soil N (0.23%), N storage (2.64 Mg ha⁻¹), available P (15.53 mg kg⁻¹), K (235.55 mg kg⁻¹), Ca (171.92 mg kg⁻¹), and Mg (36.57 mg kg⁻¹), fulvic acid (282.7 mg/100 g) as well as humic acid (134.5 mg/100 g) were found under the FE plantation compared to the other trees plantation, but were still least than those of the values under the CB-PP over a 30-year period after soil compaction. Following the establishment of different tree plantations, the significantly greatest earthworm density (1.82 n m⁻²) and dry mass (22.57 mg m⁻²), fine root biomass (76.31 g m⁻²), soil microbial respiration (0.33 mg CO₂-C g soil⁻¹ day⁻¹), MBC (438.7 mg kg⁻¹), NH₄⁺ (16.54 mg kg⁻¹), NO₃⁻ (15.92 mg kg⁻¹), N mineralization (27.04 mg N kg soil⁻¹), and MBN (28.35 mg kg⁻¹) were found under the FE plantation, whereas the values did not return to the pre-harvest levels as observed under the CB-PP over a 30-year period. Litterfall on the soil surface by planted tree species (FE and PA in particular) considered as food resource (i.e., soil C and soil organic matter) that is a primary driver to the biological and microbial activities. Results of the current study can promote our knowledge to select suitable tree species, which leads to maintain soil quality and nutrients pool in the ecosystem restoration programs and reforestation in the degraded forest area.