Soil physio-chemical and biological indicators to evaluate the restoration of compacted soil following reforestation

Reforestation of degraded forests with small to medium-scale monoculture plantation following mechanized ground-based harvesting has been widely applied in the past decades in Hyrcanian forests, but no studies have clearly characterized the restoration of soil properties following tree plantation in the temperate forests. We aimed to assess the efficacy of different plantation types (treatments) including Caucasian alder (Alnus subcordata; AS), Velvet maple (Acer velutinum; AV), and Italian cypress (Cupressus sempervirens var. horizontalis; CS) on the recovery of litter and soil properties over a 25-year period in the Tangar district in the Tyrumrud region of the Hyrcanian forests (Iran), compared to levels at the natural hornbeam-oak stand as a control. In each treatment (plantations with average length of 250 m and width of 200 m), three sample plots were measured and provided 81 soil samples. Tree species had significant differences on litter thickness and quality with the lowest litter thickness (4.2 cm), carbon (C) content (32.75%), and C/N ratio (15.56) and highest nitrogen (N) content (2.11%) in the Alnus subcordata (AS) plantation. Significantly higher recovery values of soil physical properties including soil bulk density, total porosity, macroporosity, penetration resistance, and aggregate stability were found in the AS plantation. However, soil physical properties did not fully recover in AS, Acer velutinum (AV), and Cupressus sempervirens (CS) plantation even after a 25-year period, compared to the control area. The significantly higher recovery of pH, EC, N, N storage, available nutrient, earthworm density, biomass, soil microbial respiration, and the lowest C, C/N ratio, C storage were observed in the AS plantation. The fine root biomass among the three plantation types did not fully recover compared to natural stands. Our study revealed that the roots of the planted trees initiated soil structure recovery, whereas the N-fixing tree species such AS significantly ameliorated the tested soil properties over a 25-year period following compaction.