The to 1,000 m above sea level (a.s.l.) (Sabeti,

The declining trend in the natural forest areas has long
raised a global concern, mainly due to worry about economical poverty reduction
as well as loss of forest environmental services including water filtration,
soil erosion control, and microclimate modification (Trumbore Brando and Hartmann, 2015). Iran is categorized as a Low Forest Cover Country (LFCC)
covered with only 7.6% forest ecosystems (IUFRO, 2004). Hyrcanian forests are
one of the most antique and momentous ecoregions in Iran and the last remains
of natural deciduous forests in the world. These forests are located at the
northern slopes of the Alborz Mountains, with expanding about 1.9 million ha in
the southern Caspian Sea (IUFRO,
2004), but unfortunately, the majority of these forests has been
deforested because of human manipulation, urbanization, land-use changes and
agricultural purposes (Kooch, et
al. 2015). Given the high destruction rate of
the world’s natural forests and population growth as well as the growing need
for wood and other forest services, the importance and necessity of forestation
is highly obvious to regenerate the degraded areas and to provide supplies of
community (Yousefi, et al.
2010; Rahimabady, et al. 2015).

       Restoration
of degraded forest areas could be accompanied by alterations in the
physicochemical and biological properties of the soil, thus improving forest
ecosystem services (Kooch,
et al. 2017a, b). Reforestation
success and soil development are mainly associated to the selection of suitable
tree species (Wo? and Pietrzykowski,
2015); for example, establishing
broadleaf species seems to be more effective and to grow well on degraded sites
in the tropics than conifer ones (Parrotta, et al. 1997). In temperate
forests, oak
(Quercus castaneifolia) is an important species with high
economic value in Iran. It possesses the capability to grow along an elevation
gradient from flood-prone area to high topographical positions up to 1,000 m above sea level (a.s.l.) (Sabeti, 2009); that is, the tree can
be found from the plateau, together with other broad-leaved trees in particular
with box tree, to where it is mixed with common hornbeam. The upper
distribution limit of this species depends on both climate and soil properties
and geomorphology, and at higher altitudes it prefers warm and sunny slopes (Talebi, et al. 2005). 

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      The optimal rotation age (ORA) might
facilitate the greatest level of soil quality. Lack of information is available
concerning ORA for oak plantation, and also no research has been conducted to
evaluate its long-term effects on litter and topsoil in oak plantation in
degraded natural forest lands. Over time, soil properties will be changed following forest aging,
and in turn alters the
soil C and N processes (Zhang, et
al. 2017). Previous studies
focused on changes in soil C and N cycles following afforestation in temperate
forest lands (Bárcena, et al.
2014; Nave, et al. 2013).
Not only the quantity and quality of soil organic matter (SOM)
but also C and N inputs are the overriding controls on soil microbial biomass
and activity (Kallenbach and Grandy, 2011).
Hence, distinct organic amendments can stimulate microbial biomass through
enhancing the labile organic matter on time frames from months to decades (Kallenbach and Grandy, 2011). However,
studies reporting the seasonal changes in microbial properties of C and N are
still lacking in the temperate tree plantation soils (Mori, et al. 2016).

        Owing to high sensitivity of the soil
microbial community to natural and anthropogenic factors, soil microbial
activities seem to be reliable indicators for soil quality (Utobo and Tewari, 2015), and it is
necessary to identify and describe bio-geochemical cycles at a regional
spatiotemporal scale. Thus, the study was aimed to evaluate physicochemical,
biological, and biochemical properties of soil and litter quality in three oak
plantations with different ages, to assess the
influence of stand age on soil C
and N microbial indices, to test the hypothesis that soil C and N microbial
indices will increase in the top soil layer following increasing stand age. 

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