The
present study supports the hypothesis that C microbial indices will be
generally enhanced following aging the afforested oak stand. Soil water content
could be introduced as a key driver of increase of BR and SIR under older
plantations (Chang, et al. 2013). Water content is appropriately
preserved in forest soils with thick humus layers (Page and Cameron, 2006); the more thickness of litter
under older plantation, the higher water content is occurred. BR is the main
phenomenon that controls the loss of C from terrestrial ecosystems (Haque, et al. 2014). The lack of organic C
input into soil is suggested to be the primary reason for BR and SIR reduction at
the first years of plantation when the trees are small and the understory
vegetation is improperly developed. BR and SIR will be enhanced in stands
following litter input elevation and in turn higher accumulation of organic C (Saurette, et al. 2006). In the three oak
plantations, litter C and litter C/N ratio showed the highest level in OA3
stand. BR and SIR represent a close
relationship between soil respiration and root biomass and that root
respiration have a significant proportion (30%-90%) in the total soil
respiration (Raich and Tufekcioglu, 2000).

           
In the present study,
FRB showed an increasing trend in the soil covered by the youngest stand to the
oldest one, and this could be ascribed to the observed increase in enzyme
activities. Similarly, Wang et al.
(2014) declared high soil fertility
effects on FRB in older plantations. This may indicate that the age of
monocultures is an important factor that determines FRB dynamics; however, it
may be related to other stand characteristics such as stand density, stand
structure, basal area, and phase of stand development, aboveground biomass, and
previous management practices, among others (Di Iorio, et al. 2013). The activity of all evaluated enzymes was significantly
higher in the afforested areas than the control soil, but no significant
difference was found between the stands with different ages. This emphasizes
the fact that changes in the enzymatic activities are unassociated with changes
in the organic C content. Soil physicochemical variables indirectly impact on
soil enzymes (López-Aizpún, et al.
2018).

       MBC greatly depends on SOM, and the
elevation of soil organic C will increase MBC (Chen, et al. 2005); that is, a decrease in MBC can result in mineralization
of nutrients, whereas an increase in that may lead to immobilization of
nutrients (McGill, et al. 1986). Thus, the observed higher MBC in
OA3 stand could be mainly attributed to its higher available SOM,
and this is also evident from the significant positive correlation between MBC
and SOM. This finding confirms with that reported by Wang and Wang (2007). The soil organic C, C/N ratio as well as carbon in macro and
micro aggregates in OA3 showed higher values compared to younger
ones and WTC area. Many researchers have reported similar studies in which succession
from agricultural land use change to afforestation increased the concentration
of carbon (Falkengren-Grerup, et
al. 2006), decreased
pH value (Falkengren-Grerup, et
al. 2006) and increased
the C/N ratio (Compton and
Boone, 2000).

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       High levels of sequestrated C was
measured in all of the three afforested oak stands, and the finding are
coincides with that noted by Dang
(2017) who reported that oak plantation
can improve the soil C sequestration. The present study demonstrated that the
soil organic C density in OA3 was significantly higher than that in
the OA1 and OA2. This difference could be attributed to
the larger biomass density observed in this oak stand, and support for this
view is come from the reported study by Dang (2017). The qCO2 illustrates the relationship between
microbial catabolic and anabolic processes and is frequently applied as a tool
for substrate use efficiency (Anderson
and Domsch, 2010).
The results of Li and Chen
(2004) showed a significant increase in
qCO2 with increasing EC values. The present research also indicated
that WTC area have higher qCO2 than that all plantations with
different ages. Soil EC in all oak stands was different (OA1 ? OA2
> OA3) and higher than the WTC area, and this difference
may be due to different litter quality as suggested by Haghdoost et al. (2011).

      Microbial entropy for
all ages of oak plantations was about 2 times higher than WTC area. This
results are in line with those reported previously (Rong and Zeng, 2010) for plantation on infertile degraded soil which shows a
significant improvement in soil microbiological characteristics and thus
supporting a relatively higher microbial entropy. Moreover, the present results
illustrated that BR is associated with the microbial entropy (MBC/C) ratios,
and this finding was fairly surprising because microbial entropy is frequently
applied as an indicator for microbial activity and for changes in soil
conditions (Yang, et al. 2010).

      The
CAI is used to evaluate the carbon availability for soil microorganism
community. If
the CAI is close to 1, microbial respiration is not limited by carbon
availability, but if it is less than 1, it limited by carbon availability (Cheng, et al. 1996). CAI less than 1 and near
zero, indicating the presence of carbon constraints in the soils. Considering
that FRB is the most important source of carbon for soil heterotrophic
organisms (Dai, et al. 2004), It can be
attributed to the decrease of CAI in soils under WTC area compare to oak
plantations soils. CMI
values increased in plantations areas compare with uncover one. Blair et al.
(1995)
pointed out that the CMI provides a good indication of the rate of change in
soil C dynamics over time. As the CMI captured
the impact of crop rotations and soil management in soil organic C restoration (da Silva, et al.
2014), it may be used to study successional vegetation
stages of the ecosystems. Accordingly, the observed significantly higher CMI in respectively OA3, OA2, and OA1
compared with WTC area is due to the enhancement in annual C inputs and the
difference in organic matter quality, thus modifying the labiality of C to an
oxidized form (Srivastava, et
al. 2016). Have
showed that CMI is significantly correlated with total C, MBC and POC (Liu, et
al. 2017).

        Particulate organic matter (POM) mainly
consists of root fragments (Cambardella
and Elliott, 1992).
Thus, significant different levels of POC between the WTC area and the three
oak plantations could be associated with their unequal root biomasses. Significantly
different levels of PON similar to POC values between three ages of oak
plantations and WTC area in this research would suggest differences in root
biomass (Cambardella and Elliott, 1992). Higher DOM includes DOC and DON concentrations were observed
in all three ages of oak plantation areas compare to WTC area. In covered
areas, more DOM concentrations has link by lower pH (Roth, et al. 2015). With regard to changes to the DOM, the increase in pH
could cause a deprotonation of weak acid functional groups in the DOM molecules
(Smebye, et al.
2016). Generally, soil chemical factors (Scheel, et al. 2008) influence the stability and
availability of DOM (both DOC and DON), and in turn the concentration and fluxe
of DOM. The observed decreasing trend in the concentration of DOC may be
stemmed from lower C allocation in the nutrient acquisition system of the
younger plantations (Vicca, et
al. 2012).

          In general, higher concentration of
NH4 +and
NO3- were detected in the three forestations in compare
to WTC area. Nitrogen dynamics in soils have been measured in a litter
manipulation experiment in Costa Rica (Wieder, et al. 2013), where NH4 +and
NO3- was significantly lower in litter removal plots.
Higher soil microbiological activity and the related mineralization process
presented in plantations area might increase the levels of available NH4 +and NO3-
(Lucas-Borja, et al. 2016). Considerable amounts of total N and
N in macro and micro aggregates were found
in the oak plantations.  The observed low
level of soil nutrients in WTC area could be ascribed to the opened canopy
cover that facilitates nutrient leaching (Haghdoost, et al. 2011). in the oak forests, to reduce the N accessibility in the
soil, the input to the soil microbial communities decreased by protecting the
internal nutrient cycles. On the other hand, C/N ratio of soil and litter was
significantly higher in oak plantations (Waring, et al. 2015).

             Poor substrate quality may cause microbial
N restriction and decelerate decomposition amounts (Moorhead and Sinsabaugh, 2006), yielding a negative feedback on
soil N cycling.
We observed a marked increase in net
Nmin along the chronosequence particularly in the plantations area.
The more abundant understory vegetation with resultant inputs of litter
fraction in oak stands may also have contributed to the higher Nmin
in oak compare to uncover area (Poirier, et al. 2016). This study reveals that rates of Nmin
increased in concordance with increasing inputs of N along the chronosequence,
suggesting that increased N leaching after 2–3 decades was also a result of
faster cycling and availability of N in soils. High Nmin and
nitrification rates in soils under oak are well in line with observations that
N retention was lower under oak within the first four decades after
afforestation (Hansen, et
al. 2007).

        We generally observed more MBN in all
plantation areas than WTC area.
Our results showed a significant
relationship between soil moisture content and MBN. In subtropical forest
soils, soil moisture is the major controlling factor of microbial biomass (Yang, et al. 2010). The composition of PON consists
mainly of root fragments (Cambardella
and Elliott, 1992).
The observed increase of DON concentrations in the soil solution is in
agreement with the predominantly increasing trends of DOC levels. Long-term
data series on DON concentrations is therefore very limited and published time
series were often shorter than the minimum of nine years that is recommended to
be able to detect clear trends (Verstraeten, et al. 2016). Our unique time series of DON concentrations brings new
insights into this matter. In general, according to the results of the current
research, within 25 years of establishment, study of these plantations progress
showed positive influences on most soil C and N microbial activities. 

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