Results higher than OA3 (p<0.05). The concentration of organic

Results

Soil and litter properties

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       The
analysis of physicochemical properties of the soils collected from the selected
sites demonstrated significant differences between the unafforested and the
afforested oak stands as well as among the second ones (Table
1). Litter thickness, C and C/N ratio in OA3 were
significantly higher than OA1 and OA2 (p<0.05). The concentration of P, Ca, and Mg were significantly higher under OA1 stand compared to the other ones, whereas litter N and K concentration showed no significant difference between all of the afforested sites. pH and EC showed equivalent values in the WTC area to OA1 and OA2 plantations but significantly higher than OA3 (p<0.05). The concentration of organic carbon, Mg, total N, N in macro aggregates, and C in macro and micro aggregates, as well as C and N sequestration, C/N ratio, FA, HA, and water content in OA3 were significantly higher than the other fields and demonstrated an age- and afforested- dependent order i.e., OA3 > OA2 > OA1 > WTC (p<0.05). However, significantly high content of N in micro aggregates was measured in OA1 plantation when compared to the other fields (p<0.05), and the concentration of P, K, and Ca in OA1 and OA2 were approximately equal and higher than OA3 and WTC area (Table 2).        A significant increase in the enzymatic activities was observed in the topsoil of the degraded forest lands following afforestation. Urease showed the highest activity in OA1 with the order of OA1 > OA2 > OA3 > WTC, whereas the activity of Acid phosphatase and invertase
demonstrated an afforestation dependent increase (i.e., OA1 ? OA2
? OA3) in compare to WTC area. Arylsulfatase activity showed the
order of OA1 ? OA2 > OA3 > WTC area in
the studied areas (Table 2).

 

Soil biological properties

     Table 2 shows soil biological characteristics in the four areas. Soil
nematode population revealed an inverse association with the forestry period;
that is, the highest population was observed in the soil of OA1, and
showed the order of OA1 > OA2 > OA3 >
WTC. However, no significant difference was observed in the earthworm density/biomass
between WTC and the three afforested fields. In addition, the content of fine
root biomass (FRB) showed a forestry-age-dependent increase when compared with
WTC area.

 

Soil
C and N microbial indices

          A number of the soil microbial
indices including  BR, SIR, MBC, POC and
DOC demonstrated significantly higher concentration in OA3 than did
the other studied fields and showed an order as OA3> OA2
> OA1 > WTC (p<0.05), whereas CAI did as OA3 ? OA2 > OA1 > WTC. Microbial entropy and CMI in the study area
exhibited an order as OA2 > OA3 > OA1 >
WTC and OA3 > OA2 ? OA1 > WTC,
respectively. In the WTC site, qCO2 index demonstrated the highest
amount compared to the afforested oak stands (Fig. 3).
The concentration of NH4+,
Nmin, MBN, PON and DON in the afforested sites were alike (OA1 ? OA2 ? OA3),
but higher than the WTC one. NO3- content illustrated an
age- and afforested- revers increase as OA1 > OA2 >
OA3> WTC (Fig. 4).

        Figure 5 illustrates
the output of PCA analysis for different locations and land covers as well as
litter and soil properties. The first and second axes respectively extracted
37.682% and 11.713% of the explained variance. The left PC1
shows a condition with accumulation of macro-element nutrients and more
microbial and enzyme activities, and this can be attributed to different ages of oak
plantations, while the right PC1 presents the positions with low macro-element
nutrients and almost low soil physicochemical and biological activities (with a
maximum of qCO2) imposed by WTC site. As a consequence, such
properties showed a significant positive relationship between C and N in macro and
micro aggregates, water content, available nutrient in soil and litter
and soil biological properties (such as nematode and FRB) (Fig. 5). These soil microbial indices (i.e.,
C and N; except qCO2) were altered by soil physicochemical and
biological characteristics, while Axis 2 show no clear
relationship between the different litter and soil characteristics and land
covers (Fig. 5).