3 Results
3.1 Key soil attributes
The time since reclamation had a substantial effect on the studied soil
key attributes (Fig. 2 ). Soil organic
matter (SOM) significantly increased after 17 years since reclamation
(Fig. 2A; χ2=0.68, p=0.410). Similarly, soil pH and EC
increased significantly in the reclaimed plots with respect to the
control plot (Figs. 2 B-C;
χ2=22.432, p<0.001;
χ2=19.557, p<0.001). Soil available
potassium (AK) was significantly higher in the plots under reclamation
than in the control plots (Fig. 2D; χ2=4.369,
p=0.037). However, it was not clear whether the time since reclamation
had an effect on the AK concentration in the soil. Both soil extractable
phosphorus (OP) and total nitrogen (TN) were substantially lower in the
reclaimed than in the control plots (Fig 2E-F;
χ2=11.938, p=0.001; χ2=0.22,
p=0.639). The enzymatic activity of soil urease (URA) in the soil was
significantly higher in the plots under reclamation than in the control
plots (Fig. 2G; χ2=16.518, p<0.001). Soil
polyphenol oxidase (PPO) activity showed a clear increasing trend along
the reclamation timeline (Fig. 2H; χ2=43.014,
p<0.001). Yet, it was not clear whether the time since
reclamation led to an increase in enzymatic activities of URA,
dehydrogenase (DHG) and fluorescein diacetate hydrolase (FDA) in the
soil (Fig. 2G and Figs 2I-J; χ2=1.16, p=0.281;
χ2=0.065, p=0.799).
3.2 Taxonomic composition of the soil microbial communities
(SMCs)
The most abundant phylum and genera identified in this study are shown
in Figs. 3A and 3B, respectively. The phyla Thaumarchaeota ,Acidobacteria, Actinobacteria, Bacteroidetes, Chloroflexi,
Gemmatimonadetes, Patescibacteria , Planctomycetes ,Proteobacteria and Verrucomicrobia accounted for more than
95% of the total genetic sequences retrieved for each analyzed soil
sample (Fig. 3). The relative abundance of the two most abundant phylum
- i.e. Bacteroidetes and Proteobacteria decreased with
increasing time after reclamation (Fig. 3A; χ2=18.339,
p<0.001; χ2=26.530,
p<0.001). The abundance of the third and fourth most
abundant phylum i.e. Acidobacteria and Chloroflexiincreased following reclamation (Fig. 3A; χ2=15.437,
p<0.001; χ2=42.547, p<0.001).
Moreover, the abundance of the phylum Thaumarchaeota ,Actinobacteria and Planctomycetes also increased with the
time after reclamation (Fig. 3A; χ2=0.000, p=0.993;
χ2=9.862, p=0.002; χ2=1.346,
p=0.246), but the abundance of the phylum Verrucomicrobia,
Cyanobacteria , Dependentiae , Patescibacteria , FBPand Firmicutes decreased. The relative abundance of other
identified phylum is shown in supplementary material – Fig. S2.
Regarding the most abundant SMCs’ genera found (Fig. 3B), the abundance
of the genera RB41 and UTCFX1 increased significantly with
the time after reclamation (Fig. 3B; χ2=18.110,
p<0.001; χ2=43.022, p<0.001).
Nonetheless, other genera, such as Flavobacterium ,Pedobacter , Sphingomonas and Luteimonas (Fig. 3B;
χ2=21.616, p<0.001;
χ2=13.077, p<0.001;
χ2=25.592, p<0.001;
χ2=12.872, p<0.001) showed the opposite
trend. The abundance of the less abundant genus is shown in
supplementary material – Fig. S2.
3.3 Molecular ecological networks
(MENs)