4.2 Soil bacterial community composition and its influencing
factors
The difference analysis of the soil bacterial community composition
between the three types of sites showed that the soil bacterial
community composition of SF and PB had the largest difference, and the
difference in the soil bacterial community between CSF and SF was
significantly greater than the difference in the soil bacterial
community between CSF and PB (Fig. 3). This suggests that the
restoration of the soil bacterial community in SF affected by drainage
and afforestation may be more difficult than that in PB.
The results of this study showed that the dominant groups of bacteria at
the phylum level in the soil of SF, PB and CSF were all Proteobacteria
and Acidobacteria (Fig. 5A), which was consistent with the results of
other peatlands (Dedysh et al. 2006; Kraigher et al. 2006; Morales et
al. 2006; Ausec et al. 2009; Pankratov al. 2011; Serkebaeva al. 2013;
Sun al. 2014; Danilova et al. 2016). However, the relative abundance of
Proteobacteria was the highest in SF, and the relative abundance of
Acidobacteriota was the highest in PB and CSF. The relative abundance of
Proteobacteria in SF was significantly higher than that in PB and CSF,
and the relative abundance of Acidobacteriota in PB and CSF was
obviously higher than that in SF, reaching a significant level in CSF.
There was no significant difference in the relative abundance of
Proteobacteria and Acidobacteria in PB and CSF. Acidobacteria are known
to prefer acidic environments and can grow under poor nutrient
conditions (Philippot et al. 2010; Dedysh et al. 2011; Andersen et al.
2013), while Proteobacteria are associated with higher C availability
(Fierer et al. 2007; Leff et al. 2015). Several studies have found a
negative response of Acidobacteria relative abundance to pH
(Hartman et al. 2008; Urbanová et
al. 2016). This study found that the relative abundance of
Proteobacteria was significantly positively correlated with
NH4+-N and SWW, while the relative
abundance of Acidobacteriota was significantly negatively correlated
with NO3--N,
NH4+-N and SWW and significantly
positively correlated with SBD. The relative abundances of
Proteobacteria and Acidobacteria were positively correlated and
negatively correlated with soil pH, respectively, which did not reach a
significant level. The difference in the relative abundance of
Proteobacteria and Acidobacteria among the three types of sites may
reflect their different environmental conditions, such as pH and
nutrient status (substrate availability).
In addition, the ratio between Proteobacteria and Acidobacteria is
considered to indicate the nutrient status of soil ecosystems and
different peatlands, and the higher the ratio is, the richer the
nutrient is, and vice versa (Smit
et al. 2001; Hartman et al. 2008;
Urbanová et al. 2014). In general, species richness and microbial
diversity in peat sediments increase with the improvement in nutritional
status (Hartman et al. 2008). In addition, differences in nutritional
status may also lead to changes in the bacterial microbiome in different
microhabitats (Hartman et al. 2008;
Urbanová et al. 2016). In this
study, the ratios of Proteobacteria and Acidobacteria were 2.02, 0.86
and 0.76 in SF, PB and CSF, respectively, and the values in SF were
significantly higher, while the values in PB and CSF had no significant
difference (Fig. 1S). The results showed that the nutrient status of SF
was significantly better than that of PB and CSF. Drainage construction
of Cryptomeria fortuneana forest will significantly reduce the
nutrient status in SF but has no significant impact on PB. The nutrient
status may also be an important factor for the significant difference
between the soil bacterial community diversity and composition of SF and
PB and CSF.
This study found that the relative abundance of Actinomycetota in PB and
CSF was significantly higher than that in SF, but there was no
significant difference between their values (Fig. 5A). This study also
found that the relative abundance of Actinomycetota was significantly
negatively correlated with soil pH,
NH4+-N, and SWW and was extremely
significantly positively correlated with AP (Fig. 6A, Table S1). Members
of Actinomycetota can produce extracellular enzymes and have the same
enzymatic ability as fungi (le Roes-Hill et al. 2011). Heterotrophic
actinomycetes can degrade recalcitrant polymer substances such as
lignin, chitin, pectin, aromatic hydrocarbon and humic acids under
aerobic conditions, so they thrive in the oxygen-bearing layer of acidic
peatland (Jaatinen et al. 2007). Tian et al. (2019) found that a
decrease in water level increased the thickness of the aerobic layer of
peat, leading to an increase in the abundance of actinomycetes,
supporting our research findings. The relative abundance of
Actinomycetota in PB and CSF was significantly higher, which may
indirectly indicate that their soil carbon quality was significantly
lower, and their stable carbon or recalcitrant carbon components were
significantly higher. Research has found that long-term drainage and
tree growth lead to a decrease in the decomposability of peat and an
increase in the content of recalcitrant compounds such as carboxylic
acids, aromatics, and phenols (Blodau et al. 2012; Mastny et al. 2016;
Urbanová et al. 2018). Acidobacterium has been found to be a dominant
phylum of bacteria under nutrient-poor conditions, and it is believed
that it is involved in the degradation of cellulose and aromatic
compounds (Ausec et al. 2009;
Pankratov et al. 2011).
Therefore, the higher abundance of Acidobacteriota in PB and CSF also
indirectly supports this hypothesis (Fig. 5A).
This study analysed the differences in soil bacterial community
composition among different treatments at the genus, family, class, and
phylum levels using the top ten relative abundance rankings of various
types of sites (Fig. 5). This analysis method is superior to the
analysis method of ”using the top ten groups with relative abundance
ranking of all samples” (Lin et
al. 2012). Because there may be significant differences in the dominant
species of soil microbial communities among different treatments, the
latter cannot clearly display the composition of dominant species in
specific treatments and the relative abundance differences of dominant
species among different treatments. The use of relative abundance
thresholds also has drawbacks, as there may be significant differences
in the dominance of soil microbial communities among different
treatments (Urbanová et al. 2016;
Tian et al. 2019). As shown in the results of this study,
Planctomycetes, Saccharibacteria, Chlamydiae, and Firmicutes were not
the dominant groups in SF (relative abundance ranking is not in the top
ten), but they were the dominant groups in PB or CSF (Fig. 5A). The
relative abundance of Planctomycetes ranked seventh and sixth in PB and
CSF, respectively. The relative abundance of Saccharibacteria ranked
10th in CSF, the relative abundance of Chlamydiae ranked 9th in PB, and
the relative abundance of Firmicutes ranked 4th and 5th in PB and CSF,
respectively. The relative abundance of Firmicutes in PB and CSF was not
significantly different but was significantly higher than that in SF.
The RDA results show that soil pH, AP,
NH4+-N, SWW, and VW are all
significant influencing factors for the composition of major groups of
soil bacterial communities at the phylum, class, family, and genus
levels (Fig. 6, Table 2). This further demonstrates the important
effects of pH, nutrient level, and water conditions on the composition
of major groups of soil bacterial communities at different
classification levels. In conclusion, this study shows that the
diversity and composition of the soil bacterial community in CSF are in
the middle of the corresponding values of SF and PB, and the difference
between CSF and SF is significantly greater than that between CSF and
PB, which is closely related to the soil pH, nutrient level and water
conditions of different types of peatlands. This supports our second
hypothesis, that is, ”Compared with that between CSF and PB, the
difference in the soil bacterial community between CSF and SF is
greater, which is caused by the difference in soil moisture and
nutrients between different types of peatlands”. Urbanová and Bárta
(2014) found that the diversity and composition of soil bacterial
communities in spruce swamp forest were between those of bogs and fens
in their study of different types of peatlands in the Czech Republic.
They believe that this reflects changes in soil pH, nutrient
availability, and peat decomposition ability. Hartman et al. (2008)
found a strong correlation between soil bacterial composition and
diversity and soil pH in swamps and bogs in North Carolina and fens in
the Everglades in Florida. Tian et al. (2019) studied the Sphagnum
palustre peatlands in Dajiuhu Lake of Shennongjia, China, and found
that the groundwater level and total nitrogen content had a significant
impact on the soil microbial community of the Sphagnum palustrepeatlands. The above studies all indicate that environmental conditions
have a strong impact on the diversity and composition of soil microbial
communities in peatlands, and significant environmental factors vary
depending on the specific research system.