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.