DISCUSSION
4.1 | The relative abundance of rhizosphere
soil microbial community changed
At the phylum level, the difference in the carbon sequestration level of
Masson pine families did not change the dominant phylum of soil
bacteria, but the relative abundance of the bacterial phyla changed
significantly. The dominant phyla mainly included Acidobacteria,
Proteobacteria and Chloroflexi (Figure 2); this result is consistent
with the results of previous studies (Bei, Moser, Wu, Müller, &
Liesack, 2019). The relative abundance of Acidobacteria increased with
increasing carbon sequestration in the different families, and the
differences among families reached significant and extremely significant
levels (Figure 2). Acidobacteria are important soil microorganisms that
play a very important role in the soil carbon cycle and in structuring
ecosystems (Wang, et al., 2016). In this experiment, the total organic
carbon content in the samples from the three families gradually
increased with increasing carbon sequestration (Table 2), showing the
same change trend as that of Acidobacteria. This result suggests that
some members of Acidobacteria use soil carbon as the substrate for their
metabolic activities and that the carbon content in the soil promotes
this process. Previous research has also confirmed this view (Pankratov,
et al., 2008). On the other hand, in our study, the relative abundance
of Proteobacteria decreased with increasing carbon sequestration in the
three families, although previous studies have shown that the relative
abundance of Proteobacteria was associated with a higher carbon
utilization rate (Fierer, Bradford, & Jackson, 2007). Zheng (2014)
explored the impact of ecological restoration on the structure of the
soil bacterial community of mangroves (Rhizophora apiculata ) and
found that the relative abundance of Proteobacteria was negatively
correlated with the content of soil organic carbon. The phenomenon
observed in our study may be due to the decisive influence of carbon
sequestration in the Masson pine on Proteobacteria; there is evidence
that the abovementioned vegetation had a greater impact on the bacterial
taxa involved in soil carbon sequestration than other factors, such as
soil environmental factors (Zhao, 2019). However, the specific mechanism
of this influence and its process of action are still unclear and need
to be further explored. Notably, Actinobacteria have been demonstrated
to use mycelia to break down recalcitrant organic carbon (Dang, Yu, Le,
Liu, & Zhao, 2017), and Chloroflexi showed strong carbon sequestration
(Li, 2019). However, the results of this study showed that the relative
abundance of these two phyla did not change significantly with the
increase in carbon sequestration in the different families (Figure 2 A).
We speculated that the cause of this phenomenon might be the low
proportion of the two bacteria in the total rhizosphere community.
In terms of fungi, the dominant phyla mainly included Basidiomycota,
Ascomycota, Mucoromycota and an unclassified phylum
(Unclassified_k_functional). The relative abundance of Basidiomycota
increased with the increase in the carbon sequestration level of the
Masson pine families. The relative abundance of Ascomycota gradually
decreased with increasing carbon sequestration, and the dominant
rhizosphere fungi changed from Ascomycota to Basidiomycota (Figure 2,
Figure S1). Wang et al. (2019) investigated the influence of grassland
restoration in a Chinese pine (Pinus tabuliformis ) forest on
fungi and found that when the carbon content in the soil decreased, the
relative abundance of Ascomycota decreased significantly, and
Basidiomycota replaced Ascomycota as the dominant fungal phylum; these
results are consistent with the results of this experiment. Although the
corresponding molecular mechanism is not fully understood, there is
evidence that up to 30% of plant photosynthetic products can be
transferred from plant roots to fungi (Gonzalez, et al., 2018; Nehls,
Grunze, Willmann, Reich, & Kuester, 2007). Moreover, some families of
Basidiomycota have been confirmed to have enzymes that degrade complex
macromolecular organic matter in the soil (Floudas, et al., 2012). When
the soil carbon content increased, the abundance of Agaricomycetes
significantly increased (Li, et al., 2020). Therefore, this might be the
reason why Basidiomycota became the dominant fungus after the soil
carbon content increased.
4.2 | Most members of the
core microorganic community
are involved in soil carbon metabolism
Among the three Masson pine families with different carbon sequestration
abilities, the core microorganisms in the rhizosphere soil were
abundant. For bacteria, the core genera mainly includedBryobacter , Acidibacter , Acidothermus andBradyrhizobium (Figure 3). Among them, Acidothermus andBryobacter can directly utilize carbon in soil as a primary
energy source and participate in the soil carbon cycle ( Du, et al.,
2017; Pankratov, et al., 2008; Kalyuzhnaya, Lidstrom, & Chistoserdova,
2008). Therefore, with the increase in carbon sequestration in the
Masson pine families, these genera both became core rhizosphere
bacteria. On the other hand, both Acidibacter andBradyrhizobium are affiliated with Proteobacteria; their relative
abundance declined with increasing carbon sequestration in the Masson
pine families because these two genera are mainly involved in the
nitrogen rather than the carbon metabolism process in soil ( Wang, Lin,
Huang, Yang, & Qu, 2019; Zhong, 2019; Liu, 2019). However, the carbon
sequestration process in Masson pine had no significant impact on the
nitrogen content of rhizosphere soil (Table 2). Therefore, both of these
genera may have become core microbial genera simply because of their
high overall abundance in soil, since Proteobacteria are common in
almost all soil types (Zhang, & Xu, 2008).
In terms of fungi, the core genera of the microorganism community were
fewer, only Russula , Geminibasidium , Saitozyma andPenicillium (Figure S2). Because the structure of the fungal
community was less stable than that of the bacterial community, the soil
environment had a more obvious influence on the fungal community (Chen,
et al., 2018; Xiao, Zhao, Yan, & Guan, 2018). In this experiment, the
core fungal genera were related to the carbon sequestration process in
the Masson pine families to a certain extent. Studies have shown that
carbon sequestration by plants stimulates the growth of Russulaand Geminibasidium and that these fungi regulate the uptake and
accumulation of carbon by plants in a feedback system (Zhou, et al.,
2011; Qiao, Zhou, Chai, Jia, & Li, 2017). The relative abundance ofPenicillium is mainly influenced by the soil carbon content (Hu,
2015; He, 2016). The carbon content of the rhizosphere soil increased
due to carbon sequestration by Masson pine (Table 2), which led toPenicillium becoming one of the core fungi in the microbial
community.
4.3 | The soil properties affect the
microorganic community
Previous studies have shown that the composition of the soil microbial
community is significantly correlated with soil nutrient levels (Tian,
et al., 2017). Nitrogen limitation is common in most terrestrial
ecosystems, and it often leads to intense competition between
microorganisms and plants (Liu et al., 2016) as well as to changes in
the classification and functional characteristics of the microbial
community (Leff et al., 2015). The most typical changes observed in this
study were decreases in the relative abundance of some rhizosphere
fungal communities, such as Acidibacter and Bradyrhizobium(Figure S2). On the other hand, soil moisture is another important
limiting factor in forests and has a strong influence on the soil
bacterial community (Zhang, Liu, Xue, & Wang, 2016), especially
Proteobacteria (Figure 2, Table 2).
Liu et al. (2020) found that the relative abundance of Proteobacteria in
rhizosphere and nonrhizosphere soil under safflower (Carthamus
tinctorius ) was significantly negatively correlated with soil moisture,
which was similar to the results of this study. However, other studies
have noted that the relative abundance of Proteobacteria in the
nonrhizosphere soil of grassland vegetation (including Stipa
bungeana and Artemisia vestita ) was positively correlated with
soil moisture (Liu, Huang, & Zeng,
2016). This suggests that the effects of
soil moisture on Proteobacteria
might also be influenced by vegetation types and rhizosphere effects.
However, the experimental results showed that soil moisture had little
influence on fungi (Figure 4 B). Zhao et al. (2016) demonstrated that in
semiarid grasslands, water restriction reduced the relative abundance of
bacteria but did not change that of fungi, which increased the ratio of
fungi to bacteria. Notably, pine forest soil has a higher carbon content
than soils of other forest types, making it possible to identify fungal
taxa that degrade low-quality substrates (Macdonald, et al., 2009). For
instance, due to the lower substrate mass in coniferous forests, the
diversity of Basidiomycetes and other lignin-degrading fungi was higher
than that in prairie soils with lower carbon content (Allison, Hanson,
& Treseder, 2007). In this study, with the increase in the carbon
content in soil, the relative abundance of Basidiomycetes also increased
(Figure S1 B). Soil pH has been reported to play a key role in
controlling microbial community composition (Cao et al., 2017). However,
in this study, the effects of differences in soil pH among the different
families were not significant; it is speculated that pH has little
influence on the bacterial and fungal communities in Masson pine
rhizosphere soil, except on genera that are relatively sensitive to pH,
such as Saitozyma (Figure 4 B, D).
Notably, previous studies have shown that there is a strong relationship
between rhizosphere microbes and plant root exudates (He, 2017). The
changes in rhizosphere microbial communities among Masson pine families
with different carbon sequestration capacities were discussed in this
study, but the specific effects of rhizosphere exudates on soil
microorganisms under these conditions were not analyzed. With the
increase in carbon sequestration, was there a dominant substance that
influenced the relative abundance of different rhizosphere bacteria or
fungi? What were the patterns and mechanisms of the influence of these
exudates? Were there any interactions between these secretions? These
topics should be addressed in further research.