4 DISCUSSION
4.1 The resistance of multifunctional and bacterial groups under
different land use intensity
Our study showed that land-use change affected the resistance of
ecosystem functions related to C, N and P cycling, resulting in changes
in ecosystem multifunctional resistance as proposed in our first
hypothesis. It is interesting that the main bacterial species
(>3%) are the same different land uses. This indicates
that the bacterial community has self-recovering ability during the
succession process (Da C Jesus et al., 2009). However, the resistance of
bacterial groups differed among different land use (Fig. 3). This is
consistent with previous studies that different groups showed particular
responses to land use change (Shange et al., 2012; Griffiths, 2012;
Gomez-Acata et al., 2016). The responses are generally classified as
copiotroph and oligotroph (Fierer et al., 2007). Proteobacteria and
Actinobacteria can be categorized as copiotrophic, being well adapted to
nutrient-rich environment (Zhang et al., 2016; Tian et al., 2017;Zheng
et al., 2019). Studies have shown that the higher the organic matter
content and pH in forest soil, the more the system tends to have larger
relative populations of Actinomycetes and Proteobacteria (De Vries et
al., 2012; Ramirez et al., 2014). In this study, the resistance of
Actinomycetes and Proteobacteria in secondary forests were the highest
phylums (Fig. 3a), which is consistent with the higher TOC content in
secondary forests than that in soils that have been cleared and used for
agriculture (AL and CL). Alphaproteobacteria, Betaproteobacteria and
Deltaproteobacteria at class level were in accord with the
Proteobacteria at phylum level (Fig. 3b). In contrast to copiotrophic
groups, Acidobacteria grow rapidly in low nutrient environments
(Navarrete et al., 2015a; Zheng et al., 2019), and is generally
classified as oligotroph. Therefore, the resistance of Acidobacteria in
abandoned land is also higher than in SF in this study (Fig. 3a). And at
the class level, the resistance of Acidobacteria is consistent with
their position at the phylum level (Fig. 3b). Verrucomicrobia has higher
relative abundance in cultivated and abandoned land (Fig. 3a), being
categorized as oligotroph and having strong resilience in low nutrient
conditions (Battistuzzi & Hedges, 2009; Pan et al., 2014). Chloroflexi
has strong adaptability in low-carbon soils (ability to survive
desiccation and low nutrient availability conditions; Battistuzzi,
2009). Bacterial resistance corresponds to its correlation with its
abundance (Fig. 2), that is, the stronger the resistance of the
bacteria, the higher the abundance in the soil, the resistance and
abundance of the bacteria have a close relationship with land use. This
indicates that microbial communities can regulate the stability of
ecosystems based on the resistance of the bacteria.
4.2 The linkage of the composition of microbial communities and
ecosystem multifunctional resistance
The SEM analysis showed important effects of bacterial community
structure on the multifunctional resistance of ecosystems, confirming
our second hypothesis. This is consistent with previous studies
emphasizing that multifunctionality resistance was regulated by changes
in microbial composition (Delgado-Baquerizo et al.,2017b). In the
response to global climate change, microbial functional capacity
displayed a positive linear relationship with multifunctionality
resistance, and buffered negative impacts on ecosystem functioning
caused by climate change via regulating rich microbial communities (Luo
et al., 2019). Our research indicates that in addition to protecting
ecosystems against impacts of climate change, microbial communities also
have a regulatory effect on the negative effects of land use change on
ecosystem functions.
Of 98 bacterial classes selected through random forest models, 20
bacterial species were identified as the main predictors of
multifunctional resistance (Fig.5a). This indicates that
multifunctionality resistance is driven by multiple key microbial taxa,
consistent with De Vries & Shade (2013). Consistent with our second
hypothesis, oligotrophic bacteria (Verrucomicrobia and Chloroflexi)
contributed more to predicting the multifunctional resistance of
ecosystems than copiotrophic bacteria (Fig. 5a). Among bacterial
species, OPB35_soil_group (phylum Verrucomicrobia) contributed the
most (Fig. 5a), and were also selected as main drivers of soil single
function (including DOC, TN, NO3- and TP; Fig. 6). Research shows
Verrucomicrobial abundance is extremely sensitive to changes in chemical
factors linked to soil fertility, such as soil total carbon, nitrogen,
and phosphorus (Navarrete et al., 2015b), which supports our research.
At the same time, Verrucomicrobia may exert a great impact with regard
to nitrogen availability in certain ecosystem, including oligotrophic
environments (Wertz et al., 2011). Chloroflex is another main predictor
of multifunctional resistance, together with 8 other taxa at class level
(Fig.5a). Chloroflexia and Anaerolineae were significantly positively
correlated with β-1,4-glucosidase and phosphatase, promoting starch
degradation and mineralization of organic phosphorus (Table S2).
Ardenticatenia was significantly positively related to
N-acetylglucosaminidase, which promotes the degradation of chitin. These
findings support previous studies that oligotrophic bacteria promote the
resistance of functions related to C cycle (De Vries & Shade 2013;
Trivedi et al., 2013), but our research further shows that oligotrophic
bacteria also contribute to the nitrogen and phosphorus cycles of the
ecosystem. Actinobacteria and Proteobacteria, as copiotrophic bacteria
(Navarrete et al., 2015c; Zheng et al., 2019), also play an important
role in predicting multifunctional resistance (Fig. 5a). Thermoleophilia
affected ecosystem functions by affecting starch degradation, leucine
decomposition and organic phosphorus mineralization (Table S2). Our
research showed Thermoleophilia not only significantly predicted
multifunctional resistance (Fig. 5a), but also TOC, TN, TP (Fig. 6).
Furthermore, Actinobacteria are significantly related to the total
nutrient content in the soil and play an important role in soil carbon
and nitrogen storage. Alphaproteobacteria contribute more for
NH4+ concentrations (Fig. 6),
catalyzing the rate of N cycle and mediating multifunctionality
resistance by regulating NH4+. At the
same time, cyanobacteria are not only predictors of multifunctional
resistance (Fig. 5a), but also predictors of TOC and DOC (Fig. 6), which
indicates that they also participate in the soil carbon cycle. Chamizo
et al. (2018) considered that they play multiple roles in soil including
synthesis of extracellular polysaccharides and contributing to
accumulation of total organic carbon and total nitrogen. In addition,
previous studies have shown that functional genes associated with
nitrification and denitrification are the major predictors of
multifunctional resistance (Luo et al., 2019); consistent with this, our
research also shows that the nitrifier Nitrospira (phylum Nitrospirae)
can predict multifunctionality (Fig. 5a).
4.3 Soil pH and elements driving multifunctional resistance
Besides soil bacterial communities, the SEM further indicated a
significant effect of soil pH and certain elemental concentrations on
multifunctional resistance (Fig. 4). pH is an important driving factor
affecting many soil properties and microbial populations and acttivities
(Fierer et al., 2007) and could affect multifunctional resistance via
direct and indirect effects (Li et al., 2019). In this study, pH had a
significant negative effect on N-related function resistance (Fig. 4e).
This may be because pH significantly influenced substrate availability
(Kemmitt et al., 2006).Pearson correlation analysis shows that pH has a
negative effect on NO3- and NAG (Fig. S2), indicating that in neutral or
slightly alkaline soils (average pH of the soil in the study area is
7.09), pH can reduce NO3- content and
nitrogen functional enzyme activity to reduce N-functional resistance.
Soil vegetation cover and land management directly influence the cycling
of chemical elements, and are key factors for soil biogeochemistry and
also Al behavior in soil (Tejnecký et al., 2020). We found significant
differences in soil Al, Mg and Ca concentration between land uses and
these had significant effects on functional resistance (Fig. 4). Don et
al. (2011) considered that the accumulation of some mineral elements has
a negative impact on multifunctional resistance. Our study also showed
that Al had a negative effect on multifunctional resistance (Fig. 4),
which may be because Al reduced the activity of soil functional enzymes
responsible for carbon, nitrogen and phosphorus transformations (Fig.
S2). At the same time, the differences in soil organic matter content
affect the differences in cation exchange capacity, especially
Al3+ (Jiang et al., 2018). In our
study, aluminum was positively correlated with TOC, DOC (Fig. S2), this
explained the promoting effect of aluminum on C-functional resistance
(Fig. 4). Similar to Al, Mg also promotes resistance to carbon function.
This is because Mg can increase the content of DOC in the soil (Zhu et
al., 2019), significantly correlated with DOC content (Fig. S2).We also
found that soil Ca was significantly related to N-functional resistance
(Fig. 4) and positively related to soil nitrogen content (Fig.
S2).Previous studies have shown a positive correlation between soil Ca
and nitrogen metabolism (Tang et al., 2019), soil
Ca2+ was beneficial to nitrogen
saturation and nitrate leaching in long-term soil nitrogen enrichment
(Perakis et al., 2013), and Ca is related to the process of soil
microbial nitrogen-fixation (Xie et al., 2016).