Structure
equation modeling (SEM) analysis showed that total soil nitrogen and
total soil phosphorus had direct effects on fungal community diversity,
while grazing exclusion, grassland type and SOC had indirect effects on
fungal community diversity. Fraction had a significant effect on fungal
community diversity by directly affecting total soil phosphorus
(P < 0.05) and grassland type had a significant effect
on fungal community diversity by directly affecting SOC, which in turn
affected KN and TP (P < 0.05). These variables
explained approximately 40% of the variation in fungal community
diversity (Fig. 7a). In addition, combining direct and indirect effects
revealed that soil KN was the most important predictor of change in
fungal community diversity (Fig. 7b).
Fig. 7. (a) Structural equation modeling exploring the direct
and indirect effects of the fraction(grazing and grazing exclusion),
grassland type, plant index(coverage and Patrick index), and soil
chemical properties on fungal diversity and (b) standardized effects of
fraction, grassland type, plant index, and soil chemical properties on
fungal diversity. Line thickness indicates the correlation strength: the
thicker the line, the stronger the correlation. The red and blue arrows
indicate negative and positive relationships, respectively. The numbers
adjacent to the arrows represent the standardized path coefficients. The
proportion of variance explained (R2) appears
alongside each response variable in the model.
Discussion
4.1. Effect of grazing exclusion on soil physicochemical properties in
different grassland types
Whether grazing exclusion can change the diversity and composition of
soil fungal communities, soil chemistry is the dominant factor (Li et
al., 2019; Qin et al., 2021). There is growing evidence that fungal
communities are more sensitive than bacterial communities to carbon and
nitrogen sources. (Zhang et al., 2018). Our study found that C:N did not
differ significantly among the three grassland types in either the 0-5
cm or the 5-10 cm soils before grazing exclusion, probably because
grazing can promote root secretions and fallen leaf decomposition
through trampling, which can increase the amount of C and N entering the
soil and keep the soil C and N in dynamic equilibrium (Bi et al., 2018).
In the 5-10 cm soil layer, whereas the C:N was significantly higher in
temperate deserts than in temperate steppes and mountain meadows after
grazing exclusion, the interaction of exclusion and grassland type
significantly altered the C:N, indicating that grazing exclusion reduced
the soil C and N pools (Sun et al., 2020), and this result was prominent
in steppes and meadows. In addition, we found that grazing significantly
increased SOC in the 0-5 cm soil layer of temperate deserts and
temperate steppes and in the 5-10 cm soil layer of mountain meadows;
moreover grazing significantly increased SWC in the 0-5 cm soil layer of
temperate steppes and in the 5-10 cm soil layer of mountain meadows, due
to a higher soil fertility associated with good plant cover and greater
apoplastic accumulation (Marzaioli et al., 2010; Jing et al., 2014).
Aboveground biomass has a direct effect on apoplastic accumulation,
which increases soil moisture. This explains the increase in SWC in both
temperate steppes and mountain meadows. Furthermore, apoplastic
decomposes more rapidly in moist soils, and large amounts of decomposing
apoplastic can increase soil nutrients (Jing et al., 2014).
Furthermore, soil TP showed a close correlation with temperate desert,
and grazing exclusion significantly increased the TP content in
temperate desert, which is inconsistent with previous studies (Yang et
al., 2018), This difference may be because soil nutrients still had not
returned to their original levels after 9 years of grazing exclusion but
were in an unstable and fluctuating state. This may be related to the
change in the main vegetation types on the surface, the rate of
decomposition and the amount of nutrient uptake and return by plant
roots. The effects of grazing exclusion on pH, C:N, C:P and N:P in the
three grasslands were not significant regardless of the soil stratum.
The reasons for this phenomenon may be the removal of grazing pressure
after grazing exclusion in degraded grasslands, the disappearance of
conditions that maintained the spatial pattern of soil nutrients or
vegetation, an increase in the spatial heterogeneity of soil nutrients,
and an increase in the proportion of stochastic factors, making the
research results more stochastic and the study results appear to be
different (David, 1988). The grazing areas have a higher soil nutrient
content due to the trampling of livestock, which leads to the crushing
of apoplastic and increasing its entry into the soil, thus accelerating
the circulation of organic matter; additionally, nutrients are added
from the excrement of grazing livestock(Zhao, 2022). Regional
differences in the soil itself may also be responsible for this
difference(Yang et al., 2022).
4.2. Effects of grazing exclusion on the composition and diversity of
soil fungi
Our results go further than the commonly reported differences in fungal
composition between grassland types (Zhang et al., 2010; Yang et al.,
2022; Wang et al., 2021). In different grassland types, the response of
fungal composition to grazing exclusion was basically the same. It is
worth mentioning the changes in Mortierellomycota, which we found to be
significantly lower after grazing exclusion in the 0-5 soil layer in
mountain meadows, and highly significant higher in the 5-10 soil layer
in temperate deserts. This is consistent with previous findings that
grazing exclusion is more favorable to the relative abundance of
Mortierellomycota(Wang et al., 2023), as a relatively unique group in
the soil, it may play a role in promoting material circulation in
grazing exclusion grasslands with low nutrient content(Dai et al.,
2021). This answered the second scientific question of the study.
Consistent with previous results (Maestre et al., 2015; Ren et al.,
2017), we found that at the phylum level, Ascomycota andBasidiomycota were the dominant pre-grazing exclusion fungi in
all three grassland types. After the grazing exclusion Ascomycotadecreased by 14.72% and 9.74% in temperate deserts and mountain
meadows, respectively, while it increased by 4.82% in temperate
steppes. In contrast to Ascomycota , the Basidiomycotaincreased by 238.8% and 47.68% in temperate deserts and mountain
meadows, respectively, while it decreased by 23.12% in temperate
steppes. This result may be due to the higher soil pH in temperate
desert, the acidic soils of temperate steppes and mountain meadows, and
the positive responses of Ascomycota and Basidiomycotafungi to changes in soil pH, with a higher abundance ofAscomycota in soils with higher pH (Tedersoo et al., 2014). The
correlation analysis results also confirmed the above view, and the
results showed that pH was significantly positively correlated withDothideomycetes (a class of Ascomycota subphylum), which
also explained the higher relative abundance of Ascomycota in
temperate deserts. Additionally, studies of soil fungi have found that
the soil fungal community has a strong spatial structure along the
ecological gradients, and some fungi can dominate many environments.
Wind-dispersed Ascomycota taxa dominated the soil fungal communities,
followed by Basidiomycota and other fungi(Bucbe et al., 2009; Egidi et
al., 2019). Next, significant differences were found between among the
three grassland types for the Glomeromycota andMucoromycota , and it was found that the Mortierellomycotawas not influenced by either grazing exclusion or grassland type, but
the interaction between the two had a significant effect on theMortierellomycota . It has been shown that the activity ofGlomeromycota is reduced in highly acidic environments and is not
efficient at taking up nitrogen or phosphorus from organic matter and
converting it to inorganic components for plant use; eventually, plants
gradually form new symbiotic relationships with other organic matter
mycorrhizal fungi and replace the Glomeromycota (Smith et al.,
2008). In this study, the organic matter content of the temperate desert
soil was low and the relative abundance of Glomeromycota was
lower than that in the other two types of grassland. This study
validates the findings of most studies regarding the relationship
between Glomeromycota and soil physicochemical properties, and
supports previous studies suggesting that soil organic matter content
has a suppressive effect on Glomeromycota (Pellissier et al.,
2015; Smith et al., 2008). Furthermore, at the class level, the dominant
fungi in the three grassland types before and after exclusion included
Dothideomycetes, Archaeorhizomycetes, Sordariomycetes and
Agaricomycetes, while in the temperate desert, Archaeorhizomycetes
almost disappeared. This result indicates that fungi display incredible
functional diversity at the class levels. Some fungi, which were just as
sensitive as bacteria, disappeared (and were replaced by something else)
in response to the treatment or grassland type. When some taxa
disappear, other taxa are well adapted to the new niches that are
created. The different responses of various fungal classes to
environmental change reflect the role of environmental niche separation
in selecting fungal communities (Pellissier et al., 2014).
Unlike the effect of grazing exclusion on soil fungal composition,
grazing exclusion, grassland type and the interaction between the two
did not significantly alter soil fungal α-diversity in either the 0-5 cm
or the 5-10 cm soils, similar to previous findings (Hao et al., 2020;
Yang et al., 2019; Ding et al., 2020). The response of fungal
α-diversity to grazing exclusion and grassland type was not significant,
essentially meaning that there were no significant differences in the
taxonomic richness among the sites.(Cheng et al., 2016; Zhang et al.,
2018). These results were consistent with previous findings that soil
fungi were more stable than bacteria (Hamonts et al., 2017; Wang et al.,
2019). Although there was no effect on fungal α-diversity, we found that
the interaction of grazing exclusion and grassland type significantly
altered fungal β-diversity. Changes in the β-diversity of fungi may
indicate the ability of many soil microorganisms to colonize the
inter-rhizosphere, where root biomass and plant C:N ratios may influence
the utilization of inter-rhizosphere resources by altering root turnover
or root secretions, which may in turn strongly alter the composition of
the microbial community (Chase, 2010; Philippot et al., 2013). Moreover,
other studies have detected significant effects of grazing exclusion on
soil fungal β-diversity (Chen et al., 2020). In the current study, the
strong link between soil microbial diversity and grazing exclusion and
grassland type can be explained by the fact that microbial communities
are often directly influenced by aboveground plant biomass and community
structure (de Vries et al., 2012). For example, grazing exclusion
increases aboveground plant biomass and apoplastic content, thus
increasing fungal diversity (Hamonts et al., 2017). The increase in
species richness induced by grazing exclusion suggests that high plant
diversity results in a greater variety of organic matter ultimately
flowing into the subsurface, thereby generating more ecological niches
for use by different species of microorganisms and altering the
diversity of microbial communities (Lodge, 1997; Brodie et al., 2003).
The results showed that under the influence of different grazing methods
and different grassland types, the soil fungal community composition
changed, and the difference in β diversity was significant, but the
change in soil fungal α-diversity was not significant, which answered
the first scientific question of this study.
4.3. Potential drivers of soil fungal community diversity
SEM analysis showed that grazing exclusion had a significant effect on
fungal community diversity by directly affecting total soil phosphorus,
and that grassland type had a significant effect on fungal community
diversity by directly affecting SOC, which in turn affected KN and TP.
In line with our findings, other studies have also shown that soil TP
and KN are closely related to soil fungal diversity (Lindahl et al.,
2015; Lauber et al., 2008). In contrast, a recent experiment showed that
soil fungal abundance and diversity differed between grassland habitats,
but that this difference was not correlated with geographical distance,
and that certain environmental factors, including climate, soil pH,
nitrogen and phosphorus, were the main influences affecting the
distribution, abundance and diversity of soil fungal communities
(Pellissier et al., 2014). In our study, combining direct and indirect
effects revealed that soil KN was the most important predictor of
changes in fungal community diversity, and the strong association
between soil KN and fungal diversity was justified because KN provides
the main energy source for soil microorganisms (Lindahl et al., 2015).
For example, nitrogen in the soil provides sufficient elemental nitrogen
for fungal symbiosis, and plants no longer import too much carbon into
the subsurface for nitrogen acquisition, which can have an impact on
fungal composition and biomass (Norris et al., 2013). However, it has
been shown that fungal diversity does not correlate with soil KN and TP,
suggesting that environmental factors may not be limiting factors in the
structural diversity of soil fungal communities at the larger scales
studied (Li., 2015). In the present study, SOC had an indirect effect on
fungal community diversity, similar to the results of previous studies
(Li., 2015). SOC provides nutrients and energy for plant growth and soil
microbial life (Li et al., 2006), and its content reflects the
effectiveness of limiting resources in the soil and is the ‘source’ of
nutrients for soil fungal community growth, the level of which affects
soil fungal community structure and diversity. In addition, the natural
environment in different regions can also affect soil fungal diversity,
and harsh environments such as drought can reduce fungal diversity and
abundance (Maestre et al., 2015). In summary, the results answered a
third scientific question: soil nutrient content was a major factor
affecting fungal diversity, especially soil KN, TP and SOC. However,
this paper still has some limitations, and our results do not account
for seasonal and annual differences in soil fungal communities, which
still need to be analyzed further in depth. Future experimental studies
could explore the potential effects of climate change on soil fungal
community diversity and microbial assembly processes in different
grazing exclusion years or seasons to provide a more theoretical basis
for clearing the blind spots in this field.
Conclusions
Grazing exclusion is one of the most commonly used measures to
rehabilitate degraded grasslands globally. Therefore, assessing its
ecological impact is important for grassland conservation and
biodiversity management. As revealed by the data from our study, the
composition of soil fungal communities differed between grassland types
More importantly, the dominant fungal taxa such as Glomeromycetes,
Orbiliomycetes, Tremellomycetes, Ustilaginomycetes, Eurotiomycetes,
Orbiliomycetes, Dothideomycetes, and Archaeorhizomycetes shed
new light on the involvement of soil microbes in different grassland
types. The response of fungal composition to grazing exclusion was
basically the same in the different grassland types, except for theMortierellomycota . Under the influence of both grazing exclusion
and grassland type, there was no significant change in soil fungal alpha
diversity, but there were significant differences in fungal beta
diversity. The changes in soil KN and TP caused by grazing exclusion
were closely related to soil fungal diversity. These results suggest
that soil nutrient content was the main factor influencing fungal
diversity, especially soil KN, TP and SOC. Our research provides a
long-term perspective for better understanding and managing different
grasslands, as well as a better scientific basis for future research on
grass-soil-microbe interactions.