4 DISCUSSION
4.1
Bacterial, fungal, and protist communities differ among climate and
vegetation types
Bacterial diversity was significantly affected by MAT and vegetation
type, whereas fungal and protist diversity were influenced by the
vegetation type alone. Elevated
temperatures in humid regions enhance SOM decomposition by accelerating
microbial activity and turnover (Allison et al., 2010; Zhao et al.,
2019a). The high soil exchangeable Ca content of karst soil inhibits SOM
decomposition by forming stable aggregates (Briedis et al., 2012;
Wiesmeier et al., 2019). Our previous study found that high exchangeable
Ca during vegetation restoration improves SOC accumulation and negates
the effect of temperature increases on SOC decomposition (Hu et al.,
2021). The fact that MOC was higher under the plantation forest and
shrubland of Guangxi than it was under those of Guizhou suggests that
readily decomposable organic C was more rapidly degraded than
recalcitrant C at high temperatures (Fig. S1; Samal et al., 2020).
Hence, labile organic C decreased with increasing temperature.
Bacterial groups could be
stimulated under the condition of high labile organic C (Poll et al.,
2008). Consequently, there was higher bacterial diversity under the
plantation forest and shrubland of Guizhou than there was under those of
Guangxi. In contrast to bacteria, mycorrhizal and other fungi are highly
dependent on their mutualistic relationships with plants (Newsham et
al., 2009; Schmidt et al., 2014). Fungi utilize the polymerized litter
fraction such as lignin and cellulose (Fontaine et al., 2011). High
plant residue input under plantation forest and shrubland increases
fungal diversity by supplying abundant organic substrates (Chen et al.,
2017; Waldrop et al., 2006). This phenomenon explains our observation
that fungal richness was greater under plantation forest and shrubland
than it was under cropland except for plantation forest at Guizhou.
Conversely, the bacterial Shannon index was higher under cropland than
shrubland. Bacteria have a growth
advantage over fungi in highly fertile agricultural soils (Cai et al.,
2018). Enhanced bacterial diversity under cropland promotes protist
consumers and results in higher protist diversity under cropland than
vegetation restoration after cropland abandonment (Guo et al., 2018).
Overall, these results suggest that high fungal diversity under
vegetation restoration may contribute to SOC accumulation by stimulating
plant residue decomposition.
Temperature and vegetation type influenced bacterial, fungal, and
protist community composition.
Bacterial phylum Proteobacteria
prefer labile organic carbon and their abundance is positively
correlated with C/N (Fierer et al., 2007; Hermans et al., 2017). Here,
C/N was higher at Guizhou than Guangxi
(Fig. S1). Therefore,
Proteobacteria had higher relative abundance at the former than the
latter location. Compared with cropland, shrubland provided greater
available organic C by increasing plant residue inputs. For this reason,
Proteobacteria were more abundant under shrubland than cropland.
Basidiomycota include symbiotic fungi while Ascomycota include
free-living saprotrophs (Baldrian et al., 2011; Read and PerezāMoreno,
2003). Basidiomycota promote plant litter decomposition (Baldrian et
al., 2011; Toljander et al., 2006). Consequently, there was greater
Basidiomycota abundance under plantation forest and shrubland than
cropland. Moreover, Ascomycota abundance was greater under high-AP
conditions (Dang et al., 2017). Thus, relative Ascomycota abundance was
higher under cropland than plantation forest or shrubland because
cropland had comparatively higher AP content (Figs. S1 and S3). In
general, anthropogenic disturbance and environmental pressure are
greater in cropland than plantation forest or shrubland (Santos et al.,
2020). Protist community composition under cropland was markedly
different from that vegetation restoration (Grossmann et al., 2016;
Santos et al., 2020; Schulz et al., 2019). For this reason, Ciliophora,
Lobosa, and Ochrophyta were more abundant under cropland than they were
under plantation forest or shrubland. Certain Apicomplexa are plant
parasites and were more abundant under plantation forest and shrubland
than they were under cropland (Seppey et al., 2020). Taken together, the
foregoing observations indicate that bacterial, fungal, and protist
community composition markedly differ between cropland and vegetation
restoration.
Contrary to our hypothesis, bacterial, fungal, and protist diversity and
community composition under plantation forest and shrubland were similar
in the subtropical karst region.
The southwestern karst region is
geologically fragile, and intense anthropogenic disturbance there has
led to rapid nutrient loss (Wang et al., 2019; Wen et al., 2016; Xu et
al., 2021; Zhang et al., 2013). In general, soil nutrient levels were
limited in the early stages of plantation forest and shrubland
restoration in the southwestern karst region (Hu et al., 2021). Soil
available nutrient levels are key factors regulating microbial diversity
and community composition. As bioavailable nutrient content was similar
between plantation forest and shrubland, they did not differ in terms of
microbial diversity or community composition as vegetation restoration
progressed. Nevertheless, plant diversity was greater in shrubland than
it was in single-species plantation forest
(Hu et al., 2020). These results
suggest that in the karst region with plantation forest and shrubland
restoration, nutrient availability plays a more important role than the
plant community in determining bacterial, fungal, and protist
communities.