4.3 Main drivers of fungal community under forest conversion
According to the RDA analysis, shifts of fungal composition were
associated with changes in key soil quality properties, such as SOC
(Fig. 7, Table S5). Fungal diversity was directly regulated by the soil
pH, TP and TN, but indirectly affected by SOC according to the SEM (Fig.
8). Soil pH directly shapes the fungal community by affecting nutrients
uptake over the cell plasma membrane, secretase activity or mycorrhizal
colonization (Deacon, 2006), or affecting multiple soil chemical
parameters such as ion concentrations, base cations and available P. For
example, an increase in pH reduces bioavailable P pool via decreased
production of extracellular enzymes used by soil microorganisms to
access organic phosphate, and then affects mycorrhizal colonization,
biomass and fungal community structure (Carrino-Kyker et al., 2016).
Base cation (e.g., Ca2+) also increased AM (mostly
belonging to Glomeromycota) colonization by affecting spore germination,
mycelia production and hyphal regrowth (Coughlan et al., 2000; Hepper,
1984). Moreover, Ca2+ influences the turnover rate of
soil organic matter, and increases colonization and biomass of
Ectomycorrhizal (ECM) fungi,
particular Basidiomycota (Põlme et al., 2013). Alternatively, soil pH
had an indirect effect on the fungal community. Since the more acidic
soil in Forest inhibits the growth of bacteria, fungi are more
competitive in the use of substrates, thus establishing a variety of
niches (Gunina, Smith, Godbold, Jones, & Kuzyakov, 2017).
SOC, TN and TP were important regulators of fungal communities after
forest conversion (Fig. 8). As non-autotrophic microorganisms, fungi
mainly rely on C sources from fallen leaves, wood detritus and root
exudates, which are the three key ways that aboveground plants affect
soil fungal communities. Firstly, litter quality affected degradation
rates and composition of saprotrophic (mainly Ascomycota) and ECM fungi
(mainly Basidiomycota) (Prescott & Grayston, 2013). Secondly, the
reduction of wood detritus led to a decline in the abundance of
Basidiomycota, which mainly decomposed lignin (Cooke & Rayner, 1984).
Thirdly, some C in soil came from root exudates, including primary
metabolites (sugars, amino acids and organic acids) and secondary
metabolites (Jones, Hodge, & Kuzyakov, 2004). The reduction of organic
substrates in root exudates may alter the fungal community composition
(Prescott & Grayston, 2013). Hence, as the plant diversity decreases,
and understory plants and litter are removed after conversion, the
diversity of organic substrates entering the soil is reduced, thereby
reducing the ability to establish highly diverse heterotrophic fungi
(Peay, Baraloto, & Fine, 2013). Moreover, high levels of soil erosion
in the plantations reduced the SOC and TN stock, and accelerated SOC
mineralization and mineral N leaching, resulting in lowered C and N
contents (Guillaume et al., 2015). Dominant taxa fungal growth was
consequentially reduced. For instance, the abundance of Basidiomycota
decreased with the decrease of C and N content under plantations
(Lauber, Strickland, Bradford, & Fierer, 2008). In addition, P is a key
regulator of the biogeographical pattern of fungal communities. Higher P
content increases plant root growth, mycelia development and
mycorrhizas, especially for special functional groups such as AM and ECM
fungi, thereby enhancing fungal diversity (Baldrian, 2017). Therefore,
after the forest was converted into plantations, the synchronization
effect caused changes in the biodiversity of the fungal community.