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.