1 Introduction
Worldwide, half of primary and secondary forests, which are crucial for maintaining biodiversity and ecosystem functions, are being converted into monoculture plantations for the cultivation of timber, fuel materials and other commodities. The annual expanding rates of monoculture plantations have increased to 2% over the past 25 years (FAO, 2015). Most conversion from natural forests to plantations has occurred in subtropical regions(Lan et al., 2021) . Such conversion results in alterations of ecosystem services and functions along with changes in plant diversity and soil nutrients loss (Krashevska, Klarner, Widyastuti, Maraun, & Scheu, 2015; Yang, Zhu, Xu, & Zheng, 2018). Soil quality is defined as the capacity to sustain biological productivity, maintain environmental quality, and promote plant and animal health (Doran & Parkin, 1996). Various methods such as visual qualitative and quantitative approaches have been used to assess soil quality (Bünemann et al., 2018; Yakov et al., 2020). Quantitative evaluation incorporates chemical, physical and biological indicators into the calculation of soil quality index (SQI). Forest conversion may lead to reduced vegetation coverage and forest productivity, soil quality deterioration and erosion (Guillaume, Damris, & Kuzyakov, 2015), all of which will have a negative impact on soil quality and health (Zarafshar et al., 2020). Intensive management and short rotations are employed during conversion, including site reclamation, clear-cutting, residue mulching, herbicide weeding and fertilization. These practices can modify soil quality, but the consequences for microorganisms and nutrient cycling are not clear (Armenise, Redmile-Gordon, Stellacci, Ciccarese, & Rubino, 2013). Thus, comprehensive and quantitative studies are needed to better understand the impacts of forest conversion on soil quality and functions in subtropical areas.
In forest ecosystems, fungi play fundamental ecological roles as decomposers, symbionts, and tree pathogens (Uroz, Buée, Deveau, Mieszkin, & Martin, 2016). They regulate organic matter decomposition and nutrient cycling, and stimulate plant growth (Nakayama, Imamura, Taniguchi, & Tateno, 2019). The establishment of fungal community is determined by both abiotic (i.e. climate, spatial heterogeneity and edaphic conditions) (Steidinger et al., 2019; van der Linde et al., 2018) and biotic (i.e. host species) factors (Duhamel et al., 2019). The conversion from natural forest to plantations is accompanied by changes in the identity and diversity of host tree species and soil properties, thereby affecting the fungal community and tropical-guild composition (Mcguire et al., 2015). Compared with extensive studies on bacterial communities (Kerfahi, Tripathi, Dong, Go, & Adams, 2016; Liu et al., 2020), only a few studies have evaluated the response of soil fungi to forest conversion. In addition, microbial activity is a valuable indicator of soil quality (Bünemann et al., 2018). For instance, microorganisms can maintain soil P bioavailability (Wu et al., 2019) and alleviate N limitations (Johnson, Rowland, Corkidi, & Allen, 2008). Hence, it is necessary to evaluate the impacts of forest conversion on the fungal community, which play a vital role in shaping ecosystem functions (e.g., soil nutrient cycling, carbon sequestration).
The aim of our study was to quantify the changes in soil quality and the fungal community following forest conversion. Physico-chemical and biological properties as well as the microbial community composition were measured under a natural forest and four monoculture plantations common for subtropical climate. The objectives were as follows: (1) To investigate the impacts of forest conversion on soil quality and assess the main influencing factors; (2) To evaluate the differences in diversity, composition, and key taxa of fungal community after conversion; (3) To clarify the relationship between soil quality and the fungal community, and explore main driving factors of community changes after conversion.