4.1 Effects of forest conversion on soil quality
The soil C, N contents dropped by 83% and 59% after conversion, respectively (Fig. 1), similar to previous studies that were focused on rainforest and temperate forest conversion into monoculture plantations (Guillaume et al., 2015; Yang et al., 2018). Firstly, soil erosion increased when understory vegetation and litter layer were removed after conversion, coupled with a large amount of precipitation in the region. These factors caused a sharp drop in C and N contents (Guillaume et al., 2015). Secondly, the decomposition of SOC increased due to nutrient release from the dead or burnt biomass, and the labile organic matter released from destroyed soil aggregates (Berhe, Harte, Harden, & Torn, 2007). Therefore, soil SOC content decreased following conversion. Third, C and N were reduced in plantations because low plant diversity prevented the formation of developed organic horizons, and reduced the input of labile organic compounds, leading to a decrease in microbial biomass (Mcguire et al., 2015). In contrast, soil pH increased after conversion, which was ascribed to three factors: 1) Conversion reduces plant species, resulting in a decline in root and ectomycorrhizal exudates which contain a lot of organic acids (Grayston, Vaughan, & Jones, 1997). Thus, soil pH increased after conversion. 2) Soil erosion removes the surface layer, which is usually more acidic than the subsoil. 3) The increase of exchangeable cations results in an increase in soil pH, as a fertilizer following conversion (Tripathi et al., 2016). In addition, soil TP, AP, TK, AK were the highest in the Peach plantation (Fig. 1) because of intensive application of mineral and organic fertilizers.
Compared with Forest, the enzymatic activity of SC, ACPT, GLS, UR, ACP and ALP decreased in plantations (Fig. 1). Soil SC is a C-degrading enzyme involved in organic matter hydrolysis. Thus, the SC activities declined with the reduction of SOC after conversion. Soil ACPT, UR and GLS are key enzymes involved in N mineralization. ACPT acts on proteolysis and breaks down organic N such as proteins and peptides into amino acids (Watanabe, 2010). UR converts organic N to mineral nitrogen that can be used by plants and microorganisms (Fraser, Hallett, Wookey, Hartley, & Hopkins, 2013). In the Forest, high SOC content accelerates N mineralization, resulting in higher N mineralization activity, including ACPT and UR (Yang, Yang, & Yu, 2018). Soil GLS involves the hydrolysis of L-glutamine to produce L-glutamic acid and NH3, and its activity is positively correlated with organic C and total N (Frankenberger & Tabatabai, 1991). Thus, the decrease in GLS activity is attributed to the drop in SOC and TN contents in plantations. After conversion, the phosphatase activity decreased with the increase of AP content. Compared with plantations, lower AP stimulated the microbial community to secrete phosphatase to mobilize P from organic sources, leading to higher phosphatase activity in the Forest (Wu et al., 2019).
The decline in soil quality after conversion, revealed by the SQI, showed that GLS was the biggest contributor to SQI, followed by SOC and TP (Fig. 2, Table S2). GLS is a key enzyme of N metabolism, as it catalyzes the hydrolysis of NH3 group from amino acid and degrades L-glutamine into L-glutamic acid and ammonium ions, which maybe a reliable indicator of soil fertility (Kanazawa & Kiyota, 2000). Hence, this enzyme could be used as a biological indicator of SQI in subtropical areas. SOC is often used as a proxy of soil quality and productivity due to its key role in multiple soil processes such as nutrient cycling as well as plant and microbial growth (Raiesi, 2017). A big decrease in GLS activities and SOC contents was responsible for the sharp drop in SQI after conversion. Forest conversion accelerated soil erosion, reduced organic matter inputs and C, N contents, resulting in a decrease in SQI. TP was another key indicator that contributed to SQI (Zhang et al., 2019). Among the four plantations, SQI of Peach and Berry was superior than the other two, indicating that fertilization was beneficial for soil quality after conversion by increasing TP contents.
4.2 Effects of forest conversion on fungal communities
After forest conversion, α-diversity of fungi increased in Berry and Peach plantations (Fig. 3a-c), and there were significant differences in community structure among the five types of plantations (Fig. 3d). The increase of α-diversity was consistent with the conversion of which forest to which plantation (Nakayama et al., 2019). We explain this as follows: 1) Acidity reduced the taxonomic diversity of the arbuscular mycorrhiza fungi (AM) by decreasing sporulation (Coughlan, Dalpé, Lapointe, & Piché, 2000). Hence, the gradual increase in soil pH led to a higher diversity in plantations (Fig. 3a-c, Fig. 8). 2) Fertilization was responsible for the increase of α-diversity in Berry and Peach. The continuous application of manure increased fungal biomass and diversity by increasing substrate variety and C input (Kamaa et al., 2011). The exogenous fungi from the manure (organic resources) may increase fungal diversity of plantation (Sun et al., 2016). According to the β-diversity, notable divergence was observed in the composition between Forest and four plantations (Fig. 3d). Forest conversion affects fungal community structure via changes in soil chemical properties (e.g., pH, C, N contents), root exudates and litter input (Prescott & Grayston, 2013).
Due to specific functional attributes and survival strategies, the abundance of dominant fungi responded differently to forest conversion (Fig. 4, 6). For instance, the abundance of Basidiomycota decreased while that of Ascomycota and Zygomycota increased. We can characterize these fungi based on Grime’s C-S-R (Competitor, Stress-tolerant, Ruderal) life history strategy to explain succession dynamics and the spatial structure of fungal assemblages (Chagnon, Bradley, Maherali, & Klironomos, 2013; Grime, 1979). Zygomycota (R-strategists) are primary saprotrophic fungi with ruderal strategies, which are only active in habitats with low levels of competition and stress. They are characterized by a short growth period, high reproductive potential, and effective use of accessible substrates (Cooke & Rayner, 1984). Thus, Zygomycota are more abundant in plantations with low C and N content. Basidiomycota and Ascomycota are secondary saprotrophic fungi, and their cbhI gene is a reliable indicator of cellulolytic ability. Both fungi groups can degrade part of plant residues and promote soil C accumulation (Stursova, Zifcakova, Leigh, Burgess, & Baldrian, 2012). By contrast, most Basidiomycota (C-strategists) are white and brown rot fungi, which are late-successional and oligotrophic species with slow growth and favor resource-rich conditions and better substrate quality (high C/N). They are usually involved in decomposition of lignified plant detritus (Lundell, Makela, & Hilden, 2010). In addition, the lignin content in old forest litter is high (Osono, 2006). Therefore, C-strategists Basidiomycota decrease with reduction of C/N ratio and litter after conversion. Subsequently, Basidiomycota such as the typical ectomycorrhizal group (Russula) and other genera belonging to the Basidiomycota declined sharply. In contrast, Ascomycotas are S-strategy fungi, with survival related stress tolerance characteristics, and use resources more efficiently under low-nutrient conditions. So they exhibit a more stress-tolerant strategy to C substrate than Basidiomycetes (Wang et al., 2021). The Ascomycotas are mainly soft rot fungi that can decompose complex organic matters such as cellulose, hemicellulose and the lignin in the soil (Stursova et al., 2012). As a fast-growing copiotrophic fungi, the S-strategists Ascomycota increased with stressful conditions (low C, N contents) in plantations. Thereby, the abundance of genera affiliated Ascomycota:Pseudophialophora andRhytisma increased. Moreover, among the root-associated fungi, the change in the abundance of Ascomycota related to Basidiomycota also implied that the ratio of ericoid mycorrhizal fungi (mainly Ascomycota) and ectomycorrhizal (mainly Basidiomycota) had changed (Sterkenburg, Bahr, Brandstrom, Clemmensen, & Lindahl, 2015).