2.1 Study site and sampling
The study sites were located in Lutou National Station for Scientific Observation and Research of Forest Ecosystems, Hunan Province, Central South China (113°51′52″~113°58′24″E, 28°31′17″~ 28°38′00″N). The region has a typical mid-subtropical monsoon climate with an annual temperature of 15.8 °C and an annual precipitation of 1,450 mm. The soils are characterized as Ferric and Orthic Acrisols (Liu et al., 2020). The subtropical natural forest is dominated by Cinnamomum camphora , Liquidambar formosana , Pinus massoniana , Photinia serratifolia , Ilex chinensis ,Broussonetia papyrifera , Loropetalum chinense andCamellia japonica. A large area of the natural forest (Forest) was cleared and reclaimed to monoculture plantations includingCamellia oleifera (Oil), Amygdalus persica (Peach),Myrica rubra (Berry) and Cunninghamia lanceolata (Fir) in 2013 (Liu et al., 2020). After conversion, management practices such as site reclamation, fertilization and weeding were adopted. For the Berry and Peach plantations, 4,356 kg ha-1 yr-1 organic fertilizers (manure) were applied in October, and 545 kg ha-1yr-1 compound mineral fertilizers (N/P/K ratios: 1:1:2) were additionally applied to the Peach plantation in July each year. Only 163 kg ha-1 yr-1 compound mineral fertilizers were applied to the Oil plantation in May each year. The Fir plantation was subject to no management practice.
In November 2018, three replicate plots (10 m × 10 m each one) were randomly established for each forest type according to slope. After removing the litter layer within each plot, soil was sampled from the 0-20 cm layer, homogenized and sieved with a 2-mm mesh. In total, 15 composite soil samples were divided into three parts: one was dried at room temperature for soil physico-chemical analysis; the other two were kept at 4 ℃ and -80 ℃ for enzyme activity and molecular analysis, respectively.
2.2 Analysis of soil