Field sampling and soil analysis
Soils were collected from each site from late July to early August, which is within the plant growing season. At each site, at least one typical forest was selected. In each forest with 3-5 plots, the upper 10 cm of mineral soil was sampled from 8–10 locations within a plot (100–150 m2), coarsely sieved to 5.6 mm to minimize disturbance, and gently combined to produce a composite sample. All soil samples were transported with a low-temperature incubator with ice (< 5 °C) to laboratory, where they were sieved to 2 mm, and then homogenized. One part was stored at 4 °C for incubation and microbial measurement, and the rest was air-dried for chemical determination.
In this experiment, the initial SOC and the total N contents were determined using an elemental analyzer. Labile organic C (LOC) which is oxidized by 333 mM potassium permanganate was measured according to the method described by Blair et al. (1995). Soil pH was measured with a pH meter from a soil slurry with 1:2.5 ratio by volume of soil to distilled water. The soil texture (sand, silt, and clay contents), mineral N, total and available P, and exchangeable cations (K+, Na+, Ca2+, Mg2+) were determined according to the common methods described by Lu (2000).
Microbial biomass and community composition was determined through phospholipid fatty acid (PLFA) analysis described by Bardgett et al. (1996). Phospholipids were extracted from 1.5 g of soil fresh weight and analyzed using an Agilent 6890 Gas Chromatograph. Gram-positive bacteria were identified by the terminal and mid-chain branched fatty acids (i15:0, a15:0, i16:0, i17:0, a17:0), and cyclopropyl saturated and monosaturated fatty acids (16:1ω7c, cy-17:0, 18:1ω7c, 8cy-19:0) were considered indicative of gram-negative bacteria (Rinnan & Bååth, 2009). Fungi were the sum of 18:1ω9c, 18:1ω9t, and 18:2ω9,12c, and actinomycetes were the sum of 10Me16:0, 10Me17:0 and 10Me18:0 (Moore-Kucera & Dick, 2008). The total PLFA concentration was calculated from the identified PLFAs (15:0, 14:0, 16:1, 16:1ω5, 16:0, 17:1ω8, 7Me-17:0, br17:0, br18:0, 18:1ω5, 18:0, 19:1; and those listed above). The ratios of fungal to bacterial (F:B) PLFA and gram-positive to gram-negative (GP:GN) PLFA were taken to represent the relative abundance metrics of these groups.