2.1 Site description and soil sampling
The Jinchuan Forest Farm (42°20’N, 126°24’E) in Huinan County, southwest
of Changbai Mountain, is located in Northeast China (Fig. S1). The total
area is 22.5 km2; the average altitude is 655 m; the
annual average temperature is 3.4 °C, and the annual average
precipitation is 1053.9 mm. Soil samples were collected in July 2020. We
selected three L. gmelinii plantations with different restoration
years (10, 30, and 50 years). Homogenous climatic conditions throughout
the study site made it possible to compare and represent plantations
with a different number of restoration years and the associated soil
microbial communities.
Three sample plots (30 × 30 m) were randomly selected at each study
site. Soil samples were collected from five layers (10, 20, 30, and 40
cm) along with the soil profile. The samples were screened to 2 mm,
stored at 4 °C, and transported to the laboratory for immediate
analyses. A total of 36 samples were collected (3 cultivation years of
plantation × 3 repeated samples × 4 layer depths). Each sample was
divided into three sub-samples, namely, (1) those air-dried at room
temperature (20 °C) for analyses of pH, total nitrogen (TN),
soil water content (SWC), SOC and
DOM; (2) those stored at 4 °C for analyses of soil ammonia nitrogen
(NH4+) and nitrate nitrogen
(NO3-); and (3) those stored at -80 °C
for analysis of eukaryotic and prokaryotic microorganisms.