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.