Study area
We collected 238 soil samples from 29 forest sites with a wide array of forest ecosystems (tropical, subtropical, temperate and boreal forests) across 3,800 km (Fig. 1). These sites span a latitudinal range from 18.7°N to 51.5°N and have distinct soil and site characteristics with the mean annual temperature (MAT) ranging from –5.2 °C to 24.4 °C and the mean annual precipitation (MAP) ranging from 468 mm to 1810 mm (Table S1). These forests were divided according to their dominant tree species into broadleaved (n = 86), coniferous (n = 93), and broadleaved-coniferous mixed forests (n = 59). Forest was also categorized into four biomes according to latitudes of the study sites, including tropical, subtropical, temperate and boreal forests (e.g., Feng & Zhu, 2019). For each site, information relating to geography (latitude and longitude), climate (MAT and MAP), forest types, and soil chemical properties was obtained (Table S1).