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).