Spatial variation in soil microbial respiration
Across all soil samples, the mean of Rm was 20.7 mg C kg-1 soil for 3-day incubation with a wide range of 3.3 to 98.2 mg C kg-1 soil (Fig. 2a). No simply linear correlation of Rm with latitude along the transect across all forests (r = 0.118, p > 0.05), instead, a hump-shaped one with the maximum at latitude = 32.5°N was observed. Interestingly, Rm significantly increased with latitude in the latitude < 32.5°N region (r = 0.243, p < 0.05) and tended to decrease in the latitude > 32.5°N region (r = -0.166, p < 0.05) (Fig. 2a). Thus, latitude = 32.5°N was the threshold where latitude and Rm interactions diverged. Furthermore, we also found that hump-shaped pattern of Rm with latitude (r = 0.488, p < 0.01) was observed across broadleaved forest soils (Fig. 2b), but no relationship was observed across coniferous (Fig. 2c) and mixed forests (Fig. 2d). Soils in broadleaved forests and coniferous forests had significantly higher Rm values than those in mixed forests (Fig. 3a), while soils in tropical forest had significant lower Rm than those in other biomes (Fig. 3b).