Spatial variations in soil microbial respiration along latitude at different scales
In this study, we investigated Rm along a 4,200 km and found that Rm ranged from 3.3 to 98.2 mg C kg-1 soil with a mean of 20.7 mg C kg-1 soil within 3-day incubation (Fig. 2a), which was within the general range of the Rm measured in laboratory (Li et al., 2018; Priess & Fölster, 2001). Our results suggested that Rm values had large spatial variations across China forest ecosystems. To the best of our knowledge, we for the first time revealed a hump-shaped relationship between Rm and latitude with a threshold at latitude = 32.5 °N at the continental scale (Fig. 2a), which was contrary to the general pattern previously reported (e.g., Colman & Schimel, 2013; Wang et al., 2016) and our first hypothesis that Rm would linearly change with increasing latitude. The hump-shaped relationship between Rm and latitude in broadleaved forests (Fig. 2b) may have large contribution to the opposite relationship between Rm and latitude for below and above latitude = 32.5 °N at the continental scale across China forest ecosystems. The opposite relationship between Rm and latitude for below and above latitude = 32.5 °N suggests that climate has less importance in shaping the Rm’s spatial variation, and controlling factors and potential mechanisms of Rm are different in areas below and above this threshold.
The threshold at latitude = 32.5 °N occurring at the continental scale can be potentially explained by the following reasons. The biome near to the latitude = 32.5 °N was a sensitive and transitional zone that climate is from subtropics to temperate and dominated vegetations from evergreen species to deciduous species (Jing et al., 2019). This ecotone will lead to a peak of soil bacterial diversity (i.e., richness) (Liu et al., 2020). Alternatively, soils in this zone were characterized with higher C availability and SOC quality, which was supported or reflected by the latitudinal pattern of LOC contents (Fig. S2) and lower C:N ratios of leaves and roots in mid-latitudinal regions (Zhang et al., 2020), resulting in the higher Rm than other regions.