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.