Results
Biome-level FBC, BBC, and F:B
Ratio
There are large variations in biome-level FBC, BBC, and F:B ratio
(Table 1; P < 0.001 for FBC, BBC, and F:B
ratio among biomes). Deserts exhibited the lowest FBC of 16.9 (95%
range: 14.4~19.9) mg kg-1 and BBC of
6.8 (6.1~7.7) mg kg-1, while tundra
habitats displayed the highest FBC of 3683.6
(1678.5~8083.9) mg kg-1 and BBC of
428.4 (237.0~774.3) mg kg-1. Boreal
forests had significantly higher FBC than tropical/subtropical forests
and temperate forests (1234.0 mg kg-1 for boreal
forests vs. 258.4 mg kg-1 for temperate forests and
451.4 mg kg-1 for tropical/subtropical forests).
Boreal forest and tropical/subtropical forests have significantly higher
BBC than temperate forests (226.4 mg kg-1 for boreal
forest, 210.9 mg kg-1 for tropical/subtropical forest
vs. 53.0 mg kg-1 for temperate forest), with no
significant differences in BBC were found between boreal forests and
tropical/subtropical forests (Table 1 ). Pastures had
significantly higher FBC and BBC than grasslands (632.2 mg
kg-1 soil vs. 215.2 mg kg-1 soil for
FBC and 270.7 mg kg-1 soil vs. 62.7 mg
kg-1 soil for BBC); While we did not find differences
in FBC across unvegetated ground, cropland, shrub, savanna, and natural
wetlands; BBC was significantly higher in wetlands than in unvegetated
ground (Table 1 ).
The F:B ratio varied less across biomes, with the lowest values in
savannas and greatest values in tundra habitats (1.8 for savanna vs. 8.6
for tundra). We also found significantly higher F:B ratio in boreal
forests and temperate forests than that in tropical/subtropical forests
(5.0 for boreal forest, 4.9 for temperate forest vs. 2.2 for
tropical/subtropical forest). No significant differences in F:B ratio
were found across natural wetlands, unvegetated grounds, deserts, and
shrubs (Table 1 ).
Biogeography of FBC, BBC, and F:B
ratio
Both FBC and BBC exhibited inverse unimodal relationships with latitude,
with lowest values at mid-latitudes (Fig. S2a-b ; P< 0.0001 for both FBC and BBC along latitude), whereas the F:B
ratio was positively correlated with latitude (Fig. S2c ;P < 0.0001). Of climatic predictors, MAT showed an
inverse unimodal relationship with FBC, with the lowest at 14-15°C
(Fig. S3a ; P < 0.0001). Conversely, BBC showed
no significant correlation with MAT (Fig. S3b ; P =
0.19). The F:B ratio showed a significantly negative linear relationship
with MAT (Fig. S3c ; P < 0.0001).
Both FBC and BBC showed unimodal relationships with MAP, with peak FBC
and BBC at approximately 2100-mm y-1 and 3000-mm
y-1, respectively. While F:B ratio linearly decreased
with MAP (Fig. S3d-f ; P FBC <
0.0001, P BBC < 0.0001,P F:B ratio < 0.0001). FBC increased in
a non-linear manner with SM, while BBC linearly increased with SM
(Fig. S4a-b ; P FBC < 0.0001,P BBC < 0.0001). Both FBC and BBC
linearly increased with ST (Fig. S4d-e;P FBC < 0.0001,P BBC < 0.0001). F:B ratio increased
with SM (Fig. S4c ; P < 0.0001) but decreased
with ST (Fig. 4f ; P < 0.0001).
Vegetation controls on microbial biomass C differed in fungi and
bacteria. While BBC significantly increased with Croot(Fig. S5a-b ; P FBC = 0.2,P BBC = 0.00035), no significant correlation
between FBC and Croot occurred. The F:B ratio exhibited
a unimodal correlation with Croot, with the peak F:B
ratio associated with the Croot of 6.9 kg
m-2 (Fig. S5c ; P < 0.0001).
Both FBC and BBC linearly increased with NPP, while F:B ratio linearly
decreased with NPP (Fig. S5d-f ; P FBC =
0.011, P BBC < 0.0001,P F:B ratio< 0.0001).
Microbial biomass was well correlated with edaphic factors. Both FBC and
BBC exhibited inverse unimodal relationships with SOC, with minimum FBC
and BBC at SOC of 142.1 and 222.7 g kg-1, respectively
(Fig. S6a and b ; P FBC <
0.0001, P BBC = 0.0017), while F:B ratio linearly
increased with SOC (Fig. S6c ; P < 0.0001).
Both FBC and BBC linearly increased with TN, while F:B ratio exhibited
unimodal relationship with TN, with the maximum F:B ratio at TN of 25.4
g kg-1 (Fig. S6d-f ;P FBC < 0.0001,P BBC = 0.011, P F:B ratio< 0.0001). Both FBC and BBC showed unimodal relationships with
SOC:TN (C:N) ratio, with the maximum FBC and BBC at C:N ratio of 20.1
and 17.7, respectively (Fig. S6g-h ; P FBC< 0.0001, P BBC < 0.0001),
while F:B ratio showed inverse unimodal relationship with C:N ratio,
with minimum F:B ratio at C:N ratio of 7.1 (Fig. S6i ;P F:B ratio < 0.0001). In addition, both
FBC and BBC showed inverse unimodal relationships with soil bulk
density, with minimum FBC and BBC at bulk density of 1.5 and 1.4 g
cm-3, respectively, while F:B ratio linearly decreased
with bulk density (Fig. S6j-l ; P FBC< 0.0001, P BBC = 0.00035,P F:B ratio < 0.0001). Furthermore, we
found that FBC, BBC, and F:B ratio all showed inverse unimodal
relationships with soil pH, with minimum FBC, BBC, and F:B ratio at soil
pH of 7.5, 7.4 and 6.3, respectively (Fig. S6m-o ;P FBC < 0.0001,P BBC < 0.0001,P F:B ratio < 0.0001). We also found the
highest FBC and BBC in clayey s, but the highest F:B ratio in sandy soil
(Fig. 6p-r; P FBC < 0.0001,P BBC < 0.0001,P F:B ratio < 0.0001).
Quantitative Assessment of Controls on Microbial
Biogeography
We constructed generalized linear models to disentangle the effects of
climate (MAP and MAT), plant (NPP and Croot), soil
microclimate (SM and ST), and edaphic properties (SOC, TN, soil pH,
clay, sand, and bulk density), on the variation in FBC, BBC, and F:B
ratio. The variance inflation factor (VIF) test revealed no
multicollinearity among variables. Environmental factors in total
explained a large proportion of variation in microbial biomass (81.9%
for FBC, 84.8% for BBC, and 71.2% for F:B ratio) (Fig. 2 ).
Notably, the edaphic properties were the most important drivers in FBC
and BBC, with 66.4% and 70.4% of the variations in FBC and BBC being
explained by edaphic properties and the interaction with other factors,
respectively (Fig. 2a-b ). Complex interactions between the
groups of variables explained 23.7% of the variation in FBC
(Fig 2a ). In contrast, variation in BBC was explained primarily
by the interactions between edaphic properties and climate (13.9%),
multiple interactions (11.91%), and edaphic properties alone (10.22%).
Climate was the most important predictor of F:B ratio. Climate alone
explained 11.6%, and climate interactions with other variables
explained 35.5% of the variation in the F:B ratio (Fig. 2c ).
Global Carbon Storage in Fungal and Bacterial
Biomass
Based on our findings of environmental controls on FBC and BBC at the
biome and global scales, we further developed an empirical model for F:B
ratio considering the higher proportion of missing data in FBC (14.8%)
and BBC (16.3%) relative to F:B ratio (1.9%) (Materials and
Methods; Table S2 ). Combined with a global microbial biomass C dataset
reported by Xu et al. (2013), we further produced global maps of
BBC and FBC in topsoil (Fig. 3 ). The global FBC and BBC are
estimated to be 12.56 (6.64~16.42) Pg C, and 4.34
(0.47~10.26) Pg C in BBC for 0-30 cm topsoil. Taking the
global estimates of SOC (684~724 Pg C in 0-30 cm),
approximately 1.8% and 0.6% of SOC is stored in soil fungi and
bacteria, respectively. The highest FBC density occurs in northern
high-latitude regions while lowest values are characteristic of
low-latitude regions (Fig. 3b ). On the contrary, the highest
BBC was found in high-latitude and equatorial regions, and the lowest in
mid-latitude regions (Fig. 3c) .
At biome-level, boreal forests stored large FBC (3.60 Pg C) and
tropical/subtropical forests had the largest BBC storage (0.85 Pg C),
while shrub contributed least to both FBC and BBC (0.39 Pg C for FBC and
0.14 Pg C for BBC) (Table 2 ). Although boreal forests do not
occupy the Earth’s largest surface area (11.82 million
km2), the high FBC density contributes to its
prominent FBC storage. The large microbial C storage in pasture was
primarily due to its large area (27.0 million km2).
Tropical/subtropical forests have relatively high BBC density, along
with the second largest area (16.44 million km2),
tropical/subtropical forests thus stored the largest BBC across the
globe. The smallest FBC and BBC storage in shrub was primarily due to
its small area (8.11 million km2) and the low FBC and
BBC densities (48.06 g C m-2 for FBC and 17.31 g C
m-2 for BBC). The small FBC and BBC storage in desert
primarily resulted from their low FBC and BBC densities (Fig
S5 ), while the small FBC and BBC storage in tundra and natural wetland
may be due to the small area (5.75 million km2 for
tundra and 6.91 million km2 for natural wetlands).
Eventually tundra has high densities of FBC and BBC (226.96 g C
m-2 for FBC and 32.65 g C m-2 for
BBC).