Main controls over the SOM formation efficiency
To compare the relative roles of litter and clay mineral types in the
formation efficiency of SOM, we constructed a structure equation
model27 using the parameters that were significantly
correlated with the formation efficiency of SOM in both linear
regression (Supplementary Fig. 1) and stepwise regression. Surprisingly,
the litter chemistry parameter (PC1 score in Fig. 2b) was not
identified, indicating no effect of selective preservation of
recalcitrant litter fractions on the SOM formation efficiency. The path
coefficient (kp ) of the optimized structure
equation model (Fig. 4) indicates that the SOM formation efficiency was
mostly controlled by mineral protection capacity (mineral specific
surface area, kp = 0.95) and mineral selectivity
to litter residues (PC2 score in Fig. 2b) (kp =
0.92) and, to a much lesser extent, by mineral selectivity to microbial
residues (ratio of fungal to bacterial residues, kp = 0.39).In addition, mineral surface area was also linked to the
compositions of soil microbial communities (fungal biomass and fungal to
bacterial biomass ratio, kp = 0.86) and
mineral-protected labile litter residues (kp =
-0.88) directly and to fungal residues (kp =
0.11) indirectly.