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.