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Figure captions
Fig. 1 | Schematic representation of two mineral protection mechanisms.a, A diagram of each mechanism: surface adsorption (top panels) and pore entrapment of SOM (bottom panels) by different types of clay minerals. Litter residues are associated with clay minerals to a higher strength through surface adsorption than through pore entrapment and decomposed to different degrees by microorganisms. b, Temporal changes in the chemical composition of labile (yellow) versus recalcitrant (brown) litter residues and the compositions of microbial functional communities and necromass (blue, bacteria; red, fungi) for each mechanism.
Fig. 2 | Chemical structures and composition of litter-derived SOM. a, CP/TOSS 13C NMR spectra of maize and soya litter and their derived SOM in four model soils.b , Differences in the chemical composition of litter-derived SOM between litter and clay mineral types. Principal component analysis of the relative abundance of functional C groups determined by13C NMR among four model soils by two litter types in comparison with original maize and soya litters (top panel) and the loadings of individual functional C groups to the first two principal components (bottom panel). Open symbols are for soya litter and filled symbols for maize litter.
Fig. 3 | Community compositions of microbial biomass and necromass. a, b, Microbial biomass (represented by total phospholipid fatty acids) and microbial necromass (represented by amino sugars) of different communities in model soils mixed with maize litter (left column) and soya litter (right column). Lower case letters indicate differences in total microbial biomass or necromass among model soils for each litter type and * indicates difference between litter types for each model soil P < 0.05 (n = 3). Error bars represent standard errors (n=3).
Fig. 4 | Main controls over the SOM formation efficiency during litter decomposition within clay mineral matrices.Optimized structure equation model shows no effects of litter chemistry and three independent (P = 0.16, n = 18) pathways from clay minerals to the SOM formation efficiency. Path coefficient (k p), with a significance at P < 0.05 (*) or P < 0.05 (**) and the proportion of the variance (R2) are presented for each pathway, with the line width proportional to kp . The mineral selectivity of litter residues is reflected by the score of the second principal component of principal component analysis of functional C groups estimated by 13C NMR (Fig. 2), showing the effects of clay mineral types irrespective of litter type.
Fig. 5 | Mineral-organic association effects on the X-ray diffractograms of clay minerals.a, b, Original minerals (thick lines) before incubation and model soils without H2O2 treatment (thin lines) and with H2O2 treatment (dotted lines) after incubation with maize and soya litter.
Fig. 6 |Quantification and application of mineral-protection strength. a, Cumulative respiration measured (symbols) and modeled (lines) using the novel model describing the mineral-protection strength (δ ) (inserted equation). Error bars represent standard errors (n=3). b, Correlation between SOM formation efficiency and mineral-protection strength (δ ).
Supplementary Figure 1: Correlations of SOM formation efficiency with different mineralogical, microbial and litter compositional properties. a, Specific surface area (SSA); b,mineral pH; c, PC2 in Fig. 2; d , fungal PLFAs;e, fungal to bacterial PLFAs; and e, fungal amimo sugars. Filled and open symbols represented maize and soya litters, respectively.
Supplementary Table 1:Particle size (Φ ), pH, iron oxide content measured using oxalate (Feo) or dithionite (Fed) extraction as well as specific surface area (SSA) measured using Brunauer–Emmett–Teller (N2-BET) adsorption method for pure clay minerals and natural soil material.
Supplementary Table 2: Measured total respiration, post-incubation soil C and C contents (defined as the SOM formation efficiency) and C loss rate due to hydrofluoric acid (HF) treatment, modeled mineral-organic association extent (δ ), pool sizes and decomposition rate constants (k 1 andk 2) of free litter and mineral-protected litter residue pools, respectively, and determination coefficient (r 2) of the best fitting to the new mineral-driven decomposition model.
Supplementary Table 3: Assignments and relative abundancess of C functional groups in soil organic matter of the model soils and original litters obtained by 13C cross polarization/total sideband suppression (CP/TOSS) nuclear magnetic renosance spectroscopy at the end of incubation.