Discussion
Here we used model soils to characterize both clay minerals and SOM for
understanding the mechanisms controlling the formation efficiency of
mineral-associated SOM during litter decomposition. We demonstrated
interactive effects of clay mineral and litter types, rather than litter
type alone, on the chemical composition (Fig. 1) and formation
efficiency (Fig. 4) of SOM. Different clay minerals discriminatively
protected litter and microbially-derived residues due to differences in
mineral-organic association mechanism (Fig. 5) and mineral-protection
strength (Fig. 6). Since the mineral-protection strength was higher for
surface adsorption by vermiculite than for pore entrapment within
domains of kaolinite or illite, vermiculite discriminatively protected
more litter-derived labile compounds and fungal residues and had a
higher SOM formation efficiency than kaolinite and illite. The
significance of mineral-discriminative protection and its control over
diversity of organic compounds in SOM has been recently reported for
long-term preservation/stabilization of SOM12. The
discriminative protection oflitter and microbial residues by different
clay mineral types explains why SOM structures often look similar for
soils with similar soil mineralogy, but different for soils with
contrasting soil mineralogy in different climate
zones32-35.
We found that clay minerals were associated with both labile (i.e.
O–alkyls and anomerics) and recalcitrant (aromatics and aromatic C–O)
litter residues and that litter-residues were dominant over microbial
residues in mineral-associated SOM (Figs. 2 and 3). These findings
suggest that litter residues, regardless of their recalcitrance, could
be associated with clay minerals as very fine SOM observed using
transmission electron microscopy36. However, these
findings do not support the hypothesis that mineral-associated SOM was
derived only from labile litter compounds10-11 or
microbially processed products26. The discrepancy
arises likely as the previous studies allowed only labile substrates
into mineral phases through leaching during litter decomposition above
the ground10, preferentially labelled labile compounds
in litter to trace SOM formation from litter decomposition in soil as
indicated by a low 13C abundance (4%)11 or used only labile substrates26.
We demonstrated that more labile litter residues and fungal residues
(Figs. 2 and 3) were better protected through surface adsorption by
vermiculite than through pore entrapment within domains of kaolinite and
illite, irrespective of litter types. Several studies have also
demonstrated a shift toward retention of more fungal than bacterial
residues in model soils consisting of vermiculite when compared to
illite25 or in natural soils dominant with
vermiculite37. We attributed this phenomenon to higher
relative abundance of fungi in the vermiculitic soil than in other soils
(Fig. 3) and higher recalcitrance of fungal residues compared to
bacterial residues, as suggested by previous study38.
Bacterial residues can be decomposed by fungi for
growth39. So bacterial residues were not protected
when they were exposed on vermiculitic surfaces but were better
protected when they were isolated in pores within domains of illite and
kaolinite.
We provided the first model to describe the feedback effects of
mineral-organic association on litter decomposition within mineral
matrices and to quantify mineral-protection strength. This novel model
consisted of two separate and interactive pools, which is fundamentally
different from the conventional SOM model, often consisting of two
discrete pools40-41. We provide a simple and reliable
approach to quantify mineral-protection strength for specific mineral
types or soils and to understand some physico-chemical and physical
protection processes of SOM. The modeled mineral-protection strength
explained well the variance of the measured SOM formation efficiency of
the model soils mixed with either litter types (Fig. 6). Although
several cutting-edge models provide a framework to describe the role of
mineral protection in controlling SOM dynamics and
stabilization13,42, no models are available to
describe the control of mineral composition over SOM formation
efficiency. In addition, those SOM models have not yet incorporated
parameters that consider mineral-protection strength.
We were able to predict the mineral-protection strength of the
carbon-free natural soil material based on the mineral-protection
strengths of the pure clay minerals and their relative abundances in the
soil. The vermiculite, regardless of its origin, had a much larger
specific surface area than the illite (Supplementary Table 1). The
natural soil material was taken at a depth below 2 m and is not highly
weathered, so its illite would have a relatively large particle size and
then a small specific surface area compared to the pure illite
(Supplementary Table 1). Contrasting X-ray diffractogram changes in
illite from the same subsoil as ours were observed in a previous
study43, suggesting that SOM was adsorbed on the
surfaces of < 2 μm illite, but entrapped within pores of 2-5
μm illite domains. However, natural soil minerals, particularly in
surface soils, may differ notably from pure minerals in their particle
size and surface properties, which will inevitably impact
mineral-organic association mechanism and strength, as shown for illite
in our study. In addition, our model soils were initially C-free.
However, SOM may be protected through adsorption to existing SOM, rather
than through mineral associations in soils with high organic carbon
contents44. Therefore, further studies are needed
using more mineral and litter types or soils with different initial C
contents and running for longer time scales to better understand mineral
protection of SOM. With better knowledge about soil mineralogy and
mineral-organic association, our novel model can likely be incorporated
into next generation soil and terrestrial C cycling models to reliably
predict and compare SOM dynamics among different soil types.