Mineral-organic association mechanisms
X-ray diffraction peak intensities reflect the order and average
thicknesses of
elementary
stacks of clay layers or of clay particles of 2:1 and 1:1 clay minerals.
SOM may change the thicknesses of the clay minerals by influencing the
entries of water or cations into interlayers or inner spaces between
elementary stacks of clay layers when it is adsorbed on the mineral
surface or through promoting the aggregation of mineral particles as
mineral domains or quasicrystals when it is entrapped28. So, we compared the X-ray diffraction spectra of
the pre- and post-incubation model soils and the post-incubation models
soils with and without association with SOM (through
H2O2 treatment) to gain insights into
how SOM was protected by different clay minerals (Fig. 5).
Compared with the original clay minerals, the X-ray diffraction peak
intensities increased after incubation with maize litter for vermiculite
and illite in the natural soil mineral and the pure kaolinite, as well
as with soya litter for vermiculite in the natural soil material. Peak
intensities decreased after incubation wither either litter type for
kaolinite in the natural soil material, the pure vermiculite and illite
as well as with soya litter for illite in the natural soil material.
Since the natural soil material was heated to remove SOM before the
incubation experiment, the increased peak intensities of its vermiculite
and illite during the incubation period may be attributed to the
re-entry of water to the oven-dried natural soil material, rather than
to SOM entrapment. SOM entrapment within interlayers is very rarely
observed for very fine 2:1 clay minerals28. Meanwhile,
SOM formation might bind free kaolinite particles together, leading to
the reduction of the thickness of kaolinite domains in the natural soil
material. In contrast, the increased peak intensity of the pure
kaolinite is consistent with earlier demonstration of SOC entrapment
X-ray diffractometry and scanning electron
microscopy29.
We also compared X-ray diffraction spectra of the model soils after
incubation with and without hydrogen peroxide
treatment30 to determine whether hydrogen peroxide
itself altered the minerals. We observed that the hydrogen peroxide
treatment had little effects on the positioning of all X-ray diffraction
peaks, but it changed their peak intensities compared with the untreated
minerals. After the removal of SOM, the X-ray diffraction peak intensity
increased for vermiculite and decreased for illite and kaolinite
irrespective of their origins and litter types. These findings support
the evidence provided above that SOM was associated with vermiculite
through surface adsorption, but with illite and kaolinite through pore
entrapment within mineral domains. Removal of SOM adsorbed on the
surfaces of vermiculite would allow water re-entry and expansion of
interlayers, while removal of SOM entrapped within inner spaces within
illite and kaolinite domains would reorganize their arrangements and
decrease the average thicknesses of these mineral domains.