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.