Global soil carbon cycling plays a key role in regulating and stabilizing the earth's climate change because of soils with amounts of carbon at least three times greater than those of other ecological systems. Soil minerals have also been shown to underlie the persistence of soil organic matter (SOM) through both adsorption and occlusion, but the microscopic mechanisms that control the latter process are poorly understood. Here, using time-resolved in situ atomic force microscopy (AFM) to observe how calcite, a representative mineral in alkaline soils, interacts with humic substances, we show that following adsorption, humic substances are gradually occluded by the advancing steps of spirals on the calcite (1014) face grown in relatively high supersaturated solutions, through the embedment, compression, and closure of humic substance particles into cavities. This occlusion progress is inhibited by phytate at high concentrations (10−100 μM) due to the formation of phytate-Ca precipitates on step edges to prevent the step advancement, whereas phytate at relatively low concentrations (≤1 μM) and oxalate at high concentrations (100 μM) have little effect on this process. These in situ observations may provide new insights into the organo−mineral interaction, resulting in the incorporation of humic substances into minerals with a longer storage time to delay degradation in soils. This will improve our understanding of carbon cycling and immobilization in soil ecological systems.
Biominerals can exhibit exceptional mechanical properties owing to their hierarchically‐ordered organic/inorganic nanocomposite structure. However, synthetic routes to oriented artificial biominerals of comparable complexity remain a formidable technical challenge. Herein we design a series of soft, deformable nanogels that are employed as particulate additives to prepare nanogel@calcite nanocomposite crystals. Remarkably, such nanogels undergo a significant morphological change—from spherical to pseudo‐hemispherical—depending on their degree of cross‐linking. This deformation occurs normal to the growth direction of the (104) face of the calcite and the underlying occlusion mechanism is revealed by in situ atomic force microscopy studies. This model system provides new mechanistic insights regarding the formation of oriented structures during biomineralization and offers new avenues for the design of synthetic nanocomposites comprising aligned anisotropic nanoparticles.
Intracellular membrane-bound vesicles play important roles in the formation of biominerals, such as calcium oxalate monohydrate (COM) crystals, through the interactions of the vesicles and different crystal faces. However, in situ kinetics and the mechanism of occlusion of diverse vesicles, which have similar compositions, into the (1̅ 01) and ( 010) faces of COM remain unknown. Here, using time-resolved in situ atomic force microscopy (AFM), we observe that negatively charged phosphatidylcholine vesicles are adsorbed preferentially and then occluded into the (1̅ 01) face, whereas positively charged (2,3-dioleoyloxy-propyl)trimethylammonium vesicles are only occluded into the (010) face, and zwitterionic 1,2-dipalmitoyl-sn-glycero-3-phosphocholine vesicles are rarely incorporated into COM crystals. The free energies of binding between the lipid vesicles and COM crystal faces measured by AFM-based single-molecule dynamic force spectroscopy account for the vesicle-crystal face interaction through an electrostatic attraction. These in situ kinetics and energetic analyses may improve our understanding of the mechanisms of lipid occlusion.
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