The effects of microstructural changes on montmorillonite–microbial interactions

Clay minerals are important natural adsorbents of soil organic matter (SOM) and therefore are natural modulators of soil-atmospheric carbon fluxes. Although such effects have been reported, little is known about the spatial distribution of organic matter (OM) on the surfaces of soil minerals and even less is known about the effects of microstructural changes on clay–organo interactions. Here we employ acid hydrolysis to induce varying degrees of microstructural changes to montmorillonite clay mineral as a function of time and combine IR spectroscopy, X-ray diffraction, and SEM-EDX as primary techniques to independently provide molecular-level information on the effects these changes on microbial interactions with the mineral. We observed that progressive dissolution of octahedral cations and the simultaneous enrichment of amorphous silica are prominent structural changes induced by hydrolysis, and that the adsorption of microbial-derived components (in particular lipids) on the surfaces of acid-treated clay decreases with increasing acid dissolution time. Although the precise mechanism(s) of interactions remains unclear, we speculate that this adsorption behavior is most likely due to spatial co-variation of microbial-derived OM with octahedral cations in the mineral, acid erosion of biochemically active binding sites, and/or a progressive increase in the hydrophilicity of the mineral surfaces by acid attack over time.