Surface characterization of natural and Ca-saturated soil humic-clay composites at the micrometer scale: Effect of Calcium

Calcium is a pivotal element in the interactions between humic material and clay minerals favoring the stabilization of the formed micro-aggregates. Yet, morphological and structural knowledge of the humic-clay mineral surface is limited. In the present paper we employed the scanning electron microscopy (SEM) as a potent tool to analyze the surface texture of a humic-clay composite (HCC) fractionated from a prairie field soil and of a calcium saturated humic-clay composite (HCC-Ca) of the same soil. We aim to discern the influence of the addition of calcium ions on the surface of interaction. We applied statistical analyses to quantitatively characterize the morphology and the structure of the surface of the prepared samples. The SEM image treatment analyses revealed significant differences in the morphological structure and grains distribution in the HCC sample compared to the HCC-Ca sample. From our findings, we conclude that the presence of calcium in micro-soil plays a big role (i) in enlarging the size of the grains composing the surface of the humic-clay fraction by binding the mobile micro-particles, (ii) in readjusting the surface in a manner to homogenize the rough and random distribution of the humic-clay grains on the mineral surface, (iii) and in thickening the clay mineral layers promoting a better uptake of the humic material. This study disseminates a neat explanation of how calcium ions contribute in the stabilization of humic-clay aggregates in soil, and hence in the stabilization of carbon in the global carbon cycle.