4D imaging reveals mechanisms of clay-carbon protection and release

Abstract

Soil absorbs about 20% of anthropogenic CO2 emissions annually, and clay is the key carbon-capture material. Although sorption to clay is widely assumed to strongly retard the microbial decomposition of soil organic matter, enhanced degradation of clay-associated organic carbon has been observed under certain conditions. The conditions in which clay inhibits microbial decomposition remain uncertain because the mechanisms of clay-organic carbon interactions are not fully understood. Here we reveal the spatiotemporal dynamics of carbon sorption and release within clay aggregates and the role of enzymatic decomposition by directly imaging a transparent smectite clay on a microfluidic chip. We demonstrate that clay-carbon protection is due to the quasi-irreversible sorption of high molecular-weight sugars within clay aggregates and the exclusion of bacteria from these aggregates. We show that this physically-protected carbon can be enzymatically broken down into fragments that are released into solution. Further, we suggest improvements relevant to soil carbon models.

In situ observations of the occlusion of a clay-sugar compound within calcite

“…After mixing, sugars adsorbed to the LAPONITE® surface as shown by Raman spectra, and this similar phenomenon has been observed in previous research. 4,19 The mixed LAPONITE® and sugar compound deposited on the tinfoil were characterized by Raman spectroscopy in reaction solutions to verify the formation of LAPONITE®-sugar complexes (Fig. 1A-C).…”

“…[16][17][18] However organo-clay complexes are not stable, as their interactions are reversible, leading to the degradation of combined SOM by various soil enzymes. 19 In soils, other minerals, such as iron/aluminum oxides and Environ. Sci.…”

“…21,22 Moreover, local supersaturated conditions in soil solutions may exist for calcite growth, leading to the occlusion of the combined organo-clay complexes within calcite [23][24][25] through growing hillocks, a classical crystal growth pathway. 26 The chemical properties of organic substances, such as surface functional groups 3,4,[27][28][29][30] and the molecular weight of SOM, 19,31,32 would influence the formation of organo-clay complexes 3,4,27 and subsequent occlusion within calcite, 25,[28][29][30][31][32][33][34] The organo-clay associations are reversible for low-molecular weight organic matter and they become more stable with an increase in molecular weight. 19,31 Moreover, organic polymers with high steric stabilizer chains could be easily occluded within calcite crystals.…”

“…26 The chemical properties of organic substances, such as surface functional groups 3,4,[27][28][29][30] and the molecular weight of SOM, 19,31,32 would influence the formation of organo-clay complexes 3,4,27 and subsequent occlusion within calcite, 25,[28][29][30][31][32][33][34] The organo-clay associations are reversible for low-molecular weight organic matter and they become more stable with an increase in molecular weight. 19,31 Moreover, organic polymers with high steric stabilizer chains could be easily occluded within calcite crystals. 32 Despite these experimental observations, the molecular mechanisms and effects of molecular weight for the fixation process of claysugar-calcite compounds still remain unknown.…”

Biochar composites: Emerging trends, field successes and sustainability implications

“…Therefore, clay minerals can be used for metal adsorption/immobilization and fertility improvement (because of reversible adsorption of nutrients). Most importantly, organo‐mineral interactions in soil is considered to be a key mechanism for long‐term carbon stabilization (Hemingway et al, 2019; Yang et al, 2021). Therefore, clay–biochar composite application will promote carbon storage in ground in most cases.…”