Nanoscale Interactions of Humic Acid and Minerals Reveal Mechanisms of Carbon Protection in Soil

Abstract

The concentrations of terrestrially sourced dissolved organic matter (DOM) have expanded throughout aquatic ecosystems in recent decades. Although sorption to minerals in soils is one major pathway to sequestrate soil organic matter, the mechanisms of organic matter−mineral interactions are not thoroughly understood. Here, we investigated the effect of calcium phosphate mineralization on humic acid (HA) fixation in simulated soil solutions, either with or without clay mineral montmorillonite (Mt). We found that Mt in solution promoted nucleation and crystallization of calcium phosphate (CaP) due to amorphous calcium phosphate clustering and coalescence on Mt surface, which contributed to the long-term persistence and accumulation of HA. Organic ligands with specific chemical groups on HA have higher binding energies to CaP−Mt than to CaP/Mt, according to dynamic force spectroscopy observations. Moreover, CaP−Mt formed in solution showed a great capacity for HA adsorption with a maximum adsorption quantity of 156.89 mg/g. Our findings directly support that Mt is crucial for DOM sequestration by facilitating CaP precipitation/transformation. This has an impact on how effectively we understand the long-term turnover of DOM and highlights knowledge gaps that might assist in resolving essential soil C sequestration issues.

Understanding the Nanoscale Affinity between Dissolved Organic Matter and Noncrystalline Mineral with the Implication for Water Treatment

“…At high concentrations, multiple binding mechanisms coexist due to the polydisperse and heterogeneous properties of HA. Therefore, it is challenging to identify the main interfacial binding mechanisms as they may occur simultaneously …”

“…Our recent studies have also found that the presence of clay minerals in soil facilitates the adsorption of CaP to DOM . Nanoparticles produced on the surface of natural clay minerals are tightly bound to the clay silicate layer, preventing them from aggregating .…”

“…However, both DFS and DFT experiments investigated the interaction of single- Therefore, it is challenging to identify the main interfacial binding mechanisms as they may occur simultaneously. 19…”

Carbon Sequestration Strategies in Soil Using Biochar: Advances, Challenges, and Opportunities

“…The (released) labile OC as well as the liquid byproduct of hydrochar tends to form aggregates with clay minerals. 40,71,72 The refractory fraction of biochar can also form organometallic complexes with clay minerals 30,73 during aging in soil. These processes not only physically protect the newly formed aggregates/complexes from decomposition but also enhance the oxidation resistance of biochar surfaces and thus allow both refractory and labile fractions of biochar to persist in soil.…”

“…Notably, hydrochar usually contains high levels of humic acids in its dissolved fraction, , which can account for 43.5% of the total biomass, depending on thermochemical conditions . Humic acids are widely used functional materials for soil fertility and ecological agricultural construction due to their high physiological activity, adsorption, and N-rich composition , Therefore, it is expected that the dissolved fraction of hydrochar has stronger effects on the turnover and speciation of OC than dissolved pyrochar in soil.…”