Soil organic matter stabilization at molecular scale: The role of metal cations and hydrogen bonds

Abstract

Do humic substances exist naturally? Recently, this question has taken relevance in light of new evidence that supports the theory that soil organic matter (SOM) is an assembly of molecular species. Depending on the conditions, this assembly behaves as a macromolecular specie of high-weight or as a collection of low-weight molecular species. Nevertheless, it is an undeniable fact that some fractions of soil organic matter can be more resistant to ongoing physical and (bio)chemical degradation processes than others. This can be attributed to several factors ranging from molecular properties to the preference for digesting other molecular species by microorganisms. In this work, we explain the influence of cations and hydrogen bonds in SOM stabilization at the molecular level. By using molecular dynamics simulations and analyzing volumetric and structural properties, we elucidate the possible mechanisms of the formation of stable SOM aggregates, hardly accessible from the experimental point of view due to the molecular complexity of soil. Our results show that organic matter tends to form aggregates of big molecular size due to the presence of metal cations commonly found in soils, in agreement with other works and experimental observations. For the first time, the different complex structures of cation-SOM and their distribution are reported in models highly concentrated in organic matter. We conclude that cations serve as seeds to agglomerate organic matter and thus give rise to aggregates. The organic matter exhibits more stable aggregates with Ca2+ than with Na+ cations at different pH conditions; whereas, the absence of cations leads to the SOM destabilization and potentially also to the dispersion of the organic matter components, mainly due to electrostatic repulsion among them.

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