A contribution of molecular modeling to supramolecular structures in soil organic matter#

Abstract Background Knowledge of the stabilizing mechanisms of soil organic matter (SOM) is extremely important for numerous soil functions. For this, insight into the nature of organic matter through appropriate model concepts are crucial. Aims For several years, a heated debate has emerged on the transformation and stabilization of SOM. In the present work, we try to contribute to this debate using molecular modeling and providing a comprehensive overview of the history of application of molecular modeling tools and developing structural concepts of SOM. Methods Molecular modeling methods based on quantum and/or classical mechanics were used to model SOM and related properties including interactions with reactive surfaces of soil minerals. Results Modeling of SOM aggregates revealed that hydrogen bonds and cation bridges are the main stabilizing factors in soil solution, whereas pH modifies the stability. The modeled supramolecular SOM aggregates exhibit physicochemical properties, similar to those of humic substances (HS) described in literature. The interactions of the HS models with surfaces in kaolinite nanopores led to a partial disintegration of the aggregates into individual molecules and/or smaller subaggregates. Conclusions From the molecular modeling point of view, supramolecular microaggregate models that exhibit the properties of HS are stable in the soil solution. However, their binding to reactive mineral soil constituents can be also in the form of individual molecules or subaggregates. Thus, HS microaggregate stability is relative, depending on the interacting environment. This reconciles two points of view of HS: either as small molecules and/or supramolecular structures.