Cation binding in a soil with low exchange capacity: Implication for the structural rigidity of soil organic matter

Abstract

Two previous studies suggested that part of the cation sorption sites in soil organic matter with low exchange capacity have to be considered as “lonely”, i.e., too far from each other to allow direct cross‐linking by bivalent cations. The objective of this contribution was to understand the mechanisms controlling structural rigidity and physicochemical aging of the SOM (soil organic matter) and the role of water molecule bridges (WaMB) therein. For this, we evaluated the matrix rigidity of an organic surface layer of a Haplic Podzol on a quantitative basis, by assessing WaMB transition temperature (T*) directly after treatment with bivalent cations (Mg2+, Ca2+, or Ba2+) and after eight weeks of aging. Cation loading as well as cation type influenced matrix rigidity. Ba2+ induced the most rigid matrix and Mg2+ the weakest, which is in line with their binding strength in terms of Langmuir coefficient. The matrix rigidity increased with the cross‐linking activity, which is the product of loading and Langmuir constant of the respective cation. The aging process, however, was slowed down by the initial matrix rigidity, and the rigidity of the aged matrix decreased with increasing Langmuir constant. The degree of aging increased with increasing hydration enthalpy of the cation and decreased with increasing cation loading. Thus, directly after cation treatment, direct cross‐links by multivalent cations were most relevant, but WaMB increasingly gained influence on the matrix rigidity during aging. The untreated sample revealed a considerable number of WaMB, resulting in a fairly rigid and strongly cross‐linked matrix which, however, flexibly reacts on external influences like change in cation concentration or relative humidity. With these findings, the ideas on the relevance of indirect CaB‐WaMB associations between distant sorption sites for the rigidity and flexibility of the OM matrix as proposed in previous studies were confirmed on a mechanistic basis in this study.

Considerations on cross‐linking by bivalent cations in soil organic matter with low exchange capacity

“…We also expect that for all samples which can undergo structural reorganisation during CEC determination (case B), matrix rigidity is comparably low and these samples should show a specifically strong aging effect. These predictions have been investigated in a follow‐up study ( Kunhi Mouvenchery and Schaumann , ).…”

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

“…This behavior could not be explained by the breaking of covalent bonds, but by clusters of smaller molecules held together by weaker interactions (Haberhauer et al., 2000). According to Piccolo (2002), “hydrophobic (van der Waals, π−π, CH−π) and hydrogen bonds are responsible for the apparent large molecular size of humic substances.” Additionally, some publications demonstrated that water and cation bridges play an essential role in the development of rigidity and stabilization of HS (Aquino, Tuniga, Schaumann et al., 2011; Kuhni Mouvenchery & Schaumann, 2018; Schaumann et al., 2013). Nebbioso and Piccolo (2011) published a sophisticated sequential extraction scheme defined as humeomics to access humic fractions within the supramolecular structures, breaking interactions of different strengths.…”