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It is assumed to be common knowledge that multivalent cations cross-link soil organic matter (SOM) molecules via cation bridges (CaB). The concept has not been explicitly demonstrated in solid SOM by targeted experiments, yet. Therefore, the requirements for and characteristics of CaB remain unidentified. In this study, a combined experimental and molecular modeling approach was adopted to investigate the interaction of cations on a peat OM from physicochemical perspective. Before treatment with salt solutions of Al3+, Ca2+ or Na+, respectively, the original exchangeable cations were removed using cation exchange resin. Cation treatment was conducted at two different values of pH prior to adjusting pH to 4.1. Cation sorption is slower (>>2 h) than deprotonation of functional groups (<2 h) and was described by a Langmuir model. The maximum uptake increased with pH of cation addition and decreased with increasing cation valency. Sorption coefficients were similar for all cations and at both pH. This contradicts the general expectations for electrostatic interactions, suggesting that not only the interaction chemistry but also spatial distribution of functional groups in OM determines binding of cations in this peat. The reaction of contact angle, matrix rigidity due to water molecule bridges (WaMB) and molecular mobility of water (NMR analysis) suggested that cross-linking via CaB has low relevance in this peat. This unexpected finding is probably due to the low cation exchange capacity, resulting in low abundance of charged functionalities. Molecular modeling demonstrates that large average distances between functionalities (∼3 nm in this peat) cannot be bridged by CaB-WaMB associations. However, aging strongly increased matrix rigidity, suggesting successive increase of WaMB size to connect functionalities and thus increasing degree of cross-linking by CaB-WaMB associations. Results thus demonstrated that the physicochemical structure of OM is decisive for CaB and aging-induced structural reorganisation can enhance cross-link formation.

Purpose Nutrient release, soil wettability, water binding, and matrix rigidity of soil organic matter (SOM) can be affected by cross-links between segments of SOM, cations, and water molecule bridges (WaMB). Not all cation effects on SOM can be explained with the currently accepted idea that multivalent cations cross-link organic matter segments via direct cation bridges (CaB). The objective was to understand these interactions and their effect on SOM matrix rigidity and wettability. Materials and methods We modified cation composition of two peats and an organic surface layer (OSL) using cation exchange resin to remove cations and solutions of Na + , Ca 2+ , or Al 3+ to enrich samples with cations. SOM matrix rigidity was determined at 4 and >8 weeks after treatment via the WaMB transition temperature T*, using differential scanning calorimetry. Wettability was measured via sessile drop contact angle (CA). Results and discussionThe effect of cation removal on T* depended on cation exchange capacity and initial cation content. Cation addition to OSL increased T*. This effect increased with increasing cation loading and valency, and T* correlated with CA. Classical cross-linking can neither explain the higher heterogeneous matrix of Ca-treated than Altreated samples nor the aging-induced convergence of T* for different cations and concentrations. The latter is likely due to interaction between CaB and WaMB in SOM.Conclusions Associations of CaB and WaMB evolve slowly and form a supramolecular network in SOM. Those dynamic associations can fix molecular arrangements inducing water repellency and increase kinetic barriers for the release and uptake of water and nutrients from aged soil.

A soil's cation exchange capacity (CEC) is expected to be relatively inert against changes in cation loading. In this study, we treated a soil sample originating from the organic layer of a forest soil with various bivalent cations after removing the native cations. Sorption isotherms and cation exchange capacity were determined, the latter using the BaCl2 method. Sorption showed Langmuir characteristics, with the maximum coverage (Qmax) increasing in the order Ba2+ < Ca2+ < Mg2+, but being clearly smaller than the initial load of native exchangeable cations. The Langmuir coefficient, kMe, depended oppositely to the order obtained for Qmax. CEC increased upon cation treatment and it varied by a factor of almost two. The unexpected variation of CEC was explained by the low cation exchange capacity of the organic matter such that not all functional groups are close enough to be bridged and the second charge of a bivalent cation is not neutralized by the organic functional group. The Langmuir sorption type, and Qmax being smaller than the content of sorption sites and being largest for Mg, suggested that only a part of the sites can be cross‐linked and at least part of the cross‐links are formed by hydrated cations. Thermodynamic considerations allowed reconstruction of two contrasting processes during CEC determination by Ba2+: Case A: the disruption of cross‐links, which increases with the cationic strength and the cation load before CEC determination, but does not require structural re‐orientation in the SOM matrix, and Case B: the formation of new cross‐links during CEC determination, depending only on the content of unoccupied sites before CEC determination and requiring structural re‐organization of the matrix and thus a minimum matrix flexibility. The use of bivalent cations for CEC determination may thus result in an overestimation of CEC for organic matter with low CEC. This has, however, promising potential when comparing CEC determined with monovalent cations and bivalent cations. Using a set of bivalent cations, may allow probing distribution of distances between functional groups in the organic matter and even characterize the matrix rigidity of the cation‐cross‐linked network.