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

Abstract

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.

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

“…The soil sample and the procedure for cation treatment are described in detail in the first part of this article series ( Schaumann and Kunhi Mouvenchery , ). Briefly, the soil sample was collected from an organic surface layer of a Haplic Podzol (pH = 3.8, CEC = 303 ± 23 mmol c kg −1 ), sampled in a forest stand dominated by 80 year old Norway spruces ( Picea abies L.…”

“…This contribution is the second part of a series addressing the reactions of an organic forest soil sample on the treatment with earth alkaline cations after removal of the original cations. The first part ( Schaumann and Kunhi Mouvenchery , ) showed that the treatment cations sorbed to the organic matter follow a Langmuir model with Langmuir coefficients ( k Me ) increasing in the order k Mg < k Ca < k Ba . Furthermore, the cation exchange capacity estimated by the BaCl 2 method unexpectedly varied with the amount and type of cations present in the soil sample.…”

“…These surprising findings suggested that (1) the cation‐treated OM matrix contains unoccupied lonely sites as well as cross‐linking sites, the latter being partly occupied and responsible for the Langmuir sorption ( Schaumann and Kunhi Mouvenchery , ), and (2) the CEC determination procedure increased either the number of lonely sites (case A: for Ba 2+ and some Ca 2+ treatments) or the number of cross‐linking sites (case B: for Mg 2+ and some Ca 2+ treatments). Case B requires structural re‐organization in the matrix in order to bring lonely sites closer to each other.…”

“…Therefore, it was concluded that samples reacting according to case B should have a sufficiently flexible matrix with rigidity below a certain threshold value. In contrast, samples reacting after case A should have a rigid matrix, the rigidity of which increases with number and strength of the cross‐links ( Schaumann and Kunhi Mouvenchery , ). These hypotheses require verification by an independent method.…”

“…In order to evaluate these hypotheses, the samples of the preceding study ( Schaumann and Kunhi Mouvenchery , ) were additionally characterized for their T* before and after an 8‐week aging period.…”