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

摘要

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 (Q(max)) increasing in the order Ba2+ Ca2+ Mg2+, but being clearly smaller than the initial load of native exchangeable cations. The Langmuir coefficient, k(Me), depended oppositely to the order obtained for Q(max). 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 Q(max) 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 mon