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

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

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 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.     

Native egg parasitoids recorded from the invasive <Emphasis Type="Italic">Halyomorpha halys</Emphasis> successfully exploit volatiles emitted by the plant–herbivore complex

When an accidentally introduced pest establishes in the invaded area, native natural enemies may adapt to the new host. A decade after the accidental introduction of the brown marmorated stink bug, Halyomorpha halys, in Europe, two generalist native egg parasitoids, the eupelmid Anastatus bifasciatus and the encyrtid Ooencyrtus telenomicida, have been recorded from this invasive agricultural pest in the field. Both species are able to complete development to the adult stage within the new host. Trissolcus basalis (Platygastridae = Scelionidae), which is not associated with H. halys in the field, was reared from freeze-killed sentinel eggs placed on soybean plants in central Italy. We tested in a Y-tube olfactometer the behavioural responses of these egg parasitoids to volatiles from H. halys adults and from Vicia faba plants attacked by H. halys. Both A. bifasciatus and O. telenomicida positively responded to adult H. halys male volatiles and to H. halys-induced plant volatiles, indicating ability to exploit cues associated with the new host for egg location, whereas T. basalis only reacted to female volatiles. A. bifasciatus and O. telenomicida are generalist egg parasitoids, showing a much wider host range when compared to T. basalis. On the other hand, platygastrid egg parasitoids from the native area of H. halys, considered for classical biological control, may be too risky due to the possibility of attacking non-target species, including predaceous stink bugs. Therefore, indigenous A. bifasciatus and O. telenomicida are presently under evaluation for augmentative biological control of H. halys in Europe.