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Polygalacturonic acid (PGA) is considered as a model substance for mucilage to study mucilage–soil interactions, assuming that the gel formation mechanism of mucilage is comparable to the one of PGA. However, some studies question the accepted hypothesis, which states that, like for PGA, this mechanism relies on cross‐links between uronic acid and calcium for mucilage. The aim of this study was therefore to understand the influence of the abundance and degree of esterification of uronic acids and the influence of calcium on the gel formation mechanism in mucilage as compared to model substances. The mucilage used was from chia seeds, as it is easily available in great quantity and has gel properties shared by root mucilage. Results reported here demonstrate that, while the gel formation mechanism of PGA relied on specific cross‐links with calcium and led to heterogeneous gels, low‐methoxy pectin (LMP) formed homogeneous calcium gels also characterized by nonspecific ionic interactions with calcium. On the contrary, despite similar uronic acid content to LMP, chia seed mucilage was mostly governed by weak electrostatic interactions between entangled polymer chains, which conferred the gel poor water retention. Addition of calcium reduced repulsion and molecular expansion, resulting in a reduction of the water content in chia seed mucilage. Finally, the discrepancies between PGA, LMP and chia seed mucilage discredit the use of PGA as model for chia seed mucilage. Comparison with root mucilage is still needed. This study offers the keys for further mechanistic understanding on the influence of mucilage on soil properties. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Biohydrogels such as microbial exudates and root-derived mucilage are soil-born cross-linked polymers, able to form porous three-dimensional networks during water uptake. The gel effect is the variation of soil properties, such as soil hydrology and soil structural stability, resulting from biohydrogel swelling in soil. Conventionally, soil-water-hydrogel interactions are investigated by measuring soil bulk properties such as water retention curves and porosity, without further analyzing the effect of biohydrogel phases in soil on a quantitative basis. Therefore, the evaluation of advanced and novel methods for the characterization of biohydrogel phases in soil and soil-water-hydrogel interactions is necessary. This review evaluates currently available methods for their potential to analyze processes associated to the gel effect. A promising approach to investigate the spatio-temporal distribution of biohydrogel phases in porous media is based on Nuclear Magnetic Resonance (NMR) such as 1H-NMR relaxometry, as well as on imaging techniques such as Environmental Scanning Electron Microscopy (ESEM). Especially NMR techniques enable the identification of different water populations based on their differences in the relaxation, and thus the mobility of water molecules in biohydrogels and non-gel water in soil pores. Rheology measures the flow behavior of biohydrogels, providing information on the structural behavior of the hydrogel network and its gelling mechanism. Soil rheology further quantifies the effect of the biohydrogel phases on the interactions between soil particles, and thus the impact on soil microstructural stability. However, rheology does not elucidate the spatio-temporal distribution and structural state of biohydrogel phases in soil. All in all, a systematic combination of rheology, NMR and suitable imaging methods seems promising and necessary in order to elucidate the still widely unknown gel effect in soil. [ABSTRACT FROM AUTHOR]

Coupling of atomic force microscopy (AFM) with nanothermal analysis (nTA) has the potential to assess material characteristics in soils on the lower µm-scale, but has been shown to require additional characteristics for clear distinction of materials. The objective of this study was to evaluate to which extent the combination of AFM-nTA with AFM adhesion force analysis and structural features allows distinction of organic materials in soils. Using soil samples from a chronosequence from the Damma Glacier forefield, Switzerland, as example, we tested a grid analysis approach for assessing distribution of adhesion forces and nanothermal characteristics. This approach was compared with an approach involving pre-selection of structural features of interest via morphological criteria. Only three types of nanothermograms were distinguished in the soil samples based on different thermal expansion-compression characteristics and phase transition temperatures. Combined evaluation of nanothermal characteristics, adhesion forces and morphological characteristics allowed distinction of a larger set of materials than using nanothermal analysis, adhesion force distribution or morphological characteristics separately. Part of the analyzed features showed a combination of characteristics similar to that of fresh bacterial cells which we analyzed as a potential reference material. Their stronger occurrence in the regions of interest of older samples than in those of younger samples may underline their relevance in soil development. Achieving the long-term objective of identification of materials still requires more information on reference materials, understanding the impact of mixed layering of materials on thermal profiles and the assessment of variability of the characteristics within and between different material groups. [ABSTRACT FROM AUTHOR]

Olive oil production generates large amounts of olive mill wastewater (OMW). OMW has a high nutrient content and could serve as fertilizer, but its fatty and phenolic constituents induce soil water repellency, phytotoxicity, and acidification. An appropriate season of OMW application may mitigate negative consequences while preserving beneficial effects. In order to investigate this, a field study was conducted, in which OMW was applied to an olive orchard in Israel either in winter or summer. Soil-water interactions (water drop penetration time, hydraulic conductivity), soil physicochemical parameters, phenolic compounds, and soil biological activity (bait-lamina test) were determined 12 to 18 months after OMW application. The results showed elevated K+ contents in all treatments, but all other soil properties of winter treatments were comparable to the control, which suggested a certain recovery potential of the soil when OMW is applied in winter. By contrast, summer treatments revealed a ten-fold higher soil water repellency, a three-times lower biological activity, and a four-fold higher content of phenolic compounds, independently of whether the soil was kept moist by irrigation or not. Thus, the OMW constituents were neither degraded nor leached by winter rain when applied during the hot season. Further research is needed to distinguish leaching and biodegradation effects, and to understand the development of the composition and degradation kinetics of organic OMW constituents. [ABSTRACT FROM AUTHOR]