Water molecules in soil organic matter (SOM) can form clusters bridging neighboring molecular segments (water molecule bridges, WaMBs). WaMBs are hypothesized to enhance the physical entrapment of organic chemicals and to control the rigidity of the SOM supramolecular structure. However, the understanding of WaMBs dynamics in SOM is still limited. We investigated the relation between WaMBs stability and the physicochemical properties of their environment by treating a sapric histosol with various solvents and organic chemicals. On the basis of predictions from molecular modeling, we hypothesized that the stability of WaMBs, measured by differential scanning calorimetry, increases with the decreasing ability of a chemical to interact with water molecules of the WaMBs. The interaction ability between WaMBs and the chemicals was characterized by linear solvation energy relationships. The WaMBs stability in solvent-treated samples was found to decrease with increasing ability of a solvent to undergo H-donor/acceptor interactions. Spiking with an organic chemical stabilized (naphthalene) or destabilized (phenol) the WaMBs. The WaMBs stability and matrix rigidity were generally reduced strongly and quickly when hydrophilic chemicals entered the soil. The physicochemical aging following this destabilization is slow but leads to successive WaMBs stabilization and matrix stiffening.

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Adsorption is the main mechanism of capturing water in soil organic matter (SOM) under arid conditions. This process is governed by hydrophilic sites, which are gradually bridged via water molecule bridges (WaMB). Until now, the link between WaMB and other types of water molecules occurring in SOM during sorption has not been systematically investigated. In this work, we compared the formation and stability of WaMB simultaneously with the total water content, strength of water binding, and kinetics of water sorption in a vacuum-dried model SOM (sapric histosol) exposed to different relative water pressures. The same parameters were then determined in SOM exposed to reduced relative pressures. The adsorption resulted in an adsorption isotherm with a Langmuir-like part below a relative pressure of 0.5 and a Brunauer-Emmett-Teller-like isotherm at higher relative pressures. The WaMB formation was observed at a relative pressure of 0.32, which represented the pressure at which Langmuir-like part reached a plateau. The binding energy showed a linear decrease with an increasing pressure; the slope increased at a relative pressure of 0.46. Reduction of relative pressures above 0.46 showed that the water content remained constant, but the binding energy was lowered. In contrast, below a relative pressure of 0.46, the water content decreased, but the binding energy was not changed. The results indicate that in SOM exposed to different relative pressures, water exists in three types: first, it is strongly bound to primary sorption sites (Langmuir-like), second, it occurs in the form of WaMB water, which bridges functional groups and where predominates water-water interactions, and third, it occurs in the form of phase water, which is located in larger pores similar to the pure water phase. The latter either surrounds the WaMB and destabilizes it or, for higher water content, links individual WaMB and successively reduces their stabilizing effect. Formation of phase water leads to swelling processes including plasticizing effects and potential volume changes of SOM. Accordingly, the results suggest that at lower water relative pressures WaMB stabilizes the SOM structure, whereas at higher water relative pressures, it influences the formation of phase water and thereby the total water content in SOM.

Environmental contextImmobilisation of organic chemicals in soil organic matter can strongly influence their availability in the environment. We show that the presence of water clusters, called water molecule bridges, hampers the release of organic molecules from soil organic matter. Moreover, water molecule bridges are sensitive to changes in environmental conditions (e.g., temperature or moisture) which affect the release of organic molecules into the environment. AbstractWater molecule bridges (WaMB) can stabilise the supramolecular structure of soil organic matter (SOM) by connecting individual SOM molecular units. WaMB are hypothesised to act as a desorption barrier and thus to physically immobilise molecules in SOM. To test this hypothesis, we prepared two sets of soil samples – aged samples with WaMB developed, and vacuumed samples, in which WaMB were disrupted. The samples were spiked with phenol and then stored under controlled humidity. The degree of phenol immobilisation in SOM was assessed by desorption kinetics of phenol into a gas phase. This was compared with the thermal stability (T*) of WaMB obtained by modulated differential scanning calorimetry (MDSC) and the results were related to computer modelling, which provided the stability and solvation energies of phenol-WaMB-SOM models. The desorption kinetics of phenol was best described by a first-order model with two time constants ranging between 1 and 10h. In aged samples, the time constants correlated with T*, which showed that the desorption time increased with increasing WaMB stability. Molecular modelling proposed that phenol molecules are preferentially locked in nanovoids with polar OH groups pointed to WaMB in the most stable configurations. Both findings support the hypothesis that WaMB can act as a desorption barrier for phenol.