Do water molecules bridge soil organic matter molecule segments?

One of the most valuable ecological potentials of soil organic matter (SOM) is based on its highly dynamic nature, which enables flexible reactions to a variety of environmental conditions. SOM controls a large part of the processes occurring at biogeochemical interfaces in soil and may contribute to sequestration of organic chemicals. This contribution focuses on dynamics in SOM from a viewpoint that regards SOM as an amorphous matrix, in which weak intermolecular interactions rather than covalent binding are considered. This view is based on the conception of SOM as a supramolecular assembly, which was first suggested in studies by A. Piccolo and R. L. Wershaw. Based on our recent results on thermal analysis of SOM, our central hypothesis is that regardless of the individual molecular mass, SOM undergoes physicochemical matrix aging, driven by dynamics in intermolecular cross-linking via bridges of water molecules. In this study, we have made the first efforts to evaluate the water-bridge hypothesis with proton NMR relaxation and proton wideline NMR. The results clearly indicate changes in relaxation time and proton line shape induced by manipulations of thermal history, which suggests an increase in side-chain mobility upon heating that remains after cooling. Side-chain mobility slowly decreases again within at least 1-2 weeks. Our current results strongly suggest even longer aging periods. This observation supports the hypothesis that water molecules bridge molecular segments of SOM. The bridges may be easily disrupted, while re-formation is slow due to diffusion limitation in the SOM matrix.