Combined proton NMR wideline and NMR relaxometry to study SOM-water interactions of cation-treated soils

Focusing on the idea that multivalent cations affect SOM matrix and surface, we treated peat and soil samplesby solutions of NaCl, CaCl2 or AlCl3. Water binding was characterized with low field 1H-NMR-relaxometry (20 MHz)and 1H wideline NMR spectroscopy (400 MHz) and compared to contact angles.From 1H wideline, we distinguished mobile water and water involved in water molecule bridges (WaMB). Large partof cation bridges (CaB) between SOM functional groups are associated with WaMB. Unexpectedly, 1H NMRrelaxometryrelaxation rates suggest that cross-linking in the Al-containing peat is not stronger than that by Ca.The relation between percentage of mobile water and WaMB water in the context of wettability and 1H NMR relaxationtimes confirms that wettability controls the water film surrounding soil particles. Wettability is controlled byWaMB-CaB associations fixing hydrophilic functional groups in the SOM interior. This can lead to severe water repellency.Wettability decreases with increasing involvement of functional groups in CaB-WaMB associations. The resultsdemonstrate the relevance of CaB and WaMB for the dynamics of biogeochemical and hydrological processes underfield conditions, as only a few percent of organic matter can affect the physical, chemical, and biological functioning of the entire 3-phase ecosystem.