Chemical Structure of Soil Organic Matter in Slickspots as Investigated by Advanced Solid-State NMR

Abstract

Slickspot soils are saline, and knowledge of their humic chemistry would contribute to our limited understanding how salinity affects soil C and N stocks. We characterized humic acids (HA) from slickspot soils with solid-state 13C nuclear magnetic resonance (NMR). Expanding on previous use of cross polarization/magic angle spinning (CP/MAS) NMR, we used direct polarization (DP) and yet more advanced spectral editing techniques to identify specific functional groups, detect the connectivities of different functional groups, and selectively observe fused ring carbons. A series of soil HA was extracted from soil layers having different physical properties: silt texture, vesicular structure, and clay texture. They were compared with HA from corresponding depths in soils adjacent to the slickspots. All HA consisted of five main structural components: aliphatic chains, peptides, sugar rings, lignin residues, and aromatics/olefinics. For all soils, except one outside slickspots, the HA from the vesicular and clay layers contained less nonpolar alkyls and more aromatics than those from surface silt layers, but their spectral proportions differed when the 13C NMR was performed using CP/MAS instead of DP. Humic acids from the surface layers inside the slickspots had lower aromaticity than those from outside the slickspots. Advanced spectral editing techniques allowed for the selection of nonprotonated carbons and mobile groups, alkyls, CH, and CH2 groups, which would otherwise be buried in the heavily overlapped spectrum. They provided more structural information than was obtained by routine 13C CP/MAS or DP.