13C-NMR of forest soil lipids

Abstract

Molecular characterization of soil lipids often provides valuable biogeochemical information about the impact of vegetation, microorganisms, and abiotic factors on the soil C sequestration process. The total lipid extracted with petroleum ether from nine soils developed under three types of Mediterranean forest (stone pine (Pinus pinea L.), evergreen oak (Quercus rotundifolia L.), and Spanish juniper (Juniperus thurifera L)) has been analyzed by high-resolution 13C nuclear magnetic resonance (13C-NMR) under quantitative acquisition conditions. Tentative assignments of the spectral peaks are presented, and the spectra of soil lipids are compared with those from the lipids extracted directly from leaves of the corresponding trees. This comparison evidenced that soil lipids behaved as biomarker soil fractions when analyzed by 13C-NMR as a whole. Analysis by gas chromatography-mass spectrometry (GC/MS) reveals that the volatile fraction of the lipid extract (46%, on average, as estimated by internal reference) consisted mainly of free alkanes, alkanoic acids (<C30), and diterpene resin acids. We observed some differences between the chemical structures suggested by 13C-NMR and GC/MS. This was interpreted as a portion of soil and plant lipids consisting of extractable material that cannot be detected by standard GC methods. The complex signal pattern in the 0 to 30 ppm chemical shift range showed typical signals for carbons in acyl polymethylene chains, which overlapped with a pattern suggesting isoprenoid-like branching in long-chain structures (major signals at ca. 22, 26, and 32 ppm). In addition, periodic unsaturations suggested by signals at ca. 124 and 135 ppm are also compatible with polyprenoid-type backbones. The alkyl region coincided with those of mono- to triacyl glycerol fatty esters. It seems evident that 13C-NMR allows us to characterize structures present in nonvolatile complex material.