Molecular composition and possible transformations of labile soil organic matter fractions in Mediterranean arable soils: Relevance and implications

Authors: ABDELRAHMAN, HAMADA - Cairo University; HOFMANN, DIANA - Julich Research Center; SLEIGHTER, RACHEL - Fbsciences; Olk, Daniel - Dan; BERNS, ANNE - Julich Research Center; MIANO, TEODORO - University Of Bari; SHAHEEN, SABRY - University Of Wuppertal; COCOZZA, CLAUDIO - University Of Bari

Submitted to: Environmental Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2023
Publication Date: 6/3/2023
Citation: Abdelrahman, H., Hofmann, D., Sleighter, R.L., Olk, D.C., Berns, A.E., Miano, T.M., Shaheen, S.M., Cocozza, C. 2023. Molecular composition and possible transformations of labile soil organic matter fractions in Mediterranean arable soils: Relevance and implications. Environmental Research. 232. https://doi.org/10.1016/j.envres.2023.116315.
DOI: https://doi.org/10.1016/j.envres.2023.116315

Interpretive Summary: Natural organic matter plays important roles in soil performance and water quality. Understanding these roles depends on studying the properties and behavior of the many components of natural organic matter, but there is no widely agreed best approach for separating the components from the soil. We studied components that were separated from the soil through two competing approaches and found that they provided complementary information on the development and cycling of natural organic matter in soil. Thus, combining the two approaches appears more promising than using either alone. These results will enable more detailed information on the behavior of natural organic matter and its roles in soil processes. The results are of interest to researchers who study soil processes or components of natural organic matter.

Technical Abstract: More detailed information on soil organic matter (SOM) formation and cycling can be obtained by integrating physical and chemical extractions of SOM fractions. Soils were subjected to a physical-chemical fractionation to extract the light fraction of SOM (LFOM), the 53-µm particulate organic matter (POM) and the mobile humic acid (MHA) fraction. Fractions were characterized using 13C cross polarization magic angle spinning (CPMAS) NMR and Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS). The 13C NMR showed a decrease in the carbohydrates shift range (51–110 ppm) and an increase in the aromatic shift range (111–161 ppm) moving from the LF to the POM and finally to the MHA fraction. Molecular formulae with H/C and O/C ratios similar to condensed hydrocarbons were dominant only in the MHA while those with H/C and O/C ratios in the lignin space were found only in the POM. The molecular formulae of the LF were mainly grouped in the lipid and protein H/C–O/C spaces. The double bond equivalent (DBE), a parameter of the degree of unsaturation, was extremely high (25–35) for one part of the MHA with corresponding H/C values 0.2–0.6, representing condensed hydrocarbons. The other important substance class in MHA was lipids, as revealed in Van Krevelen plots. In contrast, molecular formulae similar to lignin were not found in the H/C–O/C plot of the MHA, which is consistent with the hypothesized nature of the MHA as a young humified fraction still undergoing the humification process. Lignin is a transitional form of SOM that undergoes different processes until it reaches the refractory forms of the humic substances. These results suggest that the three fractions might play complementary roles in SOM cycling, underlining the potential value of integrating physical and chemical fractionations of SOM.