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

With the increased global interest in sequestering carbon in soil, it is necessary to understand the composition ofdifferent pools of soil organic matter (SOM) that cycle over suitably short timeframes. To explore in detail thechemical composition of agroecologically relevant yet distinct fractions of SOM, the light fraction of SOM(LFOM), the 53-μm particulate organic matter (POM), and the mobile humic acid (MHA) fractions weresequentially extracted from agricultural soils and characterized using both 13C cross polarization magic anglespinning nuclear magnetic resonance (CPMAS NMR) spectroscopy and also Fourier transform ion cyclotronresonance mass spectrometry (FT-ICR-MS). The NMR results showed a decrease in the O-alkyl C region assignedto carbohydrates (51–110 ppm) and an increase in the aromatic region (111–161 ppm) proceeding from theLFOM to the POM and then to the MHA fraction. Similarly, based on the thousands of molecular formulaeassigned to the peaks detected by FT-ICR-MS, condensed hydrocarbons were dominant only in the MHA, whilealiphatic formulae were abundant in the POM and LFOM fractions. The molecular formulae of the LFOM andPOM were mainly grouped in the high H/C lipid-like and aliphatic space, whereas a portion of the MHA compoundsshowed an extremely high (17–33, average of 25) double bond equivalent (DBE) values, corresponding tolow H/C values of 0.3–0.6, representative of condensed hydrocarbons. The labile components appeared mostpronounced in the POM (93% of formulae have H/C ≥ 1.5) similar to the LFOM (89% of formulae have H/C ≥1.5) but in contrast to the MHA (74% of formulae have H/C ≥ 1.5). The presence of both labile and recalcitrantcomponents in the MHA fraction suggests that the stability and persistence of soil organic matter is influenced bya complex interaction of physical, chemical, and biological factors in soil. Understanding the composition anddistribution of different SOM fractions can provide valuable insights into the processes that govern carboncycling in soils, which can help inform strategies for sustainable land management and climate changemitigation