Molecular characterization of soil organic matter and its extractable humic fraction from long-term field experiments under different cropping systems

Soil Organic Matter (SOM) is essential for soil stability, fertility and crop productivity. Recent findings showed that SOM supramolecular structure is strongly influenced by cultivation and land management. In this study, we investigated the molecular dynamics of organic matter in soils from three long-term field-experiments which were subjected for 20 consecutive years to the following crop managements: i) non cultivated (Control); ii) maize mono-culture; and iii) maize-leguminous (Vicia Faba) rotation. Off-line pyrolysis TMAH-GC-MS (thermochemolysis) and solid-state 13C NMR spectroscopy were applied for the direct molecular characterization of OM components in both the bulk soils and their humic extracts. While 20 consecutive years of Maize mono-cultivation led to alteration of the SOM hydrophobic composition, with a decrease in alkyl and aliphatic compounds (e.g. 47.4% reduction in fatty acids), and an increase in hydrophilic labile components (42.2%), the crop-rotated soils showed a partial preservation of the pristine SOM composition by maintaining the content of hydrophobic and lipid constituents (only 1.8% reduction). Our results suggest that different cropping systems change the SOM molecular dynamics and stability in long-term field experiments by primarily altering the hydrophobic components of SOM. In particular, Maize mono-cultivation leads to a progressive degradation of SOM quality. Yet, the introduction of a leguminous species in crop rotation with Maize maybe an advantageous strategy to preserve SOM quality while reducing SOC losses.