Molecular dynamics of organic matter in a tilled soil under short term wheat cultivation

Soil organic matter (SOM) or humus is essential for the agricultural and environmental functionality of soils. Humus comprises small-size heterogeneous organic molecules arranged in complex meta-stable suprastructures, whose composition can be greatly affected by land management. Here, we report the molecular dynamics of the fractions extracted from an agricultural soil cropped with wheat after one and three years of tillage. The total molecular components, named Humeome, of soil under wheat found in both hydrosoluble and organosoluble fractions isolated by the Humeomic procedure, as determined by GC–MS and high-resolution Orbitrap LC–MS, were compared to the Humeome characterized in the same soil when cropped with maize. While the three-years tillage did not vary the total soil organic carbon under both wheat and maize, the carbon recovered for the sum of Humeomic fractions isolated from soil was significantly larger for maize than for wheat, thus suggesting a general destabilization of SOM under wheat cropping. Moreover, the soil Humeome under wheat resulted more hydrophilic than under maize. While fatty acids and carbohydrates were periodically replenished by crop residues, nitrogen-containing molecules, such as amides and heterocyclic nitrogen, and iron-bound molecular systems were the SOM components mostly reduced under wheat. The losses of these compound classes from the soil Humeome was possibly attributed to the exudation differences between wheat and maize cropping. These results reveal that the molecular dynamics and stability of SOM molecular components can be controlled by crops even in a short term.