The impact of long-term field experiments under different cropping systems on the molecular dynamics and stability of the soil Humeome

Cultivation practices alter the molecular status of the soil Humeome, meant as the ensemble of all heterogeneous humic molecules, whose changes need to be understood and monitored in order to maintain the sustainability of agricultural soils. We applied the Humeomics sequential chemical fractionation, coupled to characterization of separated fractions by GC-MS and high-resolution Orbitrap LC-MS, on soils subjected for 20 consecutive years to the following treatments: i) non cultivated and untilled soil (Control); ii) maize monoculture (Maize); iii) maize-leguminous (Vicia faba) rotation (Mix). Humeomics fractions revealed a greater amount of organic carbon (OC) than for the traditional alkaline humus extraction (eSOM), double chromatographic visibility and one order of magnitude more detectable empirical formulae. Humeomics indicated that the ratio of organosoluble to hydrosoluble components decreased significantly passing from Control to Mix and Maize soils, thus unveiling that the loss of humic hydrophobic compounds, such as long-chain esters and fatty acids, rendered more physically and chemically fragile soils under long-term maize monoculture than under crop rotation. Saccharides undetectable in eSOM became instead visible after cleavage of esters weakly bound to the humic matrix, confirming a mechanism of protection of polar compounds by hydrophobic humic components. The same was observed for nitrogen-containing compounds, such as amides and heterocyclic nitrogen, which were significantly detected in Humeomics fractions of cropped soils only after the HI step disrupted ether linkages and organo-mineral complexes. Most of N-containing compounds in cropped soils were found to be bound to iron, thus implying that different forms of nitrogen entering soil by either synthetic fertilizer or nitrogen fixation are progressively sequestered into recalcitrant organic pools. Our findings highlight that a detailed knowledge on the molecular dynamics of the Humeome of soils under long-term field trials allowed a further understanding of the organic matter molecular distribution and the mechanisms of its stabilization.