The Soil Humeome: Chemical Structure, Functions and Technological Perspectives

Humus or humic substances (HS) are of pivotal importance in the global ecosystem dynamics, since fluctuation in their amount affects not only the growth of both plants and soil microorganisms, but also the main biogeochemical cycles. The development of technologies aimed at controlling HS in the agroecosystem processes is hindered by the limited knowledge of their chemical structure and dynamics. The recent acknowledgement of the supramolecular nature of soil HS allowed to devise a fractionation procedure, called Humeomics, that enables a detailed characterization of the structure of humic molecules in soil. Humeomics produces homogeneous fractions by progressively breaking esters and ether C–O bonds but not carbon— carbon bonds. The molecules in fractions are then identified by means of advanced spectroscopic and mass spectrometric techniques, thereby providing a body of structures that may well represent the soil Humeome. Humeomics enabled to unravel the effects of different soil management practices on soil carbon dynamics and to explain the recalcitrance of HS in soil. Moreover, the application of Humeomics allowed to corroborate the novel concept of humification, that is unambiguously described as the progressive accumulation of hydrophobic molecular components, which are no longer biotically accessible, due to their rapid thermodynamically driven partitioning from liquid to the solid soil phases. Conceiving HS as supramolecular associations of relatively small compounds also helped to unravel the reactivity of HS with respect to plant and microbial development, as well as towards xenobiotics. Finally, the supramolecular understanding of HS encouraged the proposal of an innovative technology for the control of organic matter stabilization in soil. This is based on the in situ photo-polymerization of humic molecules catalysed by metal porphyrin biomimetic catalysts. The resulting increase in the molecular mass of humic molecules was found not only to increase soil aggregate stability but also to sequester in soil significant yearly amounts of organic carbon. It is expected that the research findings presented here will prompt novel studies on the man-driven control of t he soil Humeome in order to increase its content in soil, and contribute to positively affect both crop yields and soil microbial activity.