Dynamics of paddy soil Humeome after short term rice cultivation

Recent advances in humus chemistry indicated that the traditional macropolymeric view of humic matter is to be substituted with that of non-covalently linked supramolecular association of relatively small heterogeneous molecules of biological and plant origin, which are stabilized in soil by metal bridges, hydrogen bonds and hydrophobic interactions. The traditional alkaline extraction of humic matter from soil have hardly satisfied the complete understanding of SOM conformational status and dynamics. The recognition of a supramolecular association for humic molecules has prompted the application of Humeomics in order to increase the knowledge on the molecular composition and aggregation of SOM. Humeomics is a sequential chemical fractionation that uses in the order organic solvent extraction, transesterification with boron trifluoride in methanol, methanolic alkaline hydrolysis, cleavage of ether and glycosidic bonds with HI, and final alkaline extraction. Humeomics was successfully applied on soils to identify, in a tilled agricultural soil, 2.35 times more OC than by the common alkaline extraction, to elucidate SOM degradation in molecular level after only one year of traditional tillage and to reveal molecular alteration of humic material after just one year of soil cultivation under wheat as compared to maize. Herein the Humeomics fractionation was applied directly on a paddy soil under short term rice cultivation (soil sampled in 2014 and 2015) to elucidate whether the SOM stability reported in rice paddies is linked to specific molecular composition changes. Organic Carbon (OC) analysis revealed a significantly larger amount of humic component that was solubilised in humeomic fractions (3.7 times more OC in 2014 and 2.5 times more in 2015) than it was extracted by the traditional alkaline extraction method, increasing the knowledge on SOM. The organosoluble to hydrosoluble organic carbon ratio of the fractions revealed increased hydrophobic protection of rice paddy soils even under short term cultivation period. In fact the overall Humeome after one year of rice cultivation, lost part of the hydrophilic amines and heterocyclic nitrogen compounds and gained hydrophobic fatty acids. These findings prove that the complex supramolecular arrangement of the soil Humeome dictates the stability of SOM which is absolutely related to the aliphatic hydrophobic components.