The molecular dynamics of soil humus as a function of tillage

Soil degradation processes due to intensified agriculture may be counteracted by an extended knowledge on soil humus dynamics at molecular level. Here, we show that the molecular composition of the soil Humeome changes substantially after 1 and 3 years of conventional tillage under maize cropping. Application of Humeomics sequential chemical fractionation not only separated from soil more than twice the organic carbon extracted by traditional methods, but also enabled to identify 10 times more molecular structures using GC–MS and high-resolution ESI-Orbitrap-MS. The fine molecular description of the soil Humeome revealed a decrease of soil organic matter (SOM) hydrophobic protection with prolonged tillage and a potential loss of small nitrogen-rich compounds. We showed for the first time an abundance of heterocyclic nitrogen compounds that are persistent in SOM due to a covalent binding to soil iron. The formation of organo-Fe complexes appeared not only as a major mechanism of molecular stabilization of SOC but also as an adsorption substrate for other nitrogen-rich soil components. The detailed molecular insight of SOM composition reached by Humeomics may become the key to develop new technologies for the control of soil carbon.