Deriving the Shannon Index from the soil molecular Humeome serves as a descriptor of soil organic matter stability under different cropping systems

Soil organic matter (SOM) is essential in maintaining soil structure, nutrient content, water retention, and biodiversity. It also represents an important C pool that, if efficiently stabilized, may play a role in the mitigation of global climate changes. However, knowledge is still limited on the dynamics of soil organic carbon (SOC) molecular composition occurring with changes in land use and management practices. We calculated the Shannon diversity index (SH) for the soil molecular characteristics as obtained by the humeomic sequential fractionation applied on two different soil systems including uncultivated control: (1) Soils under a short-term cultivation of maize for 1 and 3 years; (2) Soils under a 20-year long-term cultivation of either continuous maize (MAIZE) or maize-bean rotation (MIX). The SH values were derived for each of the nine molecular classes in which the soil humeomic results were grouped. When single class SH were summed, we found that the resulting total Shannon heterogeneity (SH tot) and its value weighted for the relative OC percent, were inversely proportional to a Stability Soil Organic Matter Ratio (SOMR) equation, that, in turn, was directly proportional to the normalized Chemical Protection Ratio (nCPR) equation, based on the quantity of highly hydrophobic (organosoluble) compounds present in soils. The physical–chemical stability of SOM, expressed as SOMR and nCPR, increased with the decrease of the heterogeneity of the total molecular system (SHtot) for both short- and long-term soil systems. In fact, in the short-term soil system the molecular humeome was least heterogeneous under uncropped conditions, while SH showed increased heterogeneity with Maize cultivation for 1 and 3 years, in the order. The greatest SH homogeneity was again shown by the uncultivated control soil for the 20-year long-term system, and SH heterogeneity increased passing from the soil under Maize-Broad bean rotation, to that under Maize monocultivation. The SH values of single molecular classes revealed that nitrogenated or oxygenated aromatic compounds determined the stability of soil humus, being the nitrogenated heterocyclic compounds responsible for the residual molecular stability of SOM in both soil systems. This work indicated that the molecular complexity of a soil humeome can be represented by a Shannon descriptor that provides a handy and direct information on the dynamics of humus in soils.