Chapter 4 - The Molecular Composition of Humus Carbon: Recalcitrance and Reactivity in Soils

Abstract

The importance of soil humus carbon in the ecological and environmental domains can be hardly overestimated. The complex and heterogeneous chemical nature of the soil Humeome has for a long time baffled the development of man-made technologies for the control of soil carbon. A new paradigm was introduced in the early years of the millennium that viewed soil humus as a supramolecular association of small molecules derived from the abiotic and biotic degradation of dead cells in soil. This chapter reports on the recent advances that established a general consensus around this new chemical understanding of soil humus. Based on this new paradigm, a sequential chemical fractionation of the soil Humeome was recently introduced, by which homogeneous fractions are progressively obtained without deliberately breaking carbon-carbon bonds in humic molecules and are then subjected to structural determination by advanced NMR spectroscopy and mass-spectrometry. This Humeomics technique promises to finally allow the molecular mapping of soil humus in different soil types and explain the effects of different soil-management practices on soil carbon dynamics. The supramolecular nature of the soil Humeome and its molecular unraveling by Humeomics have also contributed to explain its recalcitrance in soil and the so-far vague concept of humification. The latter should be intended as the progressive accumulation of hydrophobic molecular components, which are no longer biotically accessible because they are rapidly partitioned from liquid to the solid soil phases due to thermodynamic reasons. The new awareness on the composition and conformational arrangement of the soil Humeome has prompted the application of an in situ photo-oxidative technique under a biomimetic catalysis that enables the oligomerization of humic molecules, thereby not only improving soil structural stability but also sequestering carbon in soil to an extent never matched by any other technique aimed to stabilize carbon in soil. It is expected that the innovative perspectives on the soil Humeome highlighted here may serve to enlarge the potentials of sustainable agriculture and develop new environmental technologies.