Thermodynamic and spectroscopic data suggesting hypothetical humic fractions evolution on carbon cycle in soils.

ABSTRACT

Soil organic matter is considered by soil scientists as the interlayer that connect alive with mineral sides of the soil. In addition, microorganisms have in soil organic matter a source of carbon as well as a source of energy. We can observe a duality that can be analyzed from a biological, physicochemical, or even thermodynamic sense. From this last point of view carbon cycle follows its evolution on burial soil, and under certain temperature and pression conditions, up to fossil fuels or coals through kerogen being humic substances the ending point of biologically linked structures. When biological aspects are minimized, physicochemical aspects are maximized and carbonaceous structures are a source of energy but resilient to microorganism actions. Under these premises, we have isolated, purified, and analyzed different humic fractions. Heat of combustion of these humic fractions here analyzed reflects this situation and fitted the list of evolution stage of carbonaceous materials that step by step accumulates energy. Theoretical value of this parameter calculated from studied humic fractions, and by combination of its biochemical macromolecules yielded an exaggerated value in comparison to the real and measured value indicating a complexity of these humic structures, more than simpler molecules. Heat of combustion and excitation-emission matrices by fluorescence spectroscopy of isolated and purified grey and brown humic materials revealed different values for each fraction. Grey fractions showed a higher heat of combustion values and shorter λexc/λem, whereas brown fractions showed a lower heat of combustion and a larger λexc/λem. These data together with previous chemical analysis indicated a deep structural differentiation that can be observed by the Pyrolysis MS-GC data of the studied samples. Authors hypothesized that this incipient distinction between aliphatic and aromatic cores could evolve independently up to fossil fuel on one hand and coals on the other hand but separately.