Question

Describe the process of soil organic matter formation, emphasizing the biological, physical and chemical drivers of litter decomposistion, and soil organic matter transformation, stabilization, and loss. Provide a figure of the process.

Answer 1

The simplest form of the action of organic substances on the mineral part of the soil is the dissolving of calcium, phosphates and magnesium carbonates and other compounds by root exudates (CO2 and organic acids) and by various products of microbial activity. Among the latter are mineral compounds (CO 2 , HNO 2 , HNO 3 , and H2S, etc.) and organic acids of low molecular weight (butyric, lactic, acetic, propionic, gluconic, oxalic, fumaric, etc.).

the dissolving of MgCO 3 and Mg ammonium phosphate in selective media occupied by nitrifying organisms; the dissolving of chalk in lactic acid produced by lactic-acid bacteria and in butyric acid formed by anaerobic butyricacid bacteria; the dissolving of calcium phosphate in the metabolic products of Bac. megatherium.

Soil development leads to the formation of aggregated structures composed of a highly complex mixture of different mineral and organic constituents. The resulting soil type specific carbon sequestration can strongly be affected by soil management, varying greatly with the type and intensity of land use. The processes of formation and stabilization of organic matter through organo-mineral interactions in aggregated soil structures are controlled at the sub-µm scale. Understanding the binding of organic matter in these fine soil structures is thus key to elucidate the biogeochemical soil processes that are part of the carbon cycle as well as to evaluate the effects of soil management on the carbon cycle.