Land use impact on chemical and spectroscopical characteristics of soil organic matter in an arid ecosystem

Soil organic matter (SOM) storage and composition in ecosystems may undergo change as a result of long-term livestock and soil cultivation, particularly in arid environments. In this work, we evaluated the alterations produced in both the quantity and quality of SOM due to productive management systems. The impact of land use change on SOM, dissolved and hot water-extractable carbon (DOC and HWC), humic substances (HS), humic acids (HA), fulvic acids (FA) and the infrared and visible spectroscopy of HS were studied at three productive sites: total and selective clearings with livestock (TC livestock and SC livestock), total clearing with irrigated agriculture (TC agriculture), and an undisturbed site located in central-western Argentina. The SOM content was higher at the undisturbed and TC agriculture sites. DOC varied among the study sites only during the dry season, while HWC decreased during the wet season, clearly indicating the lability of this fraction. The concentrations of HS, HA, and FA were reduced (50-75 %) by land use change, with the HS composition determined by infrared spectroscopy reflecting a high quantity of polysaccharides in TC agriculture, while the E-4/E-6 ratio (UV-vis) presented low values at the undisturbed site, indicating a high degree of condensation of aromatic substances. In conclusion, (a) the conversion of native woodlands to livestock systems favored soil C losses, (b) the highest SOM storage recorded in TC agriculture may reflect a greater residue accumulation at the soil surface and (c) the combination of different techniques provided a very good insight into the status of soil degradation.