Land Use Change, Organic Matter Storage and Composition in an Arid Soil of Argentina.

Soil organic matter (SOM) storage and composition in woody ecosystems may change as result of long-term grazing and soil cultivation, particularly in arid environments characterized by a high risk of desertification. The impact of land use change on the storage (0-20 cm) of SOM, humic substances (HS), humic acids (HA), fulvic acids (FA), dissolved organic carbon (DOC), and hot-water extractable carbon (HWC) were measured seasonally (dry and wet season) in a Mollic Ustifluvent within 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 (CR) located in the Chaco region of central-western Argentina. Total SOM content was higher in CR and TC-agriculture relative to livestock clearing (TC and SC) sites. The concentrations of HS, HA, and FA were all reduced substantially by land use change (i.e. conversion of native woodlands to livestock systems). The type of clearing (TC and SC) did not affect HS concentration among livestock systems. Seasonality only affected the concentration of labile SOM (i.e. HWC) with a general decrease of active-C pools during the wet season. Humic substance composition, assessed by Fourier Transform Infrared Spectroscopy (FT-IR) and UV-VIS (E₄/E₆), was affected by land use and management. Polysaccharides (C-O stretching, 1030 cm^-1 band) was 4.0 times higher in TC-agriculture than in TC-livestock (dry season). Low values were obtained for the E₄/E₆ ratio in CR, indicating a relatively high degree of condensation of aromatic substances in undisturbed soils. It would appear that, a) the conversion of native woodlands to livestock systems favors soil C losses, b) higher SOM storage observed in TC-agriculture may reflect greater residue accumulation at the soil surface. It is also concluded that FT-IR is a sensitive method to evaluate how management practices affect the storage and chemical characteristics of HS.