Land use change, organic matter storage and composition in an arid soil of Argentina

CAROLINA VÁSQUEZ; ANA G. IRIARTE; CARLOS M. ROMERO; ENRIQUE LUCINI ; ESTEBAN KOWAIJOW; JOSÉ MERILES

Minneapolis

Workshop; 2015 annual meeting; 2015

American Society of Agronomy

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 total organic matter (TOM), humic substances (HS), humic acids (HA), and fulvics acids (FA) were studied in a Mollic Ustifluvent soil 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 organic matter 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 (50-75%) 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. Humic substance composition, assessed by Fourier Transform Infrared Spectroscopy (FT-IR) and UV-VIS (E4/E6), as affected by land use and management. A pronounced peak (aliphatic C-H stretching) was reported in TC-livestock, which doubled the intensity founded for the remaining sites. The band 1268-1279 cm-1, attributed to phenols, was only detected in livestock clearing sites. Polysaccharides (C-O stretching, 1032-1023 cm-1 band) was 3.0 times higher in TCagriculture than in TC-livestock. Low values were obtained for the E4/E6 ratio in CR, indicating a relatively high degree of condensationof 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 T-IR is a sensitive method to evaluate how management practices affect the storage and chemical characteristics of HS.