Abstract

BROQUEN, Patricia; SUAREZ, Adriana; GIRARDIN, Jorge Luis  and  PERCAZ, Juan Carlos. Pedogenetic Processes In A Soil Sequence Developed On A Volcanic Ash Region Innorthwestern Neuquén, Argentina. Cienc. suelo [online]. 2013, vol.31, n.2, pp.213-222. ISSN 1850-2067.

An investigation was conducted to study Al fractions, organic matter accumulation and their influence on pedogenetic features and processes in volcanic ash soils in the Andinopatagonian region of NW Neuquén, Argentina. Five sites were selected in a climatic biosequence, ranging from Nothofagus forest with a udic soil moisture regime, to grass - shrub steppe with a xeric soil moisture regime. The morphological characteristics showed young soils with andic properties, a mollic epipedon and a cambic horizon. Bulk density was higher than expected for volcanic soils (from 0.90 Mg m^-3 under Nothofagus forest and a udic regime to medium bulk density ~ 1.3 Mg m^{-3 in under gramineous shrub -steppe and a xeric regime). Buk density variation was explained by organic carbon (R2}= 0.81, p < 0.0001). Near forty percent of soil acidity variation was explained by pH (NaF) and organic carbon, ranging from moderately acidic to neutral (R²= 0.39, p< 0.005). This showed that soil acidity was related to Al activity and organic matter content, rather than to base content. In all cases, pH (H₂O) was higher than the critical value of 4.9 presenting conditions for its evolution to allophanic Andisols. The coloidal fraction was dominated by allophanic materials rather than Al-humus (Alp/Alo < 0.5). The acid oxalate extractable Si and Al values reflected a low rate of weathering. These values decreased when mean annual precipitation diminished and a xeric soil moisture developed; showing higher values under Nothofagus forest than under grass-shrub steppe, reflecting the possible effects that vegetation types and mean annual precipitation have on soil weathering.

Keywords : Andosolization; Aluminum; Organic matter; Andinopatagonian Region.

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