Allophane, aluminum, and organic matter accumulation across a bioclimatic sequence of volcanic ash soils of Argentina

An investigation was conducted to study Al fractions, organic matter accumulation, and their effects on physicochemical properties in soils derived from volcanic ash at the Andinopatagonian region in SW Neuquén, Argentina. Five sites were selected in a climatic biosequence, ranging from Nothofagus forest with a udic soil moisture regime at the footslope of the Andes mountain range to grass-shrub steppe with a xeric soil moisture regime in the hills and plains. The morphological characteristics showed young pedons, containing andic, mollic, and cambic horizons. A change from low bulk density from <0.82 Mg m-3 in Andisols under a udic regime to medium density values of ∼1.1 Mg m-3 in Xerands and Xerolls was observed. This was related to a lower rate of weathering probably coinciding with a lower content of allophane and higher clay crystallization and to a decrease in total soil organic carbon (Corg). The rate of soil weathering was estimated from acid oxalate extractable Al, which together with Corg significantly explained the bulk density variation (R 2=0.7091, p<0.00002). Soil acidity varied from moderately and strongly acidic in Andisols with a udic soil moisture regime to slightly acid and almost neutral in Xerands and Xerolls. The higher base concentration and low soil acidity in Xerolls were attributed to less leaching, due to lower mean annual precipitation. However, pHNaF and Corg were the variables which significantly explained the pHwater variation (R 2=0.7806, p<0.0000). This showed that soil acidity was closely related to Al activity and organic matter content, rather than to base content. The lower C/N ratios, in the surface horizons of the Xerands and Xerolls versus the Andisols with a udic soil moisture regime, suggested that the rate of mineralization was faster under warmer conditions with xeric moisture regime than with udic moisture regime. There was also a remarkable difference in the humic C/fulvic C ratio in A-horizons, which may reflect the input of vegetation residues indicating a sensible contribution of the cover of grass-shrub steppe to the formation of humic substances. The acid oxalate extractable Si and Al values reflected a low rate of weathering, which decreased when mean annual precipitation decreased and a xeric soil moisture developed being also higher under Nothofagus forest than under grass-shrub steppe, which reflects the possible effect of the mean annual precipitation gradient (2000-700 mm) and vegetation types on soil weathering. The differences in type and quantity of organic matter input, with a higher organic matter content and lower humic C/fulvic C ratio under Nothofagus, indicate a more reactive organic matter which may be somewhat responsible for the weathering degree. No significant relationship between acid extractable Al and Corg was found suggesting that organically bound Al is not dominant. The detection of higher allophane estimations in soils with the highest Corg levels, may indicate that the anti-allophanic hypothesis probably does not apply to these soils.Fil: Broquen, Patricia Cecilia. Universidad Nacional del Comahue. Facultad de Ciencias Agrarias; ArgentinaFil: Lobartini, Juan Carlos. Universidad Nacional del Sur. Departamento de Agronomía; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca; ArgentinaFil: Candan, Florencia. Universidad Nacional del Comahue. Facultad de Ciencias Agrarias; ArgentinaFil: Falbo, Gabriel. Universidad Nacional del Comahue. Facultad de Ciencias Agrarias; Argentin

Relationships between soil organic carbon and siteproperties in volcanic ash soils, southwestern Neuquén (Argentina)

An investigation was conducted to study the relationships between soil organic carbon and thoseproperties that may influence carbon accumulation in soils derived from volcanic ash through a rangeof bio-climatic conditions at the Andinopatagonian region in SW Neuquén, Argentina. Pedon andclimatic data of nineteen plots were used and grouped, through principal component analysis, by soilhydric regime –udic group (U) and xeric group (X)-. Within each group, a simple and a multipleregression analysis was carried out between organic C -content (t ha-1) or concentration (g kg-1)- andmean annual precipitation, bulk density, pH(H2O) and pH(NaF), oxalate extractable Al (Alo), pyro-phosphate-extractable Al (Alp), Alp/Alo, and Alo-Alp –the last four so as content or as concentra-tion. Mean annual precipitation, explained little variation in soil organic carbon in both groups (R2 =20), and bulk density was unrelated to soil organic carbon. Alp correlated strongly with organic Ccontent in both groups (U: R2 = 0.67, p<0.00001; X: R2 = 0.90, p<0.00001), but when expressed asconcentration Alp explanation was considerably lower (U: R2 = 0.10, p<0.02; X: R2 = 0.46, p<0.0006).When all factors were combined in a multiple regression analysis, the combination of Alp contents andmean annual precipitation, explained the greatest amount of variation in soil organic C content withinU group (R2U = 0.69, p<0.00001). According with previous studies, relationships between Alp/Alo or(Alp-Alo) and soil organic C were very poor, this confirms that organically bound Al in allophane isnot dominant. The fact that the better explanations were found when variables were expressed incontent rather than in concentration was probably due to the influence of specific area on humicmatter adsorption on volcanic ash soils. Our results suggested that in volcanic ash soils in SWNeuquén, Argentina, the chemical stabilization is the key process controlling soil organic C accumu-lation in some way independent of vegetation types.Fil: Broquen, Patricia Cecilia. Universidad Nacional del Comahue. Facultad de Ciencias Agrarias; ArgentinaFil: Candan, F.. Universidad Nacional del Comahue. Facultad de Ciencias Agrarias; ArgentinaFil: Lobartini, Juan Carlos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca; Argentina. Universidad Nacional del Sur. Departamento de Agronomía; ArgentinaFil: Girardin, J. L.. Universidad Nacional del Comahue. Facultad de Ciencias Agrarias; Argentin