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

Abstract 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 Neuquen, 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 −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 (C org ). The rate of soil weathering was estimated from acid oxalate extractable Al, which together with C org significantly explained the bulk density variation ( R 2 =0.7091, p NaF and C org were the variables which significantly explained the pH water variation ( R 2 =0.7806, p 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 C org was found suggesting that organically bound Al is not dominant. The detection of higher allophane estimations in soils with the highest C org levels, may indicate that the anti-allophanic hypothesis probably does not apply to these soils.