Aluminium stabilization controls organic carbon levels in Chilean volcanic soils

Chilean volcanic soils are known to contain large amounts of organic matter (OM) and amorphous (e.g., allophanic) clay. Here, we test the hypothesis that Al, rather than clay content and climatic conditions, is the most important factor for OM levels in volcanic soils. This was achieved by compiling a total of 225 pedons from two national-wide datasets of south-Central Chilean soils sampled in the first 20 cm depth. All selected soils are of volcanic origin, previously under native Nothofagus-forest and now used as pasture or arable land. The relationships between soil organic C (from now soil C) and environmental and soil characteristics (clay-allophane, Al content) were analysed by simple and multiple linear regression analyses. Soils classify as Alfisols, Andisols, Inceptisols and Ultisols. Al was extracted by KCl (Alk), acid ammonium acetate (Ala) and acid ammonium oxalate (Alo). The simple regression models showed that Ala correlated best (R 2 =0.52) with soil C variations for the four soil orders studied, while Alk (R 2 =0.08) and Alo (R 2 =0.17) were poorly correlated in all soils types. Especially in Inceptisols, Ala was highly correlated with soil C (R 2 =0.88). Clay content, and hence allophanic content, was poorly correlated with soil C (R 2 =0.14), as was the mean annual precipitation (R 2 =0.21) and temperature (R 2 =0.15). These patterns were supported by the multiple regression analyses. Results therefore support the idea that Al is the principal factor for immobilization of OM in acid volcanic soils. As ammonium acetate extractable Al at pH 4.8 correlated well with OM, and as this is a rarely used method to interpret pedogenic processes, this extraction method warrants further attention.