Effect of aluminium on mineralization of water extractable organic matter and microbial respiration in southern temperate rainforest soils

To date the effect of aluminium (Al) on carbon (C) mineralization has not been extensively studied in Chilean temperate rainforests. Here, we examine to what extent the C mineralization of water extractable organic matter (WEOM) and mineral Ah horizon are affected by differing molar ratios of Al and C (Al:C). It has been postulated that an increase in the concentration of Al in the soil solution does not affect the mineralization of WEOM and Ah horizon until a Al:C threshold is reached, at which point mineralization decreases. We hypothesized that dissolved organic C binds Al that is otherwise toxic to soil microorganism in the soil solution. Subsequently, the organic C remaining is mineralized or lost in water sheds. A microcosm experiment was designed where Al was added to WEOM or Ah horizon (0–8 mg L−1 or kg−1). Potential biodegradation was evaluated by trapping CO2 emitted during 15 days of incubation. Additionally, the amount of living fungal cells in the WEOM and microbial biomass of Ah horizon were quantified. The Al complexation was determined by selective dissolution, and characterized by infrared spectroscopy. In general, the carbon mineralization rates and the amount of fungal cells of WEOM were similar to that of the control (0 Al addition) when the Al:C ratio was <0.16. With increasing Al concentrations, mineralization was reduced up to 80%, accompanied by the formation of organo-mineral complexes, as indicated by infrared analysis. We conclude that Al exerts fine regulation on C availability necessary for microbial growth in both, WEOM and the Ah horizon.