Amazonian Podzols – a carbon time bomb?

Abstract. It has recently been shown that the C stocks in Amazonian podzols are very large. They are much larger than was previously thought, particularly in the Bh horizon, which has been estimated to contain in excess of 10 Pg C for Amazonia alone. It is predicted that changes in the regional climate will result in a drier soil moisture regime, which may affect the C dynamics in these generally waterlogged soils. In order to determine the vulnerability to decomposition of the organic C contained in the Amazonian podzols as a result of environmental changes, we established a series of incubation experiments in which the effects of different environmental factors were measured. The direct effect of drier soil moisture regimes was tested by incubating undisturbed cores from the Bh horizon at a range of matric potentials. Contrary to what is usually found in soils, no significant difference in mineralisation was found among matric potentials, suggesting that other factors control microbial mineralisation of this organic C. In a second series of incubations, the effect of nitrogen additions, of anoxic conditions and of labile C substrate additions were also tested on undisturbed cores of the Bh horizon. Samples incubated under oxic conditions produced more than twice as much CO₂ as samples incubated under anoxic conditions, whilst the mineralisation rates of samples incubated under oxic conditions with the addition of N increased more than four-fold relative to the anoxic samples. The addition of labile C did not have a significant effect on C mineralisation. An extrapolation of the data to the whole of the Amazonian podzols suggests that changes in environmental conditions could result in an increased C-CO₂ flux to the atmosphere of up to 0.41 Pg C per annum. This is equivalent to 8 % of the current net global C flux to the atmosphere.