Water-soluble Al inhibits methane oxidation at atmospheric concentration levels in Japanese forest soil

The effect of aluminium on methane oxidation was examined from incubation experiments involving the addition of several concentrations of Al solution (0.1, 0.2, 0.5, 1, 3 and 5 mM) to two soil samples that possessed different CH4 oxidation potential. Atmospheric CH4 oxidation activity was inhibited by the addition of as little as 0.1 mM Al solution (approximately 0.5 μg of Al per gram dry weight soil) to a forest soil that contained low water-soluble Al and possessed a high CH4 oxidation potential. Our results indicate that Al inhibition of CH4 oxidation activity is concentration-dependant after a certain time and the inhibition increases gradually over time until at least 96 h have elapsed. We also found that relatively small amounts of Al additions, such as 10-20 μg per gram dry weight of soil, halved the CH4 oxidation rate compared to the control, regardless of the original CH4 oxidation potential of the soil. Since the Al concentrations used in our experiment are often observed in forest soils, we can assume that Al acts as an important inhibitor of CH4 oxidation in forest soils under natural conditions. The sharp falls and a continuous decrease in CH4 oxidation rate in other forest samples with the addition of deionized water implies that the water-soluble Al contained in soils contributes to the inhibition of CH4 oxidation rate. This result suggests that precipitation causes a relatively prolonged inhibition of CH4 oxidation in soils containing a high concentration of water-soluble Al.