Site specific and regional estimates of methane uptake by tropical rainforest soils in north eastern Australia

Methane flux from rainforest soils in northeast Queensland, Australia, was investigated using a combination of laboratory, field and simulation modelling. In aerobic laboratory incubations, CH4 uptake in the top 0.1 m of the soil (-2.5 to -7.3 mu g CH4 kg(-1) SDW day(-1)) is approximately one order of magnitude higher than CH4 production under anaerobic conditions. The highest CH4 uptake, as well as potential CH4 production is found in the uppermost C rich soil layers. Detailed measurements from three contrasting rainforest sites identified the soils to be functioning as sinks for atmospheric CH4. Fifteen months continuous measurement at one of the lowland rainforest sites showed that the seasonality of CH4 uptake was mainly driven by changes in soil moisture rather than by temperature changes. Maximum CH4 uptake (109 mu g CH4 m(-2) h(-1)) was observed during dry season conditions, whereas during the wet season, CH4 uptake decreased significantly to near zero. Based on our laboratory experiments and on published literature we developed a semi-empirical CH4 module for the biogeochemical model ForestDNDCtropica. Tests at several sites showed the robustness of our modelling approach with mean simulated values within 12% of observed values. To estimate regional CH4 uptake by rainforest soils in the region of the 'Wet Tropics', Queensland, Australia, we linked CH4 uptake and production algorithms to a regional GIS database. We estimated that the lowland and montane rainforest soils in northeast Queensland, Australia, were a net sink for CH4 with a mean uptake rate of -2.89 kg CH4 ha(-1) year(-1) during July 1996 to June 1997 period.