Regional application of PnET-N-DNDC for estimating the N2O source strength of tropical rainforests in the Wet Tropics of Australia

In contrast to the significant importance of tropical rainforest ecosystems as one of the major sources within the global atmospheric N2O budget (2.2-3.7 Tg N yr(-1)), regional estimates of their N2O source strength are still limited and highly uncertain. To contribute toward more reliable estimates of the N2O source strength of tropical rainforest ecosystems on a regional scale, we modified a process-oriented biogeochemical model, PnET-N-DNDC, and parameterized it to simulate C and N turnover and associated N2O emissions in and from tropical rainforest ecosystems. Model modifications included: (1) new parameterizations associated with plant physiology and soil hydrology and the addition of algorithms relating daily leaf litterfall to water stress as well as to daily rainfall to account for the effects of heavy rainfall damage; (2) the development of a denitrifier activity index that depends on soil moisture conditions and influences N turnover by denitrification; and (3) the addition of a biological N fixation algorithm. Daily simulated N2O emissions based on site data were in good agreement (model efficiencies up to 0.83) with field observations in the Wet Tropics of Australia and Costa Rica. The model was even able to reproduce the highly dynamic pattern of N2O emissions with short-term increases during the wet season. Sensitivity analyses demonstrated that the PnET-N-DNDC model was sensitive to changes in soil properties such as pH, clay content, soil organic carbon and climatic factors such as rainfall and temperature. By linking the PnET-N-DNDC model to a geographic information systems database, tropical rainforests in a 9000 km(2) area of the Wet Tropics of Australia are estimated to emit 962 t N2O-N yr(-1) (2.4 kg N2O-N ha(-1) yr(-1)) between July 1997 and June 1998.