A global inventory of N2O emissions from tropical rainforest soils using a detailed biogeochemical model

Beside agricultural soils, tropical rainforest soils are the main source of atmospheric N2O. Current estimates of the global N2O source strength of tropical rainforest soils are still based on rather simplistic upscaling approaches and do have a large range of uncertainty. In this study, the biogeochemical ForestDNDC-tropica model was recalibrated and intensively tested on the site scale prior to inventory calculations. For this, the model was coupled to a newly developed global GIS database holding relevant information on model initialization and driving parameters in 0.25 degrees x 0.25 degrees resolution. On average, the mean annual N2O emission source strength of rainforests ecosystems worldwide for the 10-year-period 1991-2000 was calculated to be 1.2 kg N2O-N ha(-1) yr(-1). Using a total rainforest area of 10.9 x 10(6) km(2), this amounts to a total source strength of 1.34 Tg N yr(-1). The result of an initialization parameter uncertainty assessment using Latin Hypercube sampling revealed that the global source strength of N2O emissions from tropical rainforests may range from 0.88 to 2.37 Tg N yr(-1). Our calculations also show that N2O emissions do vary substantially on spatial and temporal scales. Regional differences were mainly caused by differences in soil properties, whereas the pronounced seasonal and interannual variability was driven by climate variability. Our work shows that detailed biogeochemical models are a valuable tool for assessing biosphere-atmosphere exchange even on a global scale. However, further progress and a narrowing of the uncertainty range do crucially depend on the availability of more detailed field measurements for model testing and an improvement of the quality of spatial data sets on soil and vegetation properties.