Ralf Kiese

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. (Less)

Significance of Forests as Sources for N2O and NO

Forest ecosystems cover approximately 20% of the Earth’s land area, and forests are a major source of gases such as N2O and NO that influence Earth’s climate. Tropical forests are one of the most important sources of N2O in the global atmospheric budget. Recent results from field experiments indicate that temperate forests may be more important sources than previously expected, owing to underestimations of N2O emissions. The higher than expected emissions of N2O may result from increased availability of N in temperate forests from atmospheric N deposition and its stimulating effect on N2O but also NO emissions. High rates may also be driven by repeated freezing and thawing of forest soils. Some non-tropical forests may also have a weak capacity to act as a net sink for N2O, though the mechanism is not currently known. Tropical forest soils are major sources of NO, and the NO emission from temperate forests may increase following increased atmospheric N deposition. The large uncertainties in the budgets for these gases in forest ecosystems may be reduced if process-oriented models are developed and applied to simulate all major C and N cycling and biosphere-atmosphere exchange processes, in a spatially explicit approach that will allow scaling from points to regions to the globe.