An important feature of global change is the increased deposition of active forms of nitrogen from human activities. Nitrogen deposition has increased rapidly in recent decades and is expected to double over the next 100 years. It is intense in industrialized regions of the world but occurs to some degree everywhere. Nitrogen is a nutrient for plants and microbes and so we may expect this process to affect biological activity and consequently the fluxes of GHGs. Indeed, nitrogen supply is often a rate-limiting factor for forest growth in temperate regions, and so nitrogen deposition may be making many of our forests grow faster, hence increasing the carbon sink they represent.
De Vries et al. (Chapter 17, this volume) address this issue, referring to empirical and modelling studies of European forests. They examine the effects of nitrogen deposition on the emissions of the three main GHGs: CO2, N2O and CH4. First, they provide an overview of the processes that lead to an increase of the vegetation CO2 sink, an increase of N2O emissions and a decrease of the soil CH4 sink, in response to elevated nitrogen inputs. Secondly, they estimate the net exchange of GHGs by European forests and the impacts of nitrogen deposition on this exchange in terms of global warming potential (GWP). In performing these quantifications, they use: (i) literature data on field measurements and applications of detailed biogeochemical models; and (ii) empirical and simple process-oriented models, combined with measured and modelled nitrogen deposition data at ~500 (measurements) to 6000 (model results) forest-monitoring plots.
The authors estimate that the forest carbon sink has increased by ~10% during 1960-2000 as a result of this 'fertilization'. At the same time, there has been a stimulation of N2O emissions and a change in the CH4 sink. Results show that the average reduction in GWP due to CO2 sequestration by European forests is offset by N2O emissions by ~10%, whereas the net uptake of CH4 is negligible compared to CO2 sequestration.
On average, the effect of nitrogen deposition on increasing N2O emissions is estimated at ~10-15% of the increased CO2 sequestration. The positive effect on increasing the vegetation CO2 sink is thus much larger than the effect on increasing the N2O emissions. The effect of nitrogen deposition on reduced CH4 uptake emissions is very small and highly uncertain.
De Vries et al. stress that, although their general conclusions are robust, the complexity of the processes involved and the large scale and diversity of the forests indicate that the impact of environmental factors on the emissions and sinks of CO2 and specifically on N2O and CH4 remain highly uncertain. To improve estimates in future studies, it is deemed necessary to have an integrated approach consisting of: (i) field measurements and process studies in the laboratory; (ii) further development and testing of detailed process-oriented bio-geochemical models at plot scale; and (iii) upscaling of results by further developing, as well as using empirical models and simplified process-based models.
None the less, the work with small plots discussed here is an important start. We look forward to more published work over the next few years, following the launch of a new project called 'Nitroeurope' in 2006.
17 Impact of Atmospheric Nitrogen Deposition on the Exchange of Carbon Dioxide, Nitrous Oxide and Methane from European Forests
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