Abstract
The human-driven production of reactive nitrogen to stimulate agricultural productivity and its unintended formation in combustion processes both have major impacts on the global environment. Effects of excess reactive nitrogen include reductions in air quality, water quality, soil quality and biodiversity. One of the most controversial impacts of nitrogen, however, is on the greenhouse gas balance. While recent papers have highlighted a possible benefit of nitrogen in enhancing rates of carbon sequestration, there remain many trade-offs between nitrogen and greenhouse gas exchange. The result is that the net effect of reactive nitrogen on the global radiative balance is currently far from clear. To better quantity these relationships requires measurement data and modelling that make the link between different nitrogen forms and their fate in the environment. It is essential to measure fluxes for a wide range of ecosystems considering the biosphere-atmosphere exchange each of the reactive nitrogen components and greenhouse gases, as well as the fixation and denitrification of di-nitrogen. Long term observations are needed for representative ecosystems, together with results from experiments addressing the responses of the key nitrogen and greenhouse gas fluxes to different global change drivers. The NitroEurope Integrated Project of the 6th Framework Programme of the European Commission European has developed a strategy to quantifying these different terms on multiple scales. This paper presents the experimental approach including a) a 3-tier flux network, combining process level measurements and new method development with low-cost measurements at many sites, b) a network of manipulation experiments with different global change drivers, c) a network of contrasting European landscapes for analysis of land- use and land management interactions assessing the multiple nitrogen fluxes within and between air, land and water. The paper illustrates the new datasets emerging, and shows how these are being used to support the development of site, landscape and regional-scale models of nitrogen fluxes and net greenhouse gas exchange. Finally, the paper describes how independent verification activities are providing the basis to analyze the uncertainty in regional-scale nitrogen and greenhouse gas fluxes.