Biosphere atmosphere exchange of reactive nitrogen and greenhouse gases at the NitroEurope core flux measurement sites: Measurement strategy and first data sets.

The NitroEurope project aims to improve understanding of the nitrogen (N) cycle at the continental scale and quantify the major fluxes of reactive N by a combination of reactive N measurements and modelling activities. As part of the overall measurement strategy, a network of 13 flux ‘super sites’ (Level-3) has been established, covering European forest, arable, grassland and wetland sites, with the objective of quantifying the N budget at a high spatial resolution and temporal frequency for 4.5 years, and to estimate greenhouse gas budgets (N2O, CH4 and CO2). These sites are supported by a network of low-cost flux measurements (Level-2, 9 sites) and a network to infer reactive N fluxes at 58 sites (Level-1), for comparison with carbon (C) flux measurements. Measurements at the Level-3 sites include high resolution N2O, NO (also CH4, CO2) fluxes, wet and dry N deposition, leaching of N and C and N transformations in plant, litter and soil. Results for the first 11 months (1.8.2006 to 30.6.2007) suggest that the grasslands are the largest source of N2O, that forests are the largest source of NO and sink of CH4 and that N deposition rates influence NO and N2O fluxes in non-agricultural ecosystems. The NO and N2O emission ratio is influenced by soil type and precipitation. First budgets of reactive N entering and leaving the ecosystem and of net greenhouse gas exchange are outlined. Further information on rates of denitrification to N2 and biological N2 fixation is required to complete the N budgets for some sites. The quantitative roles played by CO2, N2O and CH4 in defining net greenhouse gas exchange differ widely between ecosystems depending on the interactions of climate, soil type, land use and management