The nitrogen (N) cycle involves a set of N compounds transformed by plants and microbes. Some of these N compounds, such as nitrous oxide (N2O) or nitrate (NO3
- ), are environmental pollutants jeopardizing biodiversity, human health or the global climate. The natural abundances of the common (14N) and rare (15N) stable N isotopes in a given compound, i.e. the isotopic composition, depend on individual production and consumption processes. As each process has an individual preference for the common or
rare isotope (isotope effect) the measurement of the isotopic composition has been identified as a powerful tool for improved process understanding and source process identification. Both are key requirements
for the development of strategies aiming at mitigating the release of environmental harmful N compounds. However, up to now, no comprehensive compilation of N cycle isotope effects is available. A compilation of isotope effects is also in high demand for testing biogeochemical models as such
models are increasingly used to study N cycling in ecosystems and across landscapes and regions. Biogeochemical models are usually calibrated and validated only with single, easily accessible quantities. The isotopic composition of N compounds has a high potential to be used as additional, integrative parameter for a more thorough assessment of simulation results. For instance, the isotopic composition of soil N is determined by fractionation of the most relevant processes and, thus, integrates several N cycle processes, some of which cannot be accessed easily by direct measurement. To implement isotopic fractionation in ecosystem models and to utilize those for model validation, the magnitude of the isotope effects associated with individual transformations in the N cycle need to be assessed...
