Atmospheric inputs of nitrogen.

The requirement for estimates of the terrestrial inputs of fixed nitrogen for Europe with a spatial resolution of between 5 km x 5 km and 50 km x 50 km places great demands on the monitoring information available and our knowledge of processes. It also provides a clear focus for the monitoring and process based research to provide both the input and an estimate of the uncertainty in inputs and its spatial variability. For the estimates of critical loads exceedance maps for nitrogen, maps of the total inputs are of course essential. In this brief review the focus is on what is known, and can be mapped, what is not known because insufficient monitoring is in place, and lastly what is not known because the processes are not understood or for which key parameters are uncertain. In addition, because the principal applications of mapped data are to estimate critical loads exceedance, the issue of the scale dependence of deposition estimates and the extent to which current approaches can be extended to provide fine scale or land use specific resolution is considered qualitatively and to a limited extent quantitatively.