Nitrogen in surface waters: time trends and geographical patterns explained by deposition levels and catchment characteristics

The report presents a statistical analysis of controls (deposition, land cover, climate) on spatial variability and long-term trends (1992-2016) in surface water concentrations of nitrate (NO3) and carbon to nitrogen (C/N) ratios of dissolved organic matter (DOM) at the ICP Water sites in Europe and North America. Nitrogen (N) deposition has declined over large parts of Europe and North America. Surface water NO3 has decreased significantly at 46% of sites, increased at a few sites (4%) and has had no significant trend at the remaining sites. The C/N ratio of DOM has increased at 52% of the sites, decreased at 5% and has no trend at 43% of the sites. The greatest decline in surface water NO3 occurred at sites with high N deposition and high NO3 concentrations. The increase in C/N ratios of DOM is mainly related to increasing trends in total organic carbon (TOC) and seems largely unaffected by N deposition levels or trends. Data from most of the ICP Waters sites do not suggest that N saturation is occurring presently, but there is still a risk that climate change and ecosystem disturbances might cause higher N leaching in the future.