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

Surface waters in Europe and North America have been exposed to several decades of elevated nitrogen (N) deposition. Since the 1990s, N deposition has declined in many regions, but not to the same extent as sulphur deposition. Ecosystem N saturation has been a concern, but so far, no sign of region-wide increase in nitrate (NO3) leaching has been observed. Results from the most recent ICP Waters trend report (Garmo et al. 2020) show that NO3 has declined in all 12 ICP Waters regions, although many of the individual sites show no trend. The greatest declines were observed in the regions with the highest NO3 levels. A few sites show increasing NO3 concentrations, and this is probably due to site-specific factors or disturbances and not caused by the ambient N deposition levels as such. 

The present report builds on the findings in the trend report by Garmo et al. (2020) and aims to explore possible explaining factors for observed trends in NO3 and the C/N ratio of DOM at ICP Waters sites in Europe and North America over the past 25 years. Data on N species concentrations NIVA 7728-2022 ICP Waters 149/2022 at the ICP Waters sites are analysed along with data on N deposition, climate and catchment properties to better understand what controls the variability in N trends and levels. More specifically, authors quantify trends and geographical patterns in NO3 concentration in surface waters. Additionally, authors analyse temporal trends and spatial variability in C/N ratios of DOM in surface waters, as a proxy for N enrichment of catchment soils and elevated risk for N leaching losses to surface waters (cf. the N saturation hypothesis). Authors test if patterns in temporal trends and spatial variability can be explained by a combination of catchment properties, climate and N deposition trends. Moreover, they investigate how surface water chemistry can indicate possible N enrichment of catchment soils and
risk of future N losses to surface waters.