Environmental Research: This research theme is devoted to tracing nitrogen (N) and sulfur (S) in terrestrial and aquatic ecosystems using isotope techniques. To assess natural and anthropogenic components of the S cycle we have exploited lake sediments and lichen records documenting the extent of anthropogenic alterations throughout the last few decades. Research by our HQP has also made major contributions to improving the understanding of S cycling in terrestrial and aquatic ecosystems by identifying various sulfate sources and elucidating sulfate transformation processes in snow, vegetation, soils, groundwater, surface waters and oil field brines. We also obtained novel insights into N cycling on the continents by identifying sources of nitrate and N transformation processes in soils, surface waters, groundwater, and oilfield brines. Among the major recent findings by our HQP that have implications for the proposed research are the following: We found that nitrate and sulfate in atmospheric deposition from long-range transport and some anthropogenic point sources have distinct isotopic compositions (e.g. δ18Onitrate, Δ17Onitrate). Preliminary characterization of isotopic compositions of synthetic fertilizers, manure and effluents from waste water treatment plants in Western Canada has demonstrated that many N compounds and sulfate from these sources are often isotopically distinct. Furthermore, we have developed approaches to determine the isotopic compositions of extractable nitrate and ammonium and of soil gas N2O from different depths enabling us to study the soil nitrogen cycle in more detail than previously possible. The obtained knowledge of isotopic compositions of N and S compounds in atmospheric deposition, soil solutions, agricultural effluents, and waste waters has enabled our research group to reveal the sources that control nitrate and sulfate contamination in aquifers, rivers, and lakes. However, a thorough knowledge of some of the processes and the transit times controlling nutrient releases into aquatic systems is still elusive. A hallmark of our research approach is that we do not use stable isotope techniques as a stand-alone technique but that we interpret isotopic information in concert with physical, chemical and mass balance information in order to independently verify postulated processes, sources and sinks. In addition, we use isotopically labeled compounds to assess the transit times of N and S in terrestrial and aquatic ecosystems.