Changes in Svalbard’s nitrogen cycle: seasonal to multi-decadal perspectives based upon studies of riverine biogeochemistry

Summary

Work undertaken in Svalbard more than 20 years ago first revealed that glacial runoff exports an excess of nitrate compared to atmospheric inputs to the land surface. This finding was unexpected in a heavily glacierised landscape and now seems critical for understanding the future seaward transport of this important productivity-limiting nutrient. We show that the additional nitrate is derived from the microbial nitrification of ammonium and (less so) organic N derived from: i) atmospheric inputs via snow, firn and glacier ice; and ii) terrestrial substrates associated with organic matter and bedrock ammonium. We argue that climate change impacts have heavily depleted the former, yet are increasing the latter. This problem and its consequences are further explored using long-term observations of inorganic nitrogen concentrations in runoff and natural abundance of 15N and 18O in nitrate collected since the early 1990’s. In so doing, we report that: 1. Atmospheric nitrate inputs (either direct or via nitrified atmospheric ammonium) only dominate the seaward flux of nitrate in runoff during the earliest parts of the summer; 2. The influence of the nitrified nitrate generally increases throughout the summer, and can in some cases generate extreme concentrations by the end of the season. 3. Spatial variability in the impact of nitrified nitrate upon runoff fluxes is extremely significant across Svalbard; 4. Deglaciation promotes nitrification, not denitrification – due to the predominance of shallow flow paths through fine sediments with modest or low organic matter. Current research is trying to establish the impact of tidewater glacier retreat onto land with some urgency, because it is believed that this transition will have the greatest influence upon nitrate availability to near-surface phytoplankton. Our work clearly demonstrates that major changes in the delivery of terrestrial nitrogen need to be considered if we are to properly understand this transition