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Training in sources, sinks and impacts of atmospheric nitrogen deposition in the Arctic

Final Report Summary - NSINK (Training in sources, sinks and impacts of atmospheric nitrogen deposition in the Arctic)

1.1 Overview and objectives
The concept behind NSINK (http://nsinkproject.group.shef.ac.uk/NSINK/Home.html) was to use a major European research facility (namely Ny Ålesund in high Arctic Svalbard) and train twelve fellows to investigate nitrogen enrichment in Arctic ecosystems. A major motivation behind this was to better understand how nitrogen pollution from European and other areas might influence the fragile ecosystems of the High Arctic as they respond to climate change. An integrated, multidisciplinary approach was therefore adopted that delivered training in atmospheric sciences, snow physics, hydrology, biogeochemistry and aquatic/terrestrial ecology using experienced practitioners in UK, Norwegian, Swedish and Austrian institutions.

The scientific objectives of NSINK were to:

1.Establish the climatology and dynamics of atmospheric nitrogen delivery to the European High Arctic at an unprecedented range of temporal scales;

2. Construct mass balance models of the biogeochemical cycling of reactive nitrogen in the resident snow, ice and ecosystem stores within this part of the European High Arctic;

3. Conduct process studies of nitrogen dynamics that include the use of natural and artificial tracers (where relevant) of the fluxes that link the above stores;

4. Determine ecosystem response to enhanced atmospheric N deposition, and consequences for ecosystem biodiversity, productivity and carbon balance;

5. Produce models with the capacity to forecast ecosystem response to scenarios of coupled climate warming and atmospheric nitrogen enrichment.

The research objectives were met successfully through the appointment of nine early stage researchers and three experienced researchers across the network. Training was delivered through a training needs analysis approach and formal instruction and experiential training in field and laboratory environments. At least two major field campaigns were conducted by most fellows, and since the field activities were centred upon Svalbard, a great deal of collaboration took place while this was being conducted. Interaction among the fellows was further enhanced by Fellows’ Meetings in Amsterdam and Prague, as well as during annual network meetings. Once the training and field data collection were successfully implemented, the emphasis shifted toward research dissemination, thesis writing and career development skills: the latter including a two day Careers Workshop at the University of Sheffield.

1.2 Research outcomes

Objective 1. The five fellows addressing Objective 1 have demonstrated the dynamics of atmospheric nitrogen at a range of temporal scales by making use of archival analysis (Kühnel), snow pit studies (Björkman and Riquelme), atmospheric modelling (Kühnel and Roberts) and ice core analysis (Riquelme and Samyn). Work by Riquelme and collaborators has shown using stable isotope measurements that a rather different nitrogen inventory has been preserved in Svalbard ice cores than in Greenland. This suggests that key differences in the sources and/or transport pathways of nitrogen exist between these two parts of the Arctic. Work by these fellows has revealed that increased nitrogen accumulation in the Svalbard ice cores has most likely been dominated by extreme events associated with the rapid advection of polluted air masses into the Arctic during favourable weather conditions. However, Riquelme has shown that the increased incidence of summer melting greatly complicates the detection of these events in the ice core record. However, Kühnel was able to use archive analysis to show that half of the nitrogen deposition has recently been caused by fewer than 20% of the precipitation events, and that such events can (and will continue to) occur at any time of the year. Their impact upon resident ecosystems will therefore differ greatly according to the timing of their incidence. For example rainfall upon warm tundra will deliver atmospheric nitrogen immediately to terrestrial ecosystems, whilst winter snowfall will not release the nitrogen until melting begins in summer. At this time however, most of it will be advected straight into aquatic ecosystems because underlying soils will remain frozen during early snowmelt runoff.

Objectives 2 and 3. Mass balance models of Arctic nitrogen biogeochemistry have revealed the timing, magnitude and composition of nitrogen that accumulates in the Arctic landscape as snow and ice. Björkman assessed the relative importance of wet and dry deposition, finding that the former represents the great majority of nitrogen deposited from the atmosphere. He also collaborated with Zarsky to assess the characteristics of nitrogen leaching from the snowpack by summer melt, when Zarsky found evidence of its utilisation within the snowpack by a resident microbial community. The mass fluxes of nitrogen in runoff at various points in the landscape were then calculated by Nowak-Zwierz and Ansari, who were able to show that wetlands and glacial sediments become net sources of nitrate soon after the retreat of the snowpack. Very high concentrations of nitrate were therefore found in late summer runoff by Nowak-Zwierz, who, along with Ansari, used stable isotopes to show how bacteria were most likely responsible, by nitrifying ammonium and organic N. Of particular significance was the discovery that mineral-derived ammonium may also be liberated by glacial erosion and help stimulate these bacteria. Changes in glacier ice cover over decadal time scales were found to be very important in this context by Nowak-Zwierz. Her archive work revealed how glacier retreat is exposing more and more fresh mineral surfaces to weathering processes that easily influence the chemistry of runoff. Holm also found changes in hydrochemistry over this timescale, this time in a lake ecosystem where she had been using sediment cores to reveal increasing nitrogen accumulation. Biogeochemistry was therefore a key tool for detecting changes in landscape ecology during NSINK.

Objectives 4 and 5. The sensitivity of terrestrial ecosystems to atmospheric deposition was examined directly through plot experiments on the tundra by Choudhary and Blaud. Oulehle then used these data and the hydrological data collected by Nowak-Zwierz to explore ecosystem impacts using a new version of the MAGIC model. Early outcomes reveal that little of the deposited nitrogen is incorporated into the microbial pool of the soil because plants, especially bryophytes, are more successful at utilising the incoming nutrients. The mineral soil’s capacity to store incoming nitrogen is also limited by low, pre-existing C/N ratios in its organic matter. Monthly simulations using MAGIC were then easily capable of showing how the timing of acute N deposition is crucial, including how the greatest loss to runoff occurs in association with winter deposition (see above). Chronic, elevated deposition of N (0.4 g m-2 yr-1; spread throughout the year) was found to be more likely to be immobilized (~80%) in the soil. Isotope labelling experiments also showed that the deeper roots of vascular plants led to sedge being most effective at assimilating nitrogen from deep within the mineral soil. Therefore, although nitrogen deposition would at present seem to favour bryophytes (mosses), the concomitant increase of summer active layer depths might support the vascular plants in future. Release of the N from soil organic matter in response to climatic warming may then favour these plants at the same time. As a result, the responses of the terrestrial ecosystems to warming and nitrogen deposition may be opposing, with N deposition favouring bryophyte growth, and warming favouring higher plants. Ongoing work will be crucial for trying identifying the most likely direction and magnitude of change that will take place in future.

1.3 Impact of NSINK

NSINK science has improved our understanding of the history of anthropogenic nitrogen enrichment in the European High Arctic, and revealed complex responses from its resident terrestrial and aquatic ecosystems. In addition to these major scientific achievements, there have been positive impacts upon the fellows, partners and European Research Area (ERA). The benefits to the fellows include: i) excellent training in scientific, professional and complementary skills; ii) career development guidance; iii) international mobility, and iv) being part of a project that delivered exciting science, in areas of political and public interest. The benefits to the partners include: i) the stimulation of new interdisciplinary research; ii) the exchange of best practice between partners; iii) raising the profile of our research amongst policy makers, academics and practitioners beyond NSINK; iv) development of a team of highly employable, well-trained scientists who are likely to work with us again; iv) enhancement publication outputs in international refereed journals.

The benefits at the European level include: i) relevant activity with respect to European research objectives as defined in FP7 and the “Environment (including Climate Change)” with particular relevance to ‘Activity: climate change, pollution, and risks’ and ‘Sub-activity: Pressures on environment and climate’; ii) help in overcoming fragmentation and barriers to mobility across sectors, disciplines and countries. In so doing, NSINK has enhanced the future contribution of some of ERA’s best graduates and early career researchers; iii) NSINK has helped avoid duplication of science and training and has promoted greater use of EU resources in the future; iv) wide dissemination of ITN outcomes have advertised the benefits of the MC programme; and v) attracting resources to less favoured regions within the EU. Finally, NSINK has the capacity to influence EU policy in areas that include the Convention On Long-Range Transboundary Air Pollution; EC Thematic Strategy on Air Pollution; FP7 ‘Environment (including Climate Change)’; UN Framework Convention on Climate Change (UNFCCC); and the Arctic Climate Impact Assessment (ACIA). NSINK will also assist adherence to The 1988 Sofia Protocol concerning the Control of Emissions of Nitrogen Oxides or their Transboundary Fluxes.