Use of 15N-labelled nitrogen deposition to quantify the source of nitrogen in runoff at a coniferous-forested catchment at Gårdsjön, Sweden

To determine the source of dissolved inorganic nitrogen (N) in runoff, approx. 35 kg N enriched with the stable isotope 15N (2110‰ δ15N) was added to a mature coniferous forested catchment for one whole year. The total N input was approx. 50 kg ha−1 year−1. The enrichment study was part of a long-term whole-catchment ammonium nitrate addition experiment at Gårdsjön, Sweden. The 15N concentrations in precipitation, throughfall, runoff and upper forest floor were measured prior to, during, and 3–9 years following the 15N addition. During the year of the 15N addition the δ15N level in runoff largely reflected the level in incoming N, indicating that the leached NO3− came predominantly from precipitation. Only 1.1% of the incoming N was lost during the year of the tracer addition. The cumulative loss of tracer N over a 10-year period was only 3.9% as DIN and 1.1% as DON.