Episodic behaviour of nitrate in runoff during six years of nitrogen addition to the NITREX catchment at Gardsjon, Sweden

Since 1991 nitrogen deposition to a small forested catchment has been exper imentally increased from ambient 11 to a total of 50 kg N ha(-1) year(-1) b y sprinkling small portions of dissolved ammonium nitrate on about 30 occas ions annually. Concentrations of inorganic N in runoff (streamwater) were m easured by flow-proportional sampling and during 6 shorter periods by frequ ent sampling and continuous monitoring. Shortly after the onset of the N ad ditions, nitrate (NO3) began to appear in the runoff (streamwater), after 2 years in soil solution at shallow depths and after 4 years at all depths. N concentrations in runoff were highly variable, and most of the N lost fro m the catchment occurred during a few episodes. Flux of N from the catchmen t in runoff was much larger than that calculated from concentrations in lys imeter samples and discharge from groundwater levels. The annual inorganic N loss was only about 5% of the incoming N. Episodic behaviour of NO3 in ru noff was studied by monitoring discharge episodes after 2, 3, 4 and 6 years of N addition. The nature of NO3 concentrations in runoff during these epi sodes changed as the treatment proceeded. Factors such as season, hydrology , timing and size of N additions govern the loss of NO3 in runoff during ep isodes. Nitrogen saturation at this catchment may proceed first in the disc harge areas near the outlet to uphill soils on the slopes.