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

Since 1991 nitrogen deposition to a small forested catchment has been experimentally increased from ambient 11 to a total of 50 kg N ha−1 year−1 by sprinkling small portions of dissolved ammonium nitrate on about 30 occasions annually. Concentrations of inorganic N in runoff (streamwater) were measured by flow-proportional sampling and during 6 shorter periods by frequent sampling and continuous monitoring. Shortly after the onset of the N additions, 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 from the catchment occurred during a few episodes. Flux of N from the catchment in runoff was much larger than that calculated from concentrations in lysimeter samples and discharge from groundwater levels. The annual inorganic N loss was only about 5% of the incoming N. Episodic behaviour of NO3 in runoff 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 episodes changed as the treatment proceeded. Factors such as season, hydrology, timing and size of N additions govern the loss of NO3 in runoff during episodes. Nitrogen saturation at this catchment may proceed first in the discharge areas near the outlet to uphill soils on the slopes.