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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 ofinorganic 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 alldepths. N concentrations in runoff were highly variable, and most ofthe N lost from the catchment occurred during a few episodes. Flux of N from the catchment in runoff was much larger than that calculatedfrom concentrations in lysimeter samples and discharge from groundwater levels. The annual inorganic N loss was only about 5% of the incoming N. Episodic behavior 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 uphillsoils on the slopes. [ABSTRACT FROM AUTHOR]

Nitrogen (N) deposition is globally considered as a major threat to ecosystem functioning with important consequences for biodiversity, carbon sequestration and N retention. Lowered N retention as manifested by elevated concentrations of inorganic N in surface waters indicates ecosystem N saturation. Nitrate (NO3) concentrations in runoff from semi-natural catchments typically show an annual cycle, with low concentrations during the summer and high concentrations during the winter. Process-oriented catchment-scale biogeochemical models provide tools for simulation and testing changes in surface water and soil chemistry in response to changes in sulphur (S) and N deposition and climate. Here we examine the ability of MAGIC to simulate the observed monthly as well as the long-term trends over 10-35 years of inorganic N concentrations in streamwaters from four monitored headwater catchments in Europe: Čertovo Lake in the Czech Republic, Afon Gwy at Plynlimon, UK, Storgama, Norway and G2 NITREX at Gårdsjön, Sweden. The balance between N inputs (mineralization+deposition) and microbial immobilization and plant uptake defined the seasonal pattern of NO3 leaching. N mineralization and N uptake were assumed to be governed by temperature, described by Q10 functions. Seasonality in NO3 concentration and fluxes were satisfactorily reproduced at three sites (R2 of predicted vs. modelled concentrations varied between 0.32 and 0.47 and for fluxes between 0.36 and 0.88). The model was less successful in reproducing the observed NO3 concentrations and fluxes at the experimental N addition site G2 NITREX (R2=0.01 and R2=0.19, respectively). In contrast to the three monitored sites, Gårdsjön is in a state of change from a N-limited to N-rich ecosystem due to 20 years of experimental N addition. At Gårdsjön the measured NO3 seasonal pattern did not follow typical annual cycle for reasons which are not well understood, and thus not simulated by the model., Capsule: The MAGIC model is able to simulate NO3 leaching on a monthly as well as an annual basis, and thus to reproduce the seasonal and short-term variations in N dynamics., (Copyright © 2015 Elsevier B.V. All rights reserved.)