Changes in runoff chemistry after five years of N addition to a forested catchment at Gårdsjön, Sweden

Abstract

The deposition of nitrogen from the atmosphere to coniferous forests poses the risk of N saturation and disruption of the N cycle. Beginning in April 1991, we have conducted a whole-ecosystem manipulation of N deposition at a mature coniferous forested-catchment at Gårdsjön, SW Sweden. We have added in weekly portions of about 40 kg NH 4NO 3N ha −1 yr −1 in 5% extra water to the ambient 11 kg NH 4NO 3N ha −1 yr −1 in throughfall. The Gårdsjön experiment is part of the European NITREX project (Nitrogen saturation experiments). Both the annual mean concentrations and peak concentrations of nitrate in runoff increased dramatically in response to the chronic additions of N. The seasonal pattern and the yearly amount of N loss changed during the 5 years of treatment. During the first years of treatment, concentrations of NO 3 − were low during the growing season and high during the winter; during the fourth and fifth years, elevated concentrations were present also during the summer. Ammonium concentrations, although generally much lower than NO 3 −, followed the same seasonal pattern. The inorganic N lost during the 5 years of treatment was 0.6%, 1.1%, 5.0%, 5.7% and 4.5% respectively, of the annual inorganic N input. In the fifth year, drought and generally very low runoff led to moderate N loss despite the high NO 3 − and NH 4 + concentrations. Sulphate, inorganic Al n+, H + and Na + concentrations were significantly lower during the treatment period relative to the untreated control catchment while NO 3 −, NH 4 +, K + and Ca 2+ concentrations increased. The N concentrations in the runoff indicate incipient N saturation at the manipulated catchment. But the experimental dramatic increase in N deposition occurred so rapidly that the forest ecosystem has not yet had the opportunity to adjust and respond. Although symptoms of N saturation were induced over a relatively short time by increasing atmospheric deposition of N, the nitrogen status of the ecosystem is clearly in a state of transition. The rate of response suggests that at ambient N, deposition of 11 kg inorganic N ha −1 yr −1, the ecosystem is near the threshold at which additional N inputs cause significant NO 3 − leaching.