Long-Term Nitrate Export Pattern from Hubbard Brook Watershed 6 Driven by Climatic Variation

From 1964 through 1994, the pattern of nitrate (NO₃ ^?) export from Watershed 6 at Hubbard Brook Experimental Forest (HBEF) in New Hampshire, U.S.A., exhibited 10 years of high export (1968–1977) followed by 12 years of low export (1978–1989), with four ‘spikes’ in 1970, 1973, 1976, and 1990. Disruptions of N cycling by soil freezing, insect defoliation, or drought have been suggested to explain this pattern. We developed a model of nitrogen dynamics demonstrating that most of the long-term pattern can be reproduced without explicit consideration of these events. Comparisons of simulated N fluxes between high and low export years suggested that inorganic N input to the soil, from both atmospheric N deposition and N mineralization, was significantly higher during periods of high streamflow NO₃ ^? flux than in low periods. Simulated inorganic N pools (ammonium and nitrate) and fluxes (nitrification, plant uptake, denitrification, and ammonia volatilization) were also significantly higher in these periods. By swapping the time sequences of inorganic N input between high and low export years, it was shown that N mineralization, not atmospheric N deposition, drives the simulated long-term pattern. Although simulated nitrification showed a stronger relationship with measured streamflow NO₃ ^? flux than did N mineralization, nitrification rate depended upon availability of soil ammonium supplied from N mineralization. Because N mineralization in the model varies only with soil temperature and moisture, we conclude that shifts in the interaction of these two variables over time produced the shifts in NO₃ ^? stream exports.