PARENT SESSION
Oral Session # 98: Biogeochemistry V: Forests; Nitrogen.
Presiding: RL Sanford
Friday, August 8. 8:30 AM to 12:00 PM, SITCC Meeting Room 204.

Long-term nitrate export from Hubbard Brook Watershed 6 driven by climatic variation.

Hong, Bongghi^*,1, Swaney, Dennis¹, Woodbury, Peter², Weinstein, David¹, ¹ Boyce Thompson Institute for Plant Research, Ithaca, NY, USA² US Forest Service, Durham, NH, USA

ABSTRACT- The long-term pattern of nitrate export from Watershed 6 at Hubbard Brook Experimental Forest (HBEF), a long-term ecological research (LTER) site, during 1964 to 1994 can be characterized as ten years of high export (1968-1977) followed by twelve years of low export (1978-1989), including three "spikes" in 1970, 1973, and 1976, and a subsequent "spike" in 1990. Disruptions in N cycling by events such as soil freezing, insect defoliation, and drought have been suggested to explain this pattern. We developed a nitrogen cycle model to demonstrate that this long-term pattern can be reproduced largely without consideration of these events. Comparisons of simulated N fluxes between high and low export years suggested that inorganic N input to the soil, both as atmospheric N deposition and N mineralization, was significantly higher in periods of high streamflow nitrate flux than in low periods. As a result, simulated inorganic N pools (ammonium and nitrate) and fluxes (nitrification, plant uptake, denitrification, and ammonia volatilization) were also significantly higher in periods of high nitrate streamflow losses. 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, was the actual driver for creating the observed long-term pattern. Although simulated nitrification showed a stronger relationship with measured streamflow nitrate flux than N mineralization did, the rate of nitrification was heavily dependent upon availability of soil ammonium supplied from the N mineralization process. Because N mineralization in the model is a function of soil temperature and moisture only, we conclude that shifts in the interaction of these two variables over time produced the shifts in nitrate stream exports.

Key words: nitrogen cycle, Hubbard Brook, long term nitrate leaching