HR: 09:00h
AN: NB31D-03    [Abstracts]
TI: Will the unexpected and unexplained trend towards declining nitrate concentrations in New Hampshire streams soon reverse?
AU: * Huntington, T G
EM: thunting@usgs.gov
AF: U. S. Geological Survey, 196 Whitten Rd., Augusta, ME 04330 United States
AB: A recent report describes unexpected and unexplained declines in stream nitrate concentrations in forested watersheds of New Hampshire during recent decades. These declines are unexpected because continuing elevated atmospheric nitrogen (N) deposition is believed to exceed plant nutritional requirements which should result in increasing (rather than decreasing) stream nitrate concentrations. These declines are unexplained because several hypotheses have either been ruled out or remain untested. Increased rates of tree uptake are unlikely because region-wide assessments indicate stagnant or decreasing forest growth rates. Additionally, rates of atmospheric N deposition have remained fairly constant in recent years. Forest maturation has also been rejected as a potential explanation. Increasing tree N uptake during forest recovery from disturbances like insect defoliations and severe drought were provisionally ruled out because they do not explain the regional nature of the decline. Atmospheric CO2 fertilization effects are thought to be too small, and have not been detected as increases in aboveground growth. Interannual climatic variability, including soil frost dynamics, could explain part of the declines, but these hypotheses remain untested. Regional trends towards warmer, wetter, and longer growing seasons are consistent with increasing N uptake, but there is no evidence for a corresponding increase in forest growth. Nitrogen sequestration in soil organic matter could explain the observed nitrate decline, and may have increased in recent years through stimulation of belowground carbon cycling due to CO2 fertilization. In addition, climate changes have favored an intensification of belowground N cycling that could lead to a narrowing of the C:N ratio and increased N sequestration. If increasing atmospheric CO2 and a warmer and wetter climate are at least partially responsible for increasing N sequestration, it is not likely that the current trends in stream water nitrate will soon reverse. The rate of tree N uptake is also likely to increase if current trends towards increasing atmospheric CO2, temperature, and precipitation continue as expected.
DE: 1824 Geomorphology (1625)
DE: 1860 Runoff and streamflow
DE: 1871 Surface water quality
DE: 1890 Wetlands
SC: North American Benthological Society [NB]
MN: 2005 Joint Assembly