Nitrate Sources in Discharge from the Hardwood Forests of the Northeastern U.S.A ľ Implications for Forest Management.
Kathryn B. Piatek1, Myron J. Mitchell2, Steven R. Silva3, and Carol Kendall3. (1) West Virginia Univ, Division of Forestry, P.O. Box 6125, Morgantown, WV 26506, (2) SUNY-ESF, 1 Forestry Dr., Syracuse, NY 13210-2788, (3) USGS Geological Survey, 345 Middlefield Road, MS 434, Menlo Park, CA 94025
Atmospheric N deposition has been implicated in nitrate exports from forested ecosystems. As a contaminant in drinking water, and an agent in water acidification and eutrophication, nitrate poses an environmental concern. To determine the source of nitrate in surface water during early spring snowmelt discharge, we analyzed stream δ15N-NO3- and δ18O-NO3- values, and total Al, DOC and Si concentrations between February and June of 2001 and 2002. Isotopic values in stream were compared to those of throughfall, bulk precipitation, snow, and groundwater; high total Al and DOC and low Si were indicative of preferential water flow through the forest floor and mineral soil horizons rather than ground water. The study was conducted in an unmanaged 135-ha subcatchment of the Arbutus Watershed in the Huntington Wildlife Forest in the Adirondack Region of New York State, U.S.A. Values of δ15N and δ18O of NO3- in stream water were similar in both years. Stream water, atmospherically-derived solutions, and groundwater had overlapping δ15N-NO3- values. Delta 18O-NO3- values ranged from +5.9 to +12.9 o/oo in stream and ground water, and from +58.3 to +78.7 o/oo in atmospheric solutions. The substantial separation in δ18O-NO3- values suggested that the majority of stream and ground water nitrate was not atmospheric in origin. Stream discharge in 2001 increased from 0.6 before to 32.4 mm day-1 during snowmelt, and element concentrations increased from 33 to 71 Ámol L-1 for NO3-, 3 to 9 Ámol L-1 for total Al, and 330 to 570 Ámol L-1 for DOC, indicating water movement through the forest floor/ soil complex. Discharge in 2002 was variable, with a maximum of 30 mm day-1 during snowmelt. The highest NO3-, Al, and DOC concentrations were 52, 10, and 630 Ámol L-1, respectively, and dissolved Si decreased from 148 Ámol L-1 before to 96 Ámol L-1 during snowmelt, again indicating water flow through the forest floor/ soil complex. Lack of atmospheric values of δ18O-NO3-, and presence of high Al and DOC and low dissolved Si concentrations in water moving through the forest floor and mineral soil during snowmelt suggested that the forest floor and mineral soil are major contributors of nitrate during snowmelt. The implication of nitrification as a major source of stream water nitrate discharge from these forests is that forest management planning that includes control of N exports as an objective needs to involve the understanding of factors regulating the generation and retention of nitrate in soils, and of processes that contribute to nitrate transfer to groundwater.