HR: 0800h
AN: H41E-0339    [Abstracts]
TI: A New Twist on the Seasonality of Nitrate Retention and Release in Adirondack Watersheds
AU: * Lawrence, G B
EM: glawrenc@usgs.gov
AF: U.S. Geological Survey, 425 Jordan Road, Troy, NY 12180 United States
AU: Ross, D S
EM: DROSS@.uvm.edu
AF: University of Vermont, Department of Plant and Soil Science, Burlington, VT 05405 United States
AU: Sutherland, J W
EM: jwsuther@gw.dec.state.ny.us
AF: New York State Department of Environmental Conservation, Darrin Freshwater Institute, Bolton Landing, NY 12814 United States
AU: Nierzwicki-Bauer, S
EM: nierzs@rpi.edu
AF: Rensselaer Polytechnic Institute, Darrin Freshwater Institute, Troy, NY 12180 United States
AU: Boylen, C
EM: boylec@rpi.edu
AF: Rensselaer Polytechnic Institute, Darrin Freshwater Institute, Troy, NY 12180 United States
AB: Release of nitrate to surface waters in the Northeast has a distinct seasonality that is generally explained by high retention from plant uptake during the growing season, and low retention during the non-growing season, when biological demand is low and soil- water flux is elevated in the absence of transpiration. In the Adirondack region of New York, the highest rates of release, which consistently occur during spring snowmelt, are considered to be the result of nitrate accumulation in the soil and snowpack over the winter. This explanation implies that plants out compete nitrifying bacteria for available ammonium during the growing season. Biweekly and automated high-flow sampling over five years in two tributaries of Buck Creek, in the western Adirondacks, however, has revealed inconsistencies with the conventional view of nitrate retention and release. Although low concentrations of nitrate were measured in stream water during the growing season, concentrations were lowest each year in mid October (near the completion of leaf drop) in the North tributary, and were either the lowest or second lowest each year in mid October in the South tributary. Furthermore, concentrations of nitrate in both watersheds remained elevated throughout the snowmelt periods despite sustained high flows. For example, the concentration in the South tributary on April 9th, 2001, (the initial stage of snowmelt) was 76 micromoles per liter, and on April 24th (following two of the three largest flow events over the 5 years of sampling), was 82 micromoles per liter. Flushing of nitrate stored in the soil over the winter would result in a peak concentration in the stream that would be followed by a rapid decrease. To explain these results we hypothesize a three-way competition that includes heterotrophic non-nitrifying bacteria, as well as plants and autotrophic nitrifying bacteria. Leaf drop in the fall provides a large input of labile carbon with a high C to N ratio (>20) that favors heterotrophs over autotrophs because of the energetic advantage of heterotrophic metabolism. Rapid growth of the heterotrophic community incorporates N into microbial biomass, and leads to low nitrate concentrations in stream water. As fall proceeds into winter, the availability of labile plant-derived carbon decreases, and the populations of active heterotrophs decrease in response, which leads to the release of microbially-derived organic substrate with a low C to N ratio (<10). These conditions are more favorable for nitrifying bacteria, thereby increasing nitrification rates and leaching of nitrate from soils to surface waters. With the onset of snowmelt, soil water flux increases substantially, further lowering the availability of labile carbon (which is primarily in the form of soluble organic matter) and stimulating the nitrifying populations. An inverse relationship between concentrations of dissolved organic carbon and nitrate in stream water of the Buck Creek tributaries supports this interpretation.
DE: 1806 Chemistry of fresh water
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting