HR: 14:30h
AN: H43D-05    [Abstracts]
TI: Rapid and Drastic Changes in the 15-N Signature of Forest Soil Nitrate
AU: * Ross, D S
EM: dross@uvm.edu
AF: University of Vermont, Department of Plant & Soil Science Hills Building, Burlington, VT 05405 United States
AU: Hales, H C
EM: hhales@uvm.edu
AF: University of Vermont, Department of Plant & Soil Science Hills Building, Burlington, VT 05405 United States
AU: Fredriksen, G
EM: gfredrik@uvm.edu
AF: University of Vermont, Department of Plant & Soil Science Hills Building, Burlington, VT 05405 United States
AB: Recent work has utilized the dual isotopes of nitrate as a tracer in forested catchments. This method provides the relative contribution of atmospheric and microbial sources of nitrate in stream export. Tracing nitrate movement through the soil profile in these ecosystems may be more problematic. Sampling soils to extract nitrate stimulates both net and gross nitrification with the potential to create rapid changes in the 15-N signature. We have found changes in 15-N as high as 16 per mil in soils incubated 2-3 days at cold temperature. The initial 15-N and the change in 15-N both were significantly related to the measured net nitrification rates. In organic horizons with high net nitrification rates, the initial 15-N of nitrate was depleted but increased after sampling toward the signature of the whole soil N. The opposite was true in soils with relatively low net rates; the initial 15-N of nitrate was close to that of the whole soil N and incubation caused depletion. We hypothesize that mixing the soil causes more complete utilization of available ammonium in the former case and provides a greater pool of ammonium in the latter case. The potential for these rapid changes must be recognized when sampling soil for the dual isotope method.
DE: 0400 Biogeosciences
DE: 1040 Isotopic composition/chemistry
DE: 1803 Anthropogenic effects
DE: 1860 Runoff and streamflow
DE: 1866 Soil moisture
SC: Hydrology [H]
MN: 2005 Joint Assembly