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Shallow Groundwater Denitrification Rate Measurement by Acetylene Block

Dep. of Biology, George Mason Univ., Fairfax, VA 22030;

J. L. Starr

USDA-ARS, Natural Resources Inst., Environmental Chemistry Lab., Beltsville, MD 20705;

T. B. Parkin

USDA-ARS, National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011 (parkin{at}nstl.gov).

^* Corresponding author (rjbragan{at}aol.com).

ABSTRACT

We field tested a short term, in situ method for measuring highly variable shallow groundwater denitrification rates using a methodology developed in an earlier, laboratory phase of this study. A pulse of tracer solution was injected and subsequently sampled within well clusters 50 cm in radius. The injectate contained dissolved acetylene (C₂H₂) to block denitrification at the nitrous oxide (N₂O) stage of reduction. Six well clusters were established at two depths along a riparian area transect through field, grass, and woodland ecotypes. Bromide tracer, C₂H₂, and accumulated N₂O were monitored at sample wells downgradient from each injection well. Adequate C₂H₂ concentrations (0.43 mM) to block N₂O reduction between injection and sample wells were attained in five of the six well clusters tested. Acetylene peak concentrations moved laterally an average of 19% more slowly than the Br peak, N₂O peak concentrations an average of 8% after C₂H₂ peaks. Highest denitrification rates of 2.8 and 3.3 mg N L^–1 d^–1 were observed near the surface of the water table within the grass buffer and riparian forest. Shallow groundwater denitrification rates in the adjoining field and 2 m deep into the saturated zone at all three ecotypes were <25% of the highest rates. Acetylene block procedures were thus used to determine in situ denitrification rates within a few days time with little disruption of soil structure or associated microbial communities. Portable equipment allows application at remote sites independent of line power and road access.

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