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Nitrate Depletion in a Second-Order Mountain Stream¹

ABSTRACT

The amount of NO₃-N exported in a second-order mountain stream draining a clearcut and logged mixed hardwood forest was studied over a 4-year period. Calculations based on measurements of stream chemistry and discharge rates indicated a within-stream depletion of NO₃ from the upper reaches of the stream to the watershed outlet. Within-stream depletion the first year of treatment was 127% of total NO₃-N discharged from the watershed outlet and declined in succeeding years after treatment to 99, 42, and 5%. Assays of the quantities of denitrifying enzymes in stream sediment samples suggested 1.7 kg N year^–1 were lost via this pathway, compared with 3.9 kg N year^–1 calculated from within-stream depletion for the same time period. This study suggests sediment denitrification is a major pathway by which NO₃-N is lost. Within-stream gaseous transformations are important when accounting for changes in N dynamics associated with forest management practices, and the measurement of only hydrologic discharge of NO₃ could result in underestimation of N losses.

Key Words: denitrification • nitrogen losses • stream export • stream nutrient cycling

¹ Research supported in part by Southeastern Forest Exp. Stn., USDA-FS, Rt. 1, Box 216, Otto, NC 28763, and in part by National Science Foundation, Grant no. DEB7904537.

² Principal Plant Ecologist, Coweeta Hydrologic Lab., Southeastern Forest Exp. Stn.; Microbiologist, USDA-ARS, Morris, MN 56267, formerly Research Associate, Inst. of Ecology, Univ. of Georgia, Athens.

Received for publication August 7, 1981.

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