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Headwater Stream Losses of Nitrogen from Two Coastal Plain Watersheds¹

ABSTRACT

Nitrogen concentrations and water flow were measured for a 3-yr period from two watersheds of the Atlantic Coastal Plain in North Carolina to assess sinks for NO₃-N₄ Both of the areas are typical of the largest agricultural producing area in the region. From the Middle Coastal Plain watershed with 1299 ha of predominantly well- to moderately well-drained soils, the losses of NO₃-N and total N in surface drainage water were 2.5 and 4.5 kg ha^–1 yr^–1, respectively. The Lower Coastal Plain watershed with 6998 ha of somewhat poorly- to poorly-drained soils had NO₃-N and total N losses of 0.5 and 2.5 kg ha^–1 yr^–1, respectively. These values were much lower than expected based on agricultural production, fertilizer used, and crop efficiency. The soils included in the study were Typic Paleudults to Aquic Paleudults and Aquic Hapludults to Aeric Paleaquults. Denitrification between the field and stream was concluded to be the primary reason for these low values. However, the concentrations of NO₃-N in streams near the field outlets decreased significantly as N enriched water moved through the transport system. The lowest stream elevations sampled on the Middle and Lower Coastal Plain watersheds averaged 0.6 and < 0.1 mg L^–1 NO₃-N with maximum values of 2.5 and 0.6 mg L^–1, respectively. Comparing changes in nitrate and chloride concentrations and the reducing Eh in the stream and floodplain sediments suggest denitrification as the loss mechanism in the stream transport system. Ostensibly the stream is responsible for the loss rather than the riparian area; however, the area inundated by the stream is small compared to the flood plain. The Middle and Lower Coastal Plain watersheds have 2.5 and 94 ha inundated by the stream while confined to the channel, but while in a moderate flood these values increase to 20 and 250 ha, respectively. While instream losses of NO₃-N are important in this region to maintain water quality, they are not sufficient to completely counter heavy loading by nonpoint sources.

Key Words: denitrification • riparian area • nitrogen balance • eutrophication • Eh

¹ Paper no. 9575 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695-7619. The work upon which this publication is based was supported in part by the Water Resources Research Inst. of the Univ. of North Carolina as authorized under the Water Resources Act of 1964.

² Graduate research assistant and professor, Soil Science Dep., North Carolina State Univ., Raleigh, NC 27695-7619.

Received for publication November 19, 1984.