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Nitrogen mass balance for major drainage areas of the Albemarle-Pamlico Drainage Basin, North Carolina and Vriginia

American Geophysical Union
By Gerard McMahon and Mike D. Woodside

Abstract

The source and fate of nutrients in the Albemarle-Pamlico estuary system are important water-quality concerns in North Carolina and Virginia. A 1990 nitrogen mass balance calculated for the drainage areas of eight National Stream Quality Accounting Network (NASQAN) stations in the Albemarle-Pamlico Drainage Basin indicates the importance of agricultural nonpoint sources of nitrogen and watershed nitrogen retention and processing capabilities. Land cover in these drainage areas averages 58-percent forested, 35-percent agricultural, 4-percent developed, and 3-percent wetland areas.

Estimated nitrogen contributions were calculated for atmospheric deposition, crop fertilizer, biological nitrogen fixation, animal- waste, and point sources. Total basin nitrogen contributions ranged from 11 to 44 percent from atmospheric deposition (25-percent mean), 14 to 44 percent from crop fertilizer (27-percent mean), 5 to 40 percent from biological nitrogen fixation (21-percent mean), 10 to 35 percent from animal waste (20-percent mean), and 0.2 to 7.5 percent from point sources (2.5-percent mean).

Nitrogen removal estimates were made for instream nitrogen load and crop harvest. The difference between the sum of nitrogen contributions and the sum of the instream nitrogen loads and crop- harvest nitrogen removal was assigned to storage and retention in the basin. Nitrogen removal, as a percent of the total basin nitrogen contributions, ranged from 9 to 30 percent for instream nitrogen loads (15-percent mean), from 14 to 64 percent for crop harvest 34-percent mean), and from 26 to 71 percent for retention (51-percent mean).

The highest instream nitrogen load was measured in the predominantly agricultural drainage area. Intermediate loads were observed in mixed agricultural-urban drainage areas; the lowest loads were measured in mixed agricultural-forested drainage areas. Whereas agriculture-related nitrogen contributions accounted for more than two-thirds of overall contributions, nitrogen removal by crop harvest accounted for only one-third of these same contributions, suggesting a possible mismatch between the amount of nitrogen available and the amount needed for crops. The magnitude of retention indicates the importance of factors such as the role of wetlands in denitrification and the high reported rates of nitrogen retention in forests. The retention also reflects uncertainty and error. Additional research is needed, particularly regarding estimates of animal-waste contributions to instream nitrogen loads.

McMahon, Gerard and Woodside, M.D., 1994, Nitrogen mass balance for major drainage areas of the Albemarle-Pamlico Drainage Basin, North Carolina and Virginia [abs.]: American Geophysical Union 1994 Fall Meeting, December 5-9, 1994, San Francisco, California, p. 229.