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Nitrogen budget of the Mobile–Alabama River System watershed

¹ Environmental Institute and Center for Freshwater Studies, The University of Alabama, Tuscaloosa AL 35487 Present address Department of Geological Sciences, The Ohio State University, Columbus OH 43210–1398, USA(e-mail carey@geology.ohio-state.edu)
² Department of Geological Sciences, The University of Alabama, Tuscaloosa AL 35487 Present address Department of Geological Sciences, University of Michigan, Ann Arbor MI 48104–1063, USA(e-mail cnezat@umich.edu)
³ Dauphin Island Sea Lab, The University of Alabama, Dauphin Island AL 36528 Present address Marine Studies Program, University of New Hampshire, Durham NH 03824–3505, USA(e-mail jonathan.pennock@unh.edu)
⁴ Department of Physics and Earth Sciences, University of North Alabama, Florence AL 35632, USA(e-mail tjones2@unanov.una.edu)
⁵ Department of Geological Sciences and Center for Freshwater Studies, The University of Alabama, Tuscaloosa, AL 35487 Present address Byrd Polar Research Center, The Ohio State University, Columbus OH 43210–1002, USA(e-mail lyons.142@osu.edu)

We have determined the nitrogen mass balance for the Mobile–Alabama River System (MARS) for two years of different hydrologic regimes (i.e. low flow vs. high flow). The maximum riverine export of N from the watershed is only 7%, suggesting relatively high retention and/or losses of N by denitrification within the watershed. Previous investigations of other watersheds within the USA demonstrate export percentages of c. 20–25%. Our calculations indicate that during a high flow year such as 1990, c. 13% of the new N introduced to the watershed annually is lost within the riverine system either through diatom uptake or denitrification. Another 4% is lost to the groundwater while 25–38% is sequestered by the terrestrial biomass (i.e. crop production and forest growth). Thus, as much as 51% of the N input to the landscape in the MARS is unaccounted for. We believe the location of this ‘missing’ N is probably within the soil, or the N has been lost through denitrification within the terrestrial ecosystem. The relatively low N yield from the MARS suggests that the watershed is not as saturated with respect to N as are many other U.S. drainages.

KEYWORDS: Nitrogen, nitrate, mass balance, rivers, hydrology