Nitrogen budget of the Mobile–Alabama River System watershed

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Abstract
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.