The influence of swamp floodplain on exports of nitrogen and phosphorus from North Carolina coastal plain watersheds

Nitrogen (N) and phosphorus (P) exports from channelized stream watersheds were greater than
those from natural-stream watersheds in the Coastal Plain of North Carolina. Nitrate showed the greatest
difference in exports between natural- and channelized-stream watersheds of any N or P species.
The reduced export of N and P from natural-stream watersheds is attributed to removal of these nutrients
in floodplain swamps bordering natural streams and to maintenance of natural poorly-drained soil conditions
near stream channels, conducive to nitrate removal from shallow ground waters via denitrification
and biological immobilization. In Creeping Swamp, a small floodplain swamp at the base of one
watershed, we measured a net removal of total N and P inputs, primarily as ammonium and filterable reactive
phosphorus. Some nitrate, total organic nitrogen and particulate phosphorus were also removed
(17%, 3.8%, and 13% of the amount entering, respectively), but net amounts of filterable unreactive
phosphorus were exported from the swamp floodplain ecosystem. Greater nitrate removal in natural stream
watersheds, despite low removal in swamp floodplains, probably occurred via biotic uptake and
denitrification in shallow ground waters of poorly-drained soils before waters reached stream channels
and floodplains in the watershed. These data indicate that floodplain swamps and poorly drained soils
are effective filters of N and P and thus act as buffers between pollutant producing activities in
watersheds and water quality in downstream rivers and estuaries.