SOURCES OF NITROGEN IN 3 WATERSHEDS OF NORTHERN FLORIDA, USA - MAINLYATMOSPHERIC DEPOSITION

Atmospheric deposition is estimated to be the principal source of N in water that flows to the Apalachicola River from the Chattahoochee and Flint Rivers (ACF) as well as in two nearby small rivers, Ochlockonee (Och) and Sopchoppy (Sop), that drain watersheds with different land use characteristics. By mass balance and descriptive statistics of hun dreds of rainfall and river water samples from monitoring programs sin ce the 1960s, the average nitrate and ammonium deposition flux from th e atmosphere is sufficient to account for N that flows toward Apalachi cola Bay, an estuary in which N may be a limiting nutrient. Urban and agricultural sources of N in the three watersheds ACF, Och, and Sop ap pear to be relatively smaller. The work was based on long-term data ba ses from the National Atmospheric Deposition Program (NADP) rain chemi stry monitoring network and the U.S. Geological Survey (USGS) water mo nitoring program. Average atmospheric N depositions to the three river watersheds are nearly the same as river fluxes of N in all forms moni tored. Nitrogen is not likely to be a limiting nutrient in the three w atersheds, since river water N:P exceeds the Redfield ratio. An estima te of largest possible input of urban sewage is several times lower th an the atmospheric flux of N to the ACF watershed. And N from N-fertil izer, comparable to the atmospheric deposition flux of N, is likely to be smaller if mostly retained in crops or farmland before it reaches the estuary. Annual nitrogen export from the Apalachicola River to the estuary, 1.22 X 10(9) moles N yr-1, consists of organic nitrogen 60%, nitrate 34%, and NH4+ 6%. Atmospheric nitrate and sulfate depositions are highly correlated, both being principally from fossil fuel combus tion. Hydrologic conditions, which exhibit variations on seasonal and longer time scales, play an important role in the transport of nutrien ts and other species in the rivers.