『Abstract
Increased export of biologically available nitrogen (N) to the
coastal zone is strongly linked to eutrophication, which is a
major problem in coastal marine ecosystems (NRC(2000) Clean Coastal
Waters: Understanding and Reducing the Effects of Nutrient Pollution.
National Academy Press, Washington, DC; Bricker et al.(1999) National
Estuarine Eutrophication Assessment. Effects of nutrient enrichment
in the Nation's estuaries. NOAA-NOS Special Projects Office, silver
Spring, MD). However, not all of the nitrogen input to a watershed
is exported to the coast (Howarth et al.(1996) Biogeochemistry
35:75-139; Jordan and Weller (1996) Bioscience 46;655-664). Global
estimates of nitrogen export to coasts have been taken to be 25％
of watershed input, based largely on northeastern U.S. observations
(Galloway et al.(2004) Biogeochemistry 70:153-226; Boyer et al.(2006)
Global Biogeochem Cycle 20: Art. No. GB1S91). We applied the N
budgeting methodology developed for the International SCOPE Nitrogen
project (Howarth et al.(1996) Biogeochemistry 35:75-139; Boyer
et al.(2002) Biogeochemistry 57:1137-169) to 12 watersheds in
the southeastern U.S., and compared them with estimates of N export
for 16 watersheds in the northeastern U.S. (Boyer et al.(2002)
Biogeochemistry 57:137-169). In southeastern watersheds, average
N export was only 9％ of input, suggesting the need for downward
revision of global estimates. The difference between northern
and southern watersheds is not a function of the absolute value
of N inputs, which spanned a comparable range and were positively
related to export in both cases. Rather, the proportion of N exported
was significantly related to average watershed temperature (％
N export = 58.41 e-0.11*temperature;R2 =
0.76), with lower proportionate nitrogen export in warmer watersheds.
In addition, we identified a threshold in proportionate N export
at 38゜N latitude that corresponds to a reported breakpoint in
the rate of denitrification at 10-12℃. We hypothesize that temperature,
by regulating denitrification, results in increased proportionate
N export at higher latitudes. Regardless of the mechanism, these
observations suggest that temperature increases associated with
future climate change may well reduce the amount of nitrogen that
reaches estuaries, which will have implications for coastal eutrophication.
Keywords: Climate change; Denitrification; Eutrophication; Nitrogen
budgets; Proportionate nitrogen export; Temperature; Watersheds』
Introduction
Methods
Watershed characteristics
Nitrogen input
Net atmospheric deposition
Fertilizer input
Net food & feed import
Biological N fixation
Non-food crop export
Riverine export
Results
Population density
Wetland/water area
Residence time
Proportional N input
Forest N uptake
Temperature and streamflow
Discussion
Denitrification as a potential mechanism
Implications for coastal N loading
Acknowledgments
References