『Abstract
We have studied 15 catchments supplying freshwater to a French
Atlantic coastal lagoon, where increase in nitrogen loads due
to agriculture is supposed to have destabilized the ecosystem
in the last decades. The catchment is a lowland composed of Pleistocene
sands with an average slope of 0.25%. To study the nutrient export
in relation to land-use surface waters were sampled bi-weekly
between October 2006 and January 2009 and land-use was established
by plane photographs and Geographic Information System (GIS).
Cultivated pine forests represent more than 80% of the total surface
and 7% of the catchment area has been deforested recently. Significant
areas of some catchments are used for maize crop. Housing is confined
to the coastal zone. Maize and forest crop give a robust signature
in terms of nitrate export. In view of modeling the nutrient fluxes,
we have established the mean export rate for every land-use: forested
parcels, deforested parcels, cultivated surfaces, and housing
areas export 45, 93, 2850, and 61 kg N-nitrate km-2
year-1, respectively. Exports of ammonium, dissolved
organic N (DON), and dissolved inorganic P (DIP) could not related
to land use. The mean export is 13, 100, and 0.57 kg km-2
year-1 for N-ammonium, DON, and DIP, respectively.
The modeling of nitrogen flux is in good agreement with our measures
for the largest catchment, which supplies about 90% of the total
continental DIN flux. However, small catchments are more dynamic
due to hydrological conditions and the model is less accurate.
This work has permitted to complete and unify scattered studies
about nutrient cycling in this area. Thus we have established
and compared the nitrogen budget of cornfields and cultivated
pine forest. We have emphasized that (i) fertilizer use should
be reduced in cornfields because they stock between 200 and 6400
kg DIN km-2 year-1, and (ii) the nitrogen
budget in pine forest mostly depends on tree harvesting and symbiotic
N-fixation, which is poorly constrained. Export of N by rivers
represents a small contribution to the N budget of soils
Keywords: Nitrogen; Phosphorus; River export; Land use; Coastal
catchment; Arcachon Bay; Modeling; Mass balance』
Introduction
Studied site
Methods
Catchment boundaries, soil occupation, and river discharge
Sampling
Nutrient analysis
Annual mean concentrations and annual nutrient fluxes
Calculation of the export rate
Nitrogen mass balance budget
Results
Land use
Nutrient concentrations and fluxes
Modeling the export rates
Nitrogen mass balance
Discussion
Land use impact on biogeochemistry of the catchments
Updating the eutrophication risk of the Arcachon Bay
Nitrogen mass balance validation and assessment of the durability
of maize and pine crop
Conclusion
Acknowledgments
References