A Single Law to Describe Atmospheric Nitrogen Bulk Deposition versus Rainfall Amount: Inputs at the Seine River Watershed Scale
Résumé
Atmospheric nitrogen species (NH 4-N and (NO 3 +NO 2)-N) were determined in weekly samples of atmospheric bulk deposition (dry plus wet), collected in France at seven sites over the course of a year. Rural, semi-rural and industrialised-urban sites were chosen in the Seine river watershed from the Seine estuary to upstream from Paris. Mean NH 4-N concentrations varied from 0.7 to 1.7 mg L −1. Mean (NO 3 +NO 2)-N concentrations were approximately 0.5 mg L −1 for all sites except Paris (0.7 mg L −1), which has a local impact on the fallout contamination from urban emissions. The relation between concentration and rainfall amount obeys a power law, in the form of y = ax b. When the nitrogen sources are very local, this relationship turns into a dilution law. Annual atmospheric nitrogen deposition (NH 4-N+(NO 3 +NO 2)-N) was calculated and varied from 7.8 kg ha −1 yr −1 in the neighbourhood of a rural town to 17.3 kg ha −1 yr −1 in a very industrialised harbour. 58% of the atmospheric nitrogen deposition occurred during 'spring + summer' period. The total nitrogen atmospheric input to the Seine estuary, via direct deposition + indirect input via the watershed, was estimated to about 5% of the total nitrogen load within the Seine river basin.