Variations in mineral nitrogen levels: the River Adour

Abstract

A section of the River Adour (SW France) has been investigated to determine the role of the riparian vegetative strips (rvs) in the retention of mineral nitrogen. Entering and exiting fluxes have been measured along this segment throughout a hydrological season. The study indicates gains in mineral nitrogen (∼ 28%) during periods of low flow in the summer months, with riparian vegetation utilising 2.3 to 3.7% of the entering flux. Significant losses (∼ 50%) of nitrogen are observed in autumn, whereas minor losses are observed during the two flood events of November 1991 and June 1992 (4.6 – 4.9% of the entering flux). In winter losses of mineral nitrogen due to denitrification are small varying from 0.07% in winter, to 7% in summer, of the entering flux. The input of dissolved nitrogen through rainfall is estimated to be negligible within the zone studied. Additions of fertiliser for maize cultivation in February, May, June and July of 1992 have a significant impact on the fluxes exiting, the study zone. In May, the added nitrogen is used largely in the growth of the maize, what remains is is retained in the riparian vegetation. By contrast, in June, 27.2% of the available nitrogen added as fertiliser is found in the river. Finally, during the course of the hydrological year an increase in mineral nitrogen of 519 tonnes is observed in the area studied between Pontonx and Dax, of which only 224.8 tonnes can be ascribed to the addition of fertiliser. The remainder of this enrichment is explained by mineralisation of organic nitrogen stored in the riparian vegetative strips.