The mineralisation of organic nitrogen: Relationship with variations in the water-table within a floodplain of the river adour in southwest france

The phenomenon of mineralisation of organic nitrogen has been investigated in a section of the floodplain of the River Adour, between Saubusse and Le Vimport, in southwest France. Water quality parameters have been measured at a number of piezometers within the floodplain: nitrate concentrations measured at the sites were generally negligible during the 15-month period studied, significant levels of dissolved ammonium ion were, however, observed during this period. A mechanism of partial mineralisation has been invoked that considers the breakdown of organic nitrogen within the sediments of the floodplain. The level of the water-table is observed to modulate further oxidation of ammonium ion to nitrite and nitrate ion: low groundwater levels allow aerobic oxidation but increased water levels reduce the production of nitrate. Denitrification and uptake by riparian vegetation appear to be combining rapidly to reduce the nitrate as it is produced. The fine particle size of the floodplain sediments permits the storage of considerable quantities of organic nitrogen and the high level of mineralisation of this stock, responding to variations in the water table, provides an explanation for the increasing nitrate concentrations observed in the river channel in recent years. It is estimated that ca. 24% of the sub-surface organic nitrogen is removed each year.