Effets de l'assèchement des bassins ostréicoles sur le cycle biogéochimique de nutriments azotés: ammonification préférentielle

Oyster ponds behave as organic matter accumulators, unlike wetlands which export a large part of their natural productivity towards the ocean as well as inorganic nitrogen towards the atmosphere through denitrification. Thus, to prevent eutrophication temporary drying-out is necessary, the physical effects of which are weIl known but who se biogeochemical processes are still poorly documented. The aim of the present study was to perform a drying -out operation in an oyster pond, and to describe the modifications which occurred within the nitrogen cycle: i.e. mineralization processes within the top 5 cm of the sediment; changes in nutrient pore-water concentrations; and fluxes through the water/sediment interface. Prior to the operation, ammonia was the major nitrogen form in the water column and in pore waters (top 5 cm); concentrations of free NH4+ increased as a function of depth, whereas exchangeable NH4 + decreased. At the end of drying-out, when salt precipitation first occurred at the water/sediment interface, the sediment remained reduced des pite a slight oxidation around dessication cracks, thus preventing nitrification. In consequence, ammonification was the prevailing process within the sediment. At the surface of the sediment, concentrations of free NH4 + in pore waters were 45 times as high as at the beginning of the drying-out, whereas they remained constant at 5 cm depth; exchangeable NH4+ did not increase accordingly, but tended, rather, to decrease. Strong increases of free ammonia were correlated to those of pore-water salinity. The non-adsorption of NH4 + on sediment and the concomitant defixing of NH4 + from illitic clays were due to increasing quantities of K+, as a consequence of the high salinities induced by drying. The flooding carried out a few days after the occurrence of salt precipitation had the following effects: (i) maximum ammonia fluxes through the water/sediment interface which persisted for a long time after flooding; (ii) restoration of reduced conditions which prevented nitrogen loss due to the coupling of nitrification and denitrification; (iii) disappearance of high salinities in pore waters, thus eliminating the effect of K+ on clays, and allowing NH4 + fixing and adsorption on sediments. In conclusion, drying-out might help physically to control ammonia production, independently of the biological production (bacterial mineralization) which is controlled by temperature.