NITROGEN CYCLING IN A VENEZUELAN TROPICAL SEASONALLY FLOODED FOREST - SOIL-NITROGEN MINERALIZATION AND NITRIFICATION

Seasonally flooded forests represent a transition between terrestrial and aquatic ecosystems. The Mapire river, a tributary of the Orinoco river, floods its surrounding forests during the wet season (May-December). The soils are very acid and the total nitrogen concentration (0.1%) is only half that found in nearby soils flooded by Orinoco waters. Ammonium-nitrogen predominates in the soil during the flooded period while nitrate-nitrogen concentrations are higher in the dry period. Wide fluctuations in the inorganic nitrogen fractions did not considerably affect the annual course of soil nitrogen. The predominance of mineralization versus nitrification (56 and 5 mug N(inorg) g-1 soil month-1 respectively) and possibly the synchronization of nitrogen availability with plant demand could be considered as nitrogen conserving mechanisms. In synchrony with the hydrologic cycle, the seasonally flooded forest studied shows a nitrogen-cycle where inputs and accumulation are maximized when the system is under minimum stress (dry season). During flooding, the system enters a period of dormancy making minimal use of nutrient and energy to avoid or tolerate anaerobiosis.