DYNAMICS OF NITRIFICATION IN AN OXBOW LAKE IN TROPICAL FLOODPLAIN RIVER

Abstract

Nitrification rates were estimated in the sediment and water column at two sampling stations in Infernão Lake, during two distinct hydrological periods (dry and wet seasons). This water body is a shallow oxbow lake of the Mogi Guaçu River floodplain, located at the Ecological Station of Jatai, in the State of São Paulo, Brazil. The method used was ₁₄C-bícarbonate incorporation with and without nitrapyrin (N-Serve).From February to April (wet season), these rates were 0.14 to 4.84 and 0.02 to 0.68 mgN/m³/h. for sediments and water, respectively. From July to September (dry season), these rates were 1.93 to 8.04 and 0.05 to 1.16 mgN/m³/h, for sediments and water, respectively. No statistically significant differences between nitrification rates for different depths and sampling stations were found; however, significant differences were observed between hydrological periods, independent of station and depth. Inorganic nitrogen input in the wet season was not reflected in significant nitrification rates, as a result of unfavorable environmental conditions (chiefly low oxygen) and of assimilation by producers. Greater availability of N-NH^4-, from organic matter mineralization, and adequate environmental conditions permitted higher nitrification rates in the dry season. The magnitudes of these rates demonstrated that the ecological significance of this process in Infernão Lake is quantitatively minimal compared to the mineralization and ammonium assimilation processes, which can be considered the preferential pathways for transformation of nitrogen compounds in the system.