NH₄⁺ and oxygen availability are essential variables for the nitrification. Under anoxic environment the nitrification does not occur and, therefore, the NH₄⁺ tends to accumulate. In a sandy-clayey lowland stream, we evaluated the response of the nitrification on anoxic sediments to the exposure to O₂. We hypothesized that NH₄⁺ contained in clay and anoxic sediments fuels the nitrification once it is exposed to O₂. We evaluated this hypothesis by comparing the potential nitrification on oxic and anoxic sediments and by NH₄⁺ addition experiments on sand and clay substrates.

The mean nitrification on anoxic sediments was lower (1.01 µg N ml^-1d^-1) than the observed on oxic sediments (1.86 µg N ml^-1d^-1). Moreover, nitrification was significantly reduced (mean 0.26 µg N ml^-1d^-1) on sediments with high clay content (30% or more). On sand substrate, nitrification responded positively with the addition of NH₄⁺, whereas, on clay substrate, nitrification was only observed at the highest (1 mgL^-1) NH₄⁺ addition.

From our results we concluded that in this stream the accumulated NH₄ ⁺ in anoxic sediments constitutes as a source for the nitrification when it is exposed to oxygen. However, nitrification is significantly reduced on sediments with high clay content.