Nitrification in stream sediment biofilms: the role of ammonium concentration and DOC quality

Nitrification was shown to be important in the nitrogen cycle of intact streambed sediment from the Mediterranean stream La Solana, Spain. We employed sediment biofilm reactors (SBR) with a continuous throughflow of water and solutes and used the one-dimensional advection-dispersion and transformation model to simulate SBR output ammonium and nitrate concentrations. An average input nitrogen flux of 1.7 μg NH4-N min−1 saturated ammonium retention in streambed sediments. Nitrification rates ranged between 4 and 20 ng N min−1 g−1 dry mass sediment and its efficiency amounted 85%. In order to better understand the coupled effect of both nitrogen and carbon cycles in stream sediments, we performed simultaneous and successive glucose/ammonium additions. In this experiments, nitrification efficiency decreased to 31–45%. We present first evidence, that pulses of highly available carbon can shift the microbial biofilm community from predominantly chemoautotrophy to predominantly heterotrophy in the La Solana streambed. Under these conditions, heterotrophic bacteria can potentially compete for ammonium with nitrifiers.