Nitrogen budget in a microbial mat in the Camargue (southern France)

The main processes associated with the nitrogen cycle were studied in a microbial mat and in the surrounding water during winter (January), spring (March) and summer (June) in the Camargue (southeastern France). Denitrification, nitrogen fixation, nitrification, dissimilative reduction of nitrate, together with mineralization and ammonium assimilation, were measured under dark and light conditions using inhibitor and nitrogen isotope (N-15). All these microbial processes were expressed in the mat. The isotope pairing method allowed the identification of the source of denitrified nitrate. It appeared that a major part of the nitrate formed by nitrification was denitrified and that nitrification was a considerable source of nitrate in spring and summer. From variations in net flux (fixation versus denitrification activities), we conclude that the mat acted as a nitrogen source in summer, but as a sink in winter.