SHORT-TERM EFFECT OF NITROGEN ENRICHMENT ON THE MICROBIAL COMMUNITIESOF A PEATLAND

The aim of this study was to assess the structure of the microbial loo p in a Sphagnum fallax - Carer rostrata fen of the French Massif centr al, and the impact of nitrogen supplies on the different microbial com munities. Microalgae (46% of the total microbial biomass), Protozoa (2 6%) and heterotrophic Bacteria (17%) were the dominant microorganisms. Rotifera (5%), Cyanobacteria (3%), Fungi (2%) and Nematoda (< 1%) wer e also present. Testate Amoebae were well represented in Sphagnum peat land (14% of total microbial biomass). Thus, the structure of the prot ist community in the surface of fen peatlands is notably different fro m that recorded in other environments. The input of nitrogen led to a steady increase in microbial biomasses, but only changed the structure of microbial communities significantly when the input was 50 kg ha(-1 ). At high inputs, nitrogen supply increased the relative importance o f Cyanobacteria, Euglenophyceae, Bacillariophyceae and Ciliates, and d ecreased the relative importance of heterotrophic Bacteria, other micr oalgae and testate Amoebae. The increase in the values of photosynthet ic assimilation and heterotrophic activities could also reflect change s in community functioning.