Payne R , Jassey VEJ , Leith ID , Sheppard LJ ,

17 Nitrogen pollution affects many peatlands with consequences for their biodiversity and ecosystem 18 function. Microorganisms control nutrient cycling and constitute most of the biodiversity of peatlands 19 but their response to nitrogen is poorly characterised and likely to depend on the form of deposition. 20 Using a unique field experiment we show that ammonia exposure at realistic point source levels is 21 associated with a general shift from heterotrophic (bacteria and fungi) to autotrophic (algal) dominance 22 and an increase in total biomass. The biomass of larger testate amoebae increased, suggesting increased 23 food supply for microbial predators. Results show the widespread impacts of N pollution and suggest 24 the potential for microbial community-based bioindicators in these ecosystems. 25