Nitrogen dynamics were investigated in the top centimeters of bottom sediment in three different geomorphologic units(riffles, sand bars, pools) and along two contrasted reaches (nitrogen-rich, nitrogen-poor) of a rural stream. Some predictionsof links between geomorphology and processes were verified: (i) hydrological exchanges and nitrification were lower in poolsthan in riffles and bars, whereas (ii) ammonification and denitrification were higher in pools. Studies of denitrification potentialin six other reaches of three N-rich streams support these conclusions and demonstrate that pools are spots of high microbialactivity in streams.