Defining Critical N Loads for forests remains a major challenge, as our understanding of processes determining forest ecosystem responses to increased N deposition is still poor. Acidification will mainly depend on the nitrification capacity, and coupled nitrate leaching, of the forest soil; yet factors controlling nitrification are not clearly established. As part of the CORE project (CEC), investigating nutrient dynamics in European coniferous forest soils, we studied the effects of increased (NH4)2SO4 deposition on soil solution chemistry at six sites situated across a climatic and pollution gradient. Roofed tension lysimeters were treated every two weeks with 2.88 kg N ha-1 (75 kg N ha-1 a-1) above ambient throughfall and monitored at the same interval for a total of 18 months. Responses to increased (NH4)2SO4 inputs varied from high ammonium retention, to highly increased ammonium leaching at the different sites. The important links between N cycle transformations and cation leaching were confirmed. The response of the nitrifiers to increased external ammonium supply was slow at all sites. The different responses of these six soils to increased (NH4)2SO4 deposition emphasizes that the effects of N loads have to be discussed in relation to soil N storage and nitrification capacity.