Atmospheric nitrogen (N) input to terrestrial and aquatic ecosystems has strongly increased in the past century due to human activities. This has resulted in modified soil processes and varying ecosystem responses, including increased biomass growth and carbon sequestration, but also increased N losses by emission from soils and by leaching below the rooting zone. Nevertheless, considerable N retention in forests has been observed. Forest type and tree species potentially affect the degree of N retention because of differences in organic and mineral soil characteristics, microbial soil activity and N uptake by plant roots.
 The objective of this study is to determine aboveground and belowground N retention in two contrasting forest stands in a region exposed to a chronically enhanced atmospheric N deposition. Studying the fate of added 15N isotopes and the rates of N transformation processes can explain the discrepancy between the input and loss of N that has been observed for these ecosystems.