Coniferous forests are getting too much nitrogen from the air, which can harm the ecosystem. The NITREX project studied how these forests react to different levels of nitrogen. They found that when nitrogen input is low, forests can retain most of it. But when input is high, forests release a lot of nitrogen. Between 10-25 kg/ha/year, forests reach a point of nitrogen saturation. This saturation can happen quickly, within a few years. The amount of nitrogen in tree leaves takes longer to show changes. Overall, the study shows that forests can quickly become saturated with nitrogen and recover when nitrogen levels are reduced.