Nitrogen saturation experiments (NITREX) in coniferous forest ecosystems in Europe: a summary of results

The effect of changes in dissolved inorganic nitrogen (N) deposition on eco system functioning was investigated in the NITREX (NITRogen saturation EXpe riments) project. Field-scale manipulation experiments were carried out ove r four to six years in seven coniferous forest ecosystems in northwestern E urope. At sites with low or moderate ambient N deposition, N was experiment ally added to throughfall. At sites with high N deposition, N was removed f rom throughfall. We found that the capacity of the ecosystem to retain N wa s correlated to its internal N status. Some of the components of this N sta tus like the N concentrations in foliage and forest floor are relatively ea sy to measure. The CIN ratio of the forest floor is especially closely rela ted to the onset of nitrate leaching. Changes in N input may, in the long r un, change the N status of an ecosystem due to for instance a decrease in C /N ratio in the forest floor. Decreased N input resulted in a rapid and lar ge reduction in N concentration in drainage water. Significant improvement in tree nutritional status, tree growth, fine root biomass and diversity of ground vegetation and mycorrhizal fungi population were observed in one si te only. The time period of four to six years of manipulated N deposition m ay have been too short for changes to be manifested in the other sites.