The article revisits hypotheses about the long-term effects of nitrogen deposition on temperate forest ecosystems, focusing on nitrogen saturation. It highlights the ongoing human-induced nitrogen emissions and their impacts, particularly in industrialized regions. The authors review the initial hypotheses set forth in 1989, which predicted that increased nitrogen inputs would lead to changes in soil and plant processes, including nitrate mobility, soil acidification, and forest decline. They describe a series of field experiments conducted in the United States and Europe to test these hypotheses, including the NITREX program in Europe. The results show that while initial increases in nitrogen mineralization and nitrification occur, they are followed by decreases as saturation is approached. Foliar nitrogen concentrations increase, leading to reduced Mg:N and Ca:Al ratios, and tree growth declines or mortality increases in evergreen stands. The authors also discuss the role of land-use history in determining ecosystem responses to nitrogen deposition, suggesting that previous land-use practices can precondition forests for nitrogen saturation. They propose that functional root systems and mycorrhizal symbionts may play a crucial role in nitrogen retention, and they explore possible mechanisms for nitrogen incorporation into soil organic matter, including microbial immobilization, abiotic incorporation, and mycorrhizal assimilation. The article concludes with a revised set of hypotheses that incorporate new findings and emphasizes the importance of understanding nitrogen saturation for managing forest health and ecosystem dynamics.The article revisits hypotheses about the long-term effects of nitrogen deposition on temperate forest ecosystems, focusing on nitrogen saturation. It highlights the ongoing human-induced nitrogen emissions and their impacts, particularly in industrialized regions. The authors review the initial hypotheses set forth in 1989, which predicted that increased nitrogen inputs would lead to changes in soil and plant processes, including nitrate mobility, soil acidification, and forest decline. They describe a series of field experiments conducted in the United States and Europe to test these hypotheses, including the NITREX program in Europe. The results show that while initial increases in nitrogen mineralization and nitrification occur, they are followed by decreases as saturation is approached. Foliar nitrogen concentrations increase, leading to reduced Mg:N and Ca:Al ratios, and tree growth declines or mortality increases in evergreen stands. The authors also discuss the role of land-use history in determining ecosystem responses to nitrogen deposition, suggesting that previous land-use practices can precondition forests for nitrogen saturation. They propose that functional root systems and mycorrhizal symbionts may play a crucial role in nitrogen retention, and they explore possible mechanisms for nitrogen incorporation into soil organic matter, including microbial immobilization, abiotic incorporation, and mycorrhizal assimilation. The article concludes with a revised set of hypotheses that incorporate new findings and emphasizes the importance of understanding nitrogen saturation for managing forest health and ecosystem dynamics.