Indicators and thresholds for nitrogen saturation in forest ecosystems

Nitrogen (N) saturation occurs when N inputs exceed the total N demand by plants and microbial organisms in forest ecosystems. Long-term observations, N deposition gradient investigations, N addition experiments, and model simulations can provide insights in thresholds for N saturation using various indicators. Here we give an overview of N saturation indicators and their thresholds for soil, trees (growth and nitrogen nutrition) and stream water as well as related N deposition thresholds for forest N saturation. A synthesis of literature suggests that in boreal forests, the threshold of N deposition for N saturation was 5-10kgNha−1 yr−1 based on the occurrence of N leaching. In temperate forests, the threshold of N deposition for N saturation was estimated to be 5-46kgNha−1 yr−1, 5-30kgNha−1 yr−1 and 8-15kgNha−1 yr−1 based on soil, tree and stream water indicators, respectively. In subtropical coniferous forests, the thresholds of N deposition for N saturation based on soil, tree and stream water indicators ranged from 30 to 70kgNha−1 yr−1, 26 to 36kgNha−1 yr−1, and 7 to 10kgNha−1 yr−1, respectively. However, the thresholds of N deposition for N saturation were less well quantified in other forest ecosystems. Furthermore, we briefly reviewed critical loads of N deposition for biodiversity, a forewarning threshold associated with the first appearance of negative effects. Critical loads of N deposition were generally lower than the thresholds for N saturation. Empirical critical loads based on responses of plant species diversity, showed higher values in temperate forests (10-15kgNha−1 yr−1) than in boreal forests (5-10kgNha−1 yr−1). However, estimates of empirical critical N loads for subtropical and tropical forests are lacking in literature. Future research efforts are needed to better evaluate the saturation thresholds of N deposition and critical N loads in various forest ecosystems to inform policies for N emission reduction and forest management.