From nitrogen enrichment to ecosystem recovery: mechanisms driving forest ecosystem reorganization under changing nitrogen regimes

Abstract

Anthropogenic nitrogen (N) deposition is a major global-change driver that can reorganize forest ecosystem functioning beyond the direct effects of nutrient enrichment. Yet responses vary among forests because atmospheric N inputs interact with initial nutrient limitation, retention capacity, climate, soil properties, and historical exposure. Here, we develop a structured narrative synthesis that distinguishes atmospheric N deposition from experimental N addition and evaluates how changing N regimes propagate through plant, microbial, and soil processes. We frame ecosystem responses as context-dependent state transitions rather than a universal dose–response sequence. Initial N limitation may be alleviated, but continued enrichment can alter C:N:P stoichiometry, nitrogen-use efficiency, plant carbon allocation, microbial resource-acquisition strategies, extracellular enzyme investment, and soil organic matter transformation. Secondary nutrient limitation and nitrogen saturation are treated as related but distinct outcomes: the former concerns a shift in the dominant limiting resource, whereas the latter concerns declining capacity to retain and assimilate additional N relative to supply. We further distinguish ecological legacy, delayed recovery, and ecological hysteresis, emphasizing that delayed or incomplete recovery does not by itself demonstrate hysteresis. For soil carbon, microbial carbon-use efficiency (CUE) is considered a mediator of carbon processing rather than a sufficient determinant of stabilization; persistent storage depends additionally on microbial necromass formation, particulate and mineral-associated organic carbon pools, and mineral protection. The synthesis identifies evidence-supported pathways, context-dependent mechanisms, and major uncertainties across forest functional types, mycorrhizal associations, soil mineralogy, climate, and historical N exposure. We propose a trajectory–state–feedback framework in which changing N regimes, ecosystem state, internal feedback strength, and ecological history jointly determine enrichment responses and recovery trajectories.