Historical increases in emissions and deposition of oxidized and reduced nitrogen (N) provided the impetus for global-scale research investigating effects of excess N on terrestrial ecosystems. Several forest ecosystems of eastern North America exhibited susceptibility to N saturation, wherein the supply and availability of N exceeds total biotic demand for N, resulting in altered biogeochemical cycles and loss of plant and microbial biodiversity. The Clean Air Act (CAA) in 1970 and its further amendments have demonstrated efficacy in reducing both emissions and atmospheric deposition of N, especially in eastern North America. As this represents both a research challenge and opportunity for ecosystem ecologists and biogeochemists, the purpose of this paper is to predict changes in ecosystem structure and function of North American forest ecosystems in a decreased-N future. Hysteresis is a property of a system wherein output is not a strict function of corresponding input, incorporating lag, delay, or history dependence, particularly when the response to decreasing input is different from the response to increasing input. We propose a conceptual hysteretic model predicting varying lag times in recovery of soil acidification, plant biodiversity, soil microbial communities, forest carbon and nitrogen cycling, and surface water chemistry toward pre-N impact conditions as deposition of N continues to decline.
Results/Conclusions
Although responses of soil acidification to decreased N is complicated by association with sulfur deposition (the initial target of the CAA), data suggest that response of soil acidification should generally follow the hysteretic model. Evidence is inconsistent regarding response of plant biodiversity, widely reduced by excess N, precluding broad generalizations. Recovery of soil microbial communities to their pre-N enriched state may be delayed if the microbial spore bank composition was significantly altered or if microbes lost their capacity to perform particular functions. Regarding forest carbon and nitrogen cycling, we hypothesize that soil organic matter and wood pools are likely to respond slowly to declines in N deposition and exhibit pronounced hysteresis, whereas fast pools (e.g., mineral soil N) will respond rapidly, exhibiting less hysteresis. High natural spatial variability in surface water chemistry again precludes broad generalizations. Some sites currently show significant decreases in N export, whereas other do not. Because notable decreases in N deposition in this region did not begin until the new millennium, this is a relatively novel phenomenon. Thus, we strongly urge that experiments manipulating decreased N inputs be an immediate part of the future of biogeochemical and vegetation/soil microbial research.