Synthesis of lichen response to gaseous nitrogen: Ammonia versus nitrogen dioxide
The dominant chemical species of nitrogen pollution emitted to the atmosphere in the U.S. and Europe is shifting from oxidized nitrogen (NOY) from combustion of fossil fuels to reduced nitrogen (NHX) from agricultural animal waste and fertilizer applications. Does it matter to ecosystems that the relative contribution of reduced versus oxidized forms of nitrogen is changing? In this synthesis, we characterize the state of the science on the effects of air concentrations of NO2 and NH3 on vascular plants and lichens across scales of biological organization, from physiology to community-level effects. Both atmospheric NO2 and NH3 generally serve as nutrients to plants leaves and lichens through arial surface adsorption, but as concentrations rise, both forms of nitrogen can cause physiological stress, leaf injury and growth inhibition. In general, reduced forms of nitrogen are toxic at lower concentrations than oxidized forms. Community-level changes in biodiversity of vascular plants have not been observed or thoroughly studied at concentrations of NO2 and NH3 that currently occur in the U.S. However, these concentrations do cause lichens physiological stress and alter their community composition. Often, sensitive species of lichens disappear from ecosystems. These findings are discussed in the context of U.S. and European air pollution policy and standards.