Synthesis of lichen response to gaseous nitrogen: ammonia versus nitrogen dioxide

The dominant chemical form of nitrogen pollution in the atmosphere in the U.S. is shifting from oxidized nitrogen, primarily from combustion of fossil fuels, to reduced nitrogen from agricultural animal waste and fertilizer applications. Does it matter to lichens? In this synthesis, we characterize U.S. air concentrations of the most ubiquitous gaseous forms of reduced and oxidized nitrogen, NO₂ and NH_3, respectively, and their direct effects on lichens. In the U.S., the 3-year average (2017-2019) of the annual mean for each monitoring site ranges up to 56.4 microg NO₂ m^-3 (~30 ppb) and 6 microg NH₃ m^-3 (~9 ppb). The spatial coverage of current routine monitoring of NO₂ and NH₃ likely does not accurately represent exposures of NO₂ to ecosystems in rural areas or capture spikes of NH₃ concentrations proximal to intensive agriculture, which are documented to exceed 700 microg NH₃ m^-3 (~1000 ppb) for short durations. Both NO₂ and NH₃ can act as nutrients to lichens, but as exposures rise, both can cause physiological stress and mortality that then change community composition and diversity. There is a growing body of evidence that lichen community composition is altered at current levels of exposure in the U.S. with estimated no effect or lowest effect concentrations from <1-3 microg m^-3 NO₂ and <1 microg m^-3 NH₃. Better spatial characterization of both NO₂ and NH₃ concentrations, especially near intensive agriculture, would help to characterize the extent of the impacts across the U.S. These findings are discussed in the context of U.S. air pollution policy.