The role of nitrogen in climate change and the impacts of nitrogen–climate interactions in the United States: foreword to thematic issue

Emma C. Suddick 0 1 2 Penelope Whitney 0 1 2 Alan R. Townsend 0 1 2 Eric A. Davidson 0 1 2 0 A. R. Townsend Department of Ecology and Evolutionary Biology, University of Colorado , INSTAAR, 1560 30th St., Boulder, CO 80303, USA 1 P. Whitney Resource Media, 101 Montgomery St., Suite 2600, San Francisco, CA 94104, USA 2 E. C. Suddick (&) E. A. Davidson The Woods Hole Research Center , 149 Woods Hole Road, Falmouth, MA 02450, USA Producing food, transportation, and energy for seven billion people has led to large and widespread increases in the use of synthetic nitrogen (N) fertilizers and fossil fuel combustion, resulting in a leakage of N into the environment as various forms of air and water pollution. The global N cycle is more severely altered by human activity than the global carbon (C) cycle, and reactive N dynamics affect all aspects of climate change considerations, including mitigation, adaptation, and impacts. In this special issue of Biogeochemistry, we present a review of the climate-nitrogen interactions based on a technical report for the United States National Climate Assessment presented as individual papers for terrestrial and aquatic ecosystems, agriculture and human health within the US. We provide a brief overview of each of the paper's main points and conclusions is presented in this foreword summary. - In the last 50 years, synthetic fertilizer production, widespread cultivation of leguminous crops, and a variety of industrial processes including fossil fuel use have greatly increased the release of reactive nitrogen (Nr) to the environment (Vitousek et al. 1997; Galloway et al. 2004, 2008). Globally, the N cycle is perhaps the most altered of the major biogeochemical cycles, with serious implications for human health, biodiversity, and air and water quality (Vitousek et al. 1997; Galloway et al. 2008; Townsend and Howarth 2010; Davidson et al. 2012). Moreover, Nrs unwanted consequences can be further aggravated by climate change, and vice versa. Reactive nitrogen moves easily through the atmosphere, from air to water and to soil and back to plants, therefore, in its numerous chemical forms, Nr plays a critical role in all aspects of climate change considerations, including mitigation, adaptation, and impacts (Davidson et al. 2012). The collection of papers within this Biogeochemistry special issue originated from a July 2011 workshop in Fort Collins, CO, at the US Geological Survey John Wesley Powell Center for Analysis and Synthesis. The central objective of the workshop was to provide in-depth analysis of climatenitrogen interactions in both terrestrial and aquatic systems for consideration by the United States National Climate Assessment (US-NCA) for the 2013 National Climate Assessment report. The papers in this special issue reflect much of the workshop input to the USNCA, and highlight some of the most important climateN interactions in terrestrial, aquatic and agricultural systems, as well as those relevant to human health concerns. Nitrogen cycling affects atmospheric concentrations of the three most important anthropogenic greenhouse gases in terms of total current radiative forcing: carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Therefore, mitigation of excess Nr would both reduce N2O emissions and affect CO2 and CH4 in complex ways (Pinder et al. this issue). These include how N affects C sequestration in forests and soils, and how atmospheric CH4 concentrations are affected by the chemistry of nitrogen oxides (NOx) and ozone (O3) of which NOx is a precursor in the lower atmosphere. Several of these N cycling processes have contrasting effects on the atmospheric burdens of greenhouse gases, including a possible net cooling effect on the time scale of a few decades (Pinder et al. 2012). However, most evidence suggests that minimizing Nr release to the environment would slow the rate of climate change over the next century (Townsend et al. 2012). Understanding how nitrogen cycling affects climate change is essential, especially as adaptation to climate change involves changes in energy and water use. Although considerable progress has been made in lowering nitrogen oxide (NOx) pollution from energy, industry, and transportation sectors in the US, this progress could slow or be reversed if energy use increases to adapt to climate change, such as to provide additional energy for air conditioning or to pump and treat water. Adaptations to increasing water scarcity, which many regions will experience with climate change, may include greater use of surface and groundwater (Gleick 2003). This will likely exacerbate problems of elevated nitrate (NO3-) concentrations in waters draining to rivers, lakes, groundwater and estuaries, leading to eutrophication, costly drinking water treatments (Hoagland et al. 2002), or increased incidents of nitrate-related disease (Johnson et al. 2010). In agriculture, improvements in nutrient management such as proper timing and use of fertilizer will reduce N releases and could also provide some adaptive protection to crops from climate variability (Bruulsema et al. 2009). Yet unpredictable weather will also make efficient nutrient management more difficult for farmers and may worsen downstream and downwind problems: drought will cause a buildup of NO3- in soils and estuaries, and flooding will cause fertilizer and manure that has been applied to crops to be released more rapidly downstream and into the atmosphere (Davidson et al. 2012). Climate change will significantly alter N cycling processes, which will affect both terrestrial and aquatic ecosystems, as well as human health. Higher air temperatures will complicate air quality mitigation, because larger reductions in NOx emissions will be needed to achieve the same reductions of O3 pollution under higher temperatures (Wu et al. 2008). Such a climate penalty will impose challenges to avoid harmful impacts of O3 pollution on human health (Racherla and Adams 2009) and crop productivity (Mauzerall and Wang 2001). Changes in river flow, due to summer drought and extreme precipitation events, will affect the loading and processing of N within rivers and estuaries. Lower river flows may reduce the total flux of N entering coastal regions, but would also reduce rates of flushing of estuaries, whereas higher flows will accelerate loading of N from terrestrial to aquatic systems. In either case, more frequent blooms of harmful or nuisance algal species are possible. In addition, rising ambient temperatures will increase ammonia (NH3) emissions throughout all phases of manure handling and will likely result in lower N use efficiency in livestock production systems and greater losses of Nr to the environment (Rotz 2004; Montes et al. 2009; Hristov et al. 2011). Both climate change and N inputs from air pollution (i.e., N deposition) can provoke a loss of biodiversity in aquatic and terrestrial ecosystems, due to nutrient enrichment of native ecosyste (...truncated)