Effects of Nitrogen Deposition on Greenhouse-Gas Fluxes for Forests and Grasslands of North America

Nitrogen deposition across North America has been influencing fluxes in greenhouse gas pollutants such as carbon dioxide, nitrous oxide, and ozone.  In the face of climate change, it is crucial for us to understand the spatial patterns and effects of nitrogen deposition on greenhouse gasses. Templer et al.(2012) discussed spatial patterns of nitrogen deposition and tropospheric ozone and then summarized the known effects of nitrogen deposition on greenhouse gases in North American terrestrial ecosystems.  The authors investigated the causes and implications of emissions such as nitrogen and ozone, and then created an exposure index that analyzed regions affected by both emissions.  The index showed that forests in Eastern U.S. and California’s Sierra Nevada mountains had the highest levels of nitrogen deposition and ozone exposure.—Anthony Li
Templer, P. H., Pinder, R. W., Goodale, C. L. 2012. Effects of nitrogen deposition on greenhouse-gas fluxes for forests and grasslands of North America.  Frontiers in Ecology and the Environment  10.10 547–553

In this paper, the authors investigate the spatial patterns of nitrogen deposition and tropospheric ozone and its implications on other greenhouse-gas pollutants.  Due to the industrial sector, emissions of nitrogen oxides and ammonia have been on the rise.   Both of these pollutants contribute to the formation of aerosols in the atmosphere, which are responsible for decreasing the amount of light available.  Nitrogen oxides also drive the formation of tropospheric ozone and the removal of atmospheric methane.  Nitrogen deposition also largely impacts carbon released from plants and soils, because it enhances carbon dioxide uptake in plants and reduces carbon dioxide release from soils by decomposition. Tropospheric ozone is a byproduct of the photo-oxidation process of nitrous oxides, and often occurs in areas with nitrogen deposition.  Ozone is itself a greenhouse gas and can decrease carbon dioxide uptake in plants by damaging their root production and stomates.  Tropospheric ozone currently accounts for 22% of global warming, and is predicted to reduce rates of primary productivity by up to 16%.  Methane is another greenhouse gas pollutant that was also investigated in this study.  Methane concentrations are affected by nitrogen oxides multiple ways.  In soils, nitrogen enrichment slows the consumption of atmospheric methane by bacteria whereas in wetlands, nitrogen enrichment can enhance methane production.  However, the effect that increased nitrous oxides have on removing methane from the atmosphere is more prominent than the previous two effects.  Data for these pollutants were obtained from previous studies and were then compared with each other regionally using an exposure index that the authors developed.
Collected data show that nitrogen deposition on Eastern U.S. averages 8.5 kg nitrogen per hectare year while on the West coast it averages less than 4 kg nitrogen per hectare year.  The Eastern U.S. has hotspots of nitrogen deposition near urban areas and sites of intensive livestock operations, but the differences between rural and urban areas are not as pronounced as those in the West.  Along the East Coast, an analysis revealed that the nitrogen deposition stimulated forest growth in the hardwood forests.  The forests on the West Coast are noticeably drier and limited in nitrogen, so increased fire frequency and subsequent carbon release may result from the nitrogen deposition.  Southern Canada also experienced elevated rates of atmospheric nitrogen deposition, more so in areas downwind of the agricultural and industrial regions of southern Ontario.  Nitrogen deposition on the East Coast also increased rates of nitric oxide and nitrous oxide production, with the former being produced significantly faster.  Authors note how this is for the better, as nitrous oxide is roughly 300 times more potent in warming potential than carbon dioxide is per molecule.  Nitrogen deposition causes nitrous oxide emissions to increase by 30 to 90 gigagrams while causing atmospheric methane to increase by 40 to 110 gigagrams. Nitrous oxide and methane emissions contribute to warming and can partially offset the cooling that results from increased uptake of carbon dioxide.  The index that the authors used showed forests in Eastern U.S. and California’s Sierra Nevada mountains amongst the regions with highest levels of nitrogen deposition and ozone exposure.  The index also showed that the highest exposure for grasslands occurred in California, Texas, and Kansas.
While the results of this study show how detrimental nitrogen deposition can be, the authors also note that regulation of nitrous oxide emissions has resulted in significant reductions over the past decade.  With more standards being implemented, nitrogen oxides are predicted to decrease by up to 47% by 2030 and 67% by 2050, relative to 2006 nitrogen oxide levels.   Lower levels of nitrogen oxides will likely lead to lower carbon dioxide uptake by plants and lowered nitrous oxide emitted from soils.  As opposed to nitrous oxide however, ammonia levels are unlikely to decline.  Even though standards are projected to decrease nitrogen deposition, increasing levels of atmospheric carbon dioxide are likely to increase the demand for nitrogen in vegetation.  This shows how implemented standards may mean nothing if we do not seek to lower carbon dioxide emissions.  In summary, rates of nitrous oxide, ammonia emissions, and atmospheric nitrogen deposition are elevated over much of North America.  Heightened nitrogen deposition results in greater carbon dioxide uptake by vegetation and increased nitrous oxide emissions from soils.  The net effect of nitrogen deposition in the U.S. is equivalent to an annual uptake of 170 Tg of carbon dioxide equivalent gases.  Despite the heightened nitrous oxide, ammonia, and nitrogen deposition, standards are expected to decrease nitrogen deposition in the coming years.  The impacts of this will be two-sided, as lowered rates of nitrogen deposition result in slower forest growth and carbon storage, but also result in lower emissions of nitrous oxide.

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