B21D-01 INVITED
Sources of Atmospheric Pollutants Impacting Air and Water Quality in the Lake Tahoe Basin
Starting in the second half of the 20th century, decline in Lake Tahoe's water clarity and degradation in the basin's air quality have become major concerns due to its unique scenic features. Gaseous and particulate nitrogen (N) and particulate phosphorus (P) loading via direct atmospheric deposition and sediment transport to the lake have also been implicated as responsible for its eutrophication and decline in water clarity. Estimates suggest that atmospheric N deposition contributes 55% of the total N loading to the lake, while atmospheric P deposition contributes 15% of the total P loading. In order to improve both air quality and, as a consequence, water quality, it is necessary to develop an understanding of the sources of the atmospheric pollutants. Once this is accomplished, it is possible to implement cost-effective strategies to reduce this impact. This paper summarizes the findings of a series of studies performed to determine the levels and sources of ambient air pollutants in the basin. Projects have included the development of a Tahoe-specific emissions inventory, long-term measurements of road dust resuspension, modeling to determine the fraction of pollutants coming from in-basin vs. out-of-basin sources, particulate source apportionment, and estimates of nitric acid deposition. These studies found that the pollutants most closely connected to the decline in water quality come largely from within basin sources, as opposed to those coming from the Central Valley and upwind urban areas of California. These results indicate regulators need to control pollutant emissions within the Tahoe basin in order to reduce the impact of atmospheric pollutants on both air and water quality.
B21D-02
Soil emissions of gaseous reactive nitrogen from North American arid lands: an overlooked source.
The biosphere-atmosphere exchange and transformation of nitrogen has important ramifications for both terrestrial biogeochemistry and atmospheric chemistry. Several important mechanisms within this process (e.g., photochemistry, nitrogen deposition, aerosol formation) are strongly influenced by the emission of reactive nitrogen compounds from the Earth's surface. Therefore, a quantification of emission sources is a high priority for future conceptual understanding. One source largely overlooked in most global treatments are the soil emissions from arid and semi-arid landscapes worldwide. Approximately 35-40% of global terrestrial land cover is aridland and emission of reactive nitrogen from soils in these regions has the potential to strongly influence both regional and global biogeochemistry. Here we present estimates of soil emission of oxidized (NO, total NOy including NO2 and HONO) and reduced (NH3) forms of reactive nitrogen from two North American arid regions: the Mojave Desert and the Colorado Plateau. Soil fluxes in these regions are highly dependent on soil moisture conditions. Soil moisture is largely driven by pulsed rain events with fluxes increasing 20-40 fold after a rain event. Using field measurements made across seasons under an array of moisture conditions, precipitation records, and spatially explicit cover type information we have estimated annual estimates for the Mojave Desert (1.5 ± 0.7 g N ha-1 yr-1), the shale derived (1.4 ± 0.9 g N ha-1 yr-1), and sandy soil derived (2.8 ± 1.2 g N ha-1 yr-1) regions of the Colorado Plateau. The chemical composition of soil emissions varies significantly both with season and soil moisture content. Emissions from dry soils tend to be dominated by ammonia and forms of NOy other than NO. In contrast, NO becomes a dominant portion of the flux post rain events (~30% of the total flux). This variability in chemical form has significant implications for the tropospheric fate of the emitted N. NO and other nitrogen oxides are likely to participate in photochemistry, ozone production, and production of organic nitrates and nitric acid. In contrast, NH3 is likely to locally redeposit or form secondary aerosols in the presence of sulphate. Given the vastly different influence of oxidized versus reduced forms of N on atmospheric chemistry, the variable chemical partitioning of soil emissions based on season and water availability observed in this study is likely to improve the performance of regional photochemistry models.
B21D-03
Kudzu (Pueraria montana) Invasion Doubles Emissions of Nitric Oxide, a Precursor to Tropospheric Ozone.
Nitrogen-fixing plants can increase rates of nitrogen (N) cycling in soils, fluxes of the greenhouse gas nitrous oxide (N2O), and fluxes of the ozone precursor nitric oxide (NO). Invasion by the nitrogen-fixing legume kudzu (Pueraria montana) across millions of hectares in the southeastern United States could be contributing to increased ozone concentrations. Ozone formation in the Southeast is broadly limited by atmospheric NO concentrations, so perturbations to the N cycle by kudzu have the potential to increase ozone concentrations in the region. At three sites in Madison County, Georgia, kudzu invasion increased rates of net N mineralization by up to an order of magnitude and rates of net nitrification and soil pools of NO3- and NO2- by up to 500%. Emissions of NO from soils invaded by kudzu averaged 2.81 ng NO-N cm-2 h-1, significantly higher than emissions from soil dominated by native vegetation, which averaged 1.24 ng NO-N cm-2 h-1. Emissions of N2O display a trend towards increasing under kudzu invasion, but N2O emissions at these sites were three orders of magnitude smaller than emissions of NO. Atmospheric chemical modeling suggests that these higher NO emissions could increase ozone concentrations in the region by up to 2 ppb. We propose that kudzu invasion in the southeastern United States represents a novel threat to air quality and could increase the frequency with which federal ozone standards are exceeded.
B21D-04
Nitrogen deposition budgets for Rocky Mountain National Park
The Rocky Mountain Airborne Nitrogen and Sulfur (RoMANS) study was conducted to improve our understanding of the sources and transport of airborne nitrogen and sulfur species within Rocky Mountain National Park (RMNP) as well as their deposition pathways. Two field campaigns were conducted, in spring and summer 2006, to characterize pollutant transport and deposition during seasons with historically high nitrogen inputs. Measurements at the RoMANS core study site included daily wet deposition fluxes (nitrate, ammonium, organic nitrogen and sulfate) and daily concentrations of key particle (ammonium, nitrate, and sulfate) and trace gas (ammonia, nitric acid, and sulfur dioxide) species. From these measurements and dry deposition velocities derived from co-located CASTNet data, deposition flux budgets were determined for the spring and summer measurement campaigns. During both spring and summer, wet deposition of ammonium was the largest contributor to nitrogen deposition followed by wet deposition of nitrate. The next most important pathways were wet deposition of organic nitrogen and dry deposition of ammonia. Nitric acid dry deposition was fifth in importance. Dry deposition of fine particle nitrate and ammonium made only minor contributions to total nitrogen deposition fluxes. Neither wet deposition of organic nitrogen nor dry deposition of ammonia are measured by existing monitoring networks. These two pathways together comprise approximately 30 percent of RMNP nitrogen deposition measured during RoMANS, suggesting that their contributions need to be better quantified in the future in order to fully define nitrogen inputs to the park. Dry deposition of organic nitrogen, which was not measured even during RoMANS, is an even larger unknown.
B21D-05
Nitrogen Isotopes in Tree Rings: A Record of Atmospheric Deposition or Tree Physiology?
Dendroisotopic analysis of nitrogen in tree rings has suggested that changes in nitrogen availability can be determined over time. These records match lake sediment and stream N flux measurements. Other studies suggest that tree ring nitrogen isotope data match foliar samples and not soil isotope data. In this study we compare the nitrogen isotope record from loblolly pine and American beech tree rings to a 10 year rainfall record and groundwater samples taken in isolated forested areas, next to a roadway and adjacent to agricultural fields. The influence of atmospheric nitrogen was determined by O17 analysis on groundwater nitrate. In areas where nutrient availability was low and O17 in groundwater nitrate was high, the tree ring nitrogen isotope record was similar to the nitrate nitrogen isotope trend. In areas where there was increased amounts of nitrate in groundwater due to local land use and O17 in groundwater nitrate was low, the nitrogen isotopic record in tree rings did not correlate to the atmospheric deposition record and probably represent internal recycling of nitrogen in the tree. These results suggest that nitrogen isotopes in tree rings can be used to monitor long term trends of atmospheric deposition of nitrogen in areas of low nutrient availability.
B21D-06 INVITED
Empirical and modeling approaches to setting critical loads for N deposition in southern California shrublands
Southern California deserts and coastal sage scrub (CSS) are undergoing vegetation-type conversion to exotic annual grassland, especially in regions downwind of urban areas that receive high N, primarily as dry deposition. To determine critical loads (CL) of N that cause negative impacts, we measured plant and soil responses along N deposition gradients, fertilized vegetation at different N levels, and used biomass production output from the DayCent model. N deposition gradients were identified from the CMAQ model and compared with measured N deposition values. CSS receives N deposition as high as 30 kg ha-1yr- 1, while the desert has levels up to 16 kg ha-1yr-1. Unlike more mesic ecosystems where critical loads are determined by changes in soil chemistry or biogeocycling, these arid and semiarid ecosystems are subject to increases in exotic species production, loss of native species diversity, and increased fire risk at relatively low CL's for N of 5 - 10 kg ha-1yr-1. For instance, a gradient survey in CSS showed that exotic grass increased and native plant species and arbuscular mycorrhizal species richness declined by almost half above 11 kg N ha-1yr-1. Fertilization studies in desert creosote bush scrub showed a significant increase in exotic species biomass with 5 kg N ha-1yr-1 in a wet year, and biomass output from DayCent modeling indicated an increased fire risk from exotic grasses with 1 T per ha production during years with moderate to high precipitation at 5 kg N ha-1yr-1. The difference in CL between desert and CSS may be related to the responsiveness of native vs. exotic plant species to N, as well as the degree to which precipitation and soil N limits plant growth in the two vegetation types.
B21D-07
Quantifying Nitrogen Fluxes and Their Influence on the Greenhouse gas Balance- Research Strategy and new Findings From the NitroEurope Integrated Project
The human-driven production of reactive nitrogen to stimulate agricultural productivity and its unintended formation in combustion processes both have major impacts on the global environment. Effects of excess reactive nitrogen include reductions in air quality, water quality, soil quality and biodiversity. One of the most controversial impacts of nitrogen, however, is on the greenhouse gas balance. While recent papers have highlighted a possible benefit of nitrogen in enhancing rates of carbon sequestration, there remain many trade-offs between nitrogen and greenhouse gas exchange. The result is that the net effect of reactive nitrogen on the global radiative balance is currently far from clear. To better quantity these relationships requires measurement data and modelling that make the link between different nitrogen forms and their fate in the environment. It is essential to measure fluxes for a wide range of ecosystems considering the biosphere-atmosphere exchange each of the reactive nitrogen components and greenhouse gases, as well as the fixation and denitrification of di-nitrogen. Long term observations are needed for representative ecosystems, together with results from experiments addressing the responses of the key nitrogen and greenhouse gas fluxes to different global change drivers. The NitroEurope Integrated Project of the 6th Framework Programme of the European Commission European has developed a strategy to quantifying these different terms on multiple scales. This paper presents the experimental approach including a) a 3-tier flux network, combining process level measurements and new method development with low-cost measurements at many sites, b) a network of manipulation experiments with different global change drivers, c) a network of contrasting European landscapes for analysis of land- use and land management interactions assessing the multiple nitrogen fluxes within and between air, land and water. The paper illustrates the new datasets emerging, and shows how these are being used to support the development of site, landscape and regional-scale models of nitrogen fluxes and net greenhouse gas exchange. Finally, the paper describes how independent verification activities are providing the basis to analyze the uncertainty in regional-scale nitrogen and greenhouse gas fluxes.
B21D-08 INVITED
Nitrogen Cascade: An Opportunity to Integrate Biogeochemistry and Policy
It began with micro-organisms millions of years ago, was enhanced by the burning of fossil carbon in the last several hundred years, and was magnified by a patent filed one hundred years ago. Today, the combined actions of cultivation-induced biological nitrogen fixation, fossil fuel combustion and the Haber-Bosch process have exceeded natural terrestrial processes in converting N22 to nitrogen compounds that are biologically, chemically or physically reactive (reactive nitrogen, Nr). While the benefits of Nr are well understood, many of the adverse consequences of excessive Nr are invisible from a policy perspective. Over the past century, the fundamental knowledge on nitrogen processes has advanced to the point where we have a good understanding of nitrogen's biogeochemical cycle, the role of humans in altering the cycle, and the consequences of the alterations. This knowledge has collectively led us to two conclusions—the consequences of intensive human influence on the nitrogen cycle leads to a cascade of ecosystem and human effects which need to be managed. Secondly, the management is complicated by the facts that it not only has to be integrated, but it also has to take into account that the management should not lower the ability of managed ecosystems to produce food for the world's peoples. The framework of the nitrogen cascade provides us with a structure for better identifying intervention points, and more effective policies, technologies and measures to prevent or mitigate the adverse impacts of reactive nitrogen, while enhancing its beneficial uses. We can now begin to use our understanding of science to set priorities and craft new policy strategies. For many regions of the world, the science is strong enough to manage nitrogen and there are existing tools to do so. However, the tools are not integrated, critical tools are missing and most importantly, there are nitrogen-rich regions of the world where the science is lacking, and nitrogen-poor regions where there is inadequate supply of nitrogen. After a brief review of the nitrogen cycle and the associated nitrogen cascade, this paper will identify control points in the cycle where management would be optimum, review the possible tools that are available for management, and suggest a process by which an integrated management approach might be developed.