B12A-01 INVITED 10:20h
Anthropogenic Nitrogen Mobilization: Drivers, Consequences and Opportunities for Action
Food and energy production converts N$_{2}$ to reactive N to such a degree that in many regions of the world, human action dominates the introduction and cycling of reactive nitrogen. While there are major benefits (food!), there are also numerous negative impacts on humans and ecosystems. This presentation will briefly examine the magnitude of alteration of the N cycle, and then focus on the resulting negative impacts, how they are connected, and how they might be reduced. The challenge is how to enhance the beneficial aspects of N use, while developing policies that take into account the cascading nature of nitrogen downstream and downwind from its point of entry into environmental systems.
B12A-02 10:35h
An overview of sources and species of water-soluble organic nitrogen in the global atmosphere
For several decades the atmospheric/biogeochemical community has been interested in water-soluble organic nitrogen in the atmosphere within aerosols and rain, its sources and sinks, and its species composition. However, little information is available regarding its deposition flux or its source emissions. I will present an overview of sources (biomass burning, agriculture, etc.) for water-soluble organic nitrogen in the global atmosphere as well as introduce new data for individual nitrogen species identifed in atmospheric aerosols and discuss their solubility in rain. With new data from an aerosol study conducted at a dairy in Northern Colorado, I will discuss the release of water-soluble organic nitrogen from agricultural systems.
B12A-03 INVITED 10:50h
Passive Samplers for Monitoring Insidious N Air Pollutants and Estimating N Deposition to Terrestrial Ecosystems
Ammonia (NH3), nitric acid vapor (HNO3), nitric oxide (NO) and nitrogen dioxide (NO2) are the main biologically important nitrogenous (N) air pollutants. At highly elevated concentrations, these pollutants have a potential of causing injury to sensitive plants. More importantly, gaseous N pollutants may provide significant amounts of atmospheric N to the terrestrial ecosystems. This is especially true for wildlands affected by photochemical smog and agricultural emissions (e.g. mountains near California Central Valley or Los Angeles Basin). Passive samplers developed in the 1990s and 2000s have allowed for reliable monitoring of ambient concentrations of the pollutants at large geographic scales. Information on spatial and temporal distribution of NH3, HNO3, NO and NO2 from passive samplers may allow for determining potential "hot spots" of N pollutants effects. Information on ambient concentrations of gaseous N can also be used for estimates of N deposition to various ecosystems. Monitoring of N air pollutants and estimates of N deposition have been conducted in deserts, coastal sage, serpentine grassland, chaparral, and mixed conifer forests in California. These efforts and potential future use of passive samplers will be discussed.
B12A-04 11:05h
Dry and Wet Deposition of Atmospheric Inorganic Nitrogen in a Tropical Environment (Rond\^{o}nia, Brazil: 10\deg45.44'S, 62\deg21.27'W)
Total N deposition (wet + dry) has been experimentally determined in temperate regions and few data sets exist about wet N deposition to tropical ecosystems. When global atmospheric chemistry & transport models such as MOGUNTIA had been applied, they suggested that total N deposition in the Northern Hemisphere far exceeded contemporary tropical N deposition [Holland et al., 1999]. However, quantitative experimental information about dry N deposition in tropical environments, required to test these model predictions, has been lacking. We estimated the total N deposition (wet + dry) for a pasture site in the Amazon Basin (Rond\^{o}nia, Brazil) based on field measurements covering the closing of the dry season (biomass burning), a transition period, and the onset of the wet season (clean conditions) (LBA-SMOCC). Wet N deposition was determined by collection and subsequent analyses of rainwater samples. Gaseous species, such as NO, NO$_{2}$, O$_{3}$, NH$_{3}$, HNO$_{3}$, HONO and the inorganic aerosol compounds NH$_{4}$$^{+}$ and NO$_{3}$$^{-}$ were measured at one aerodynamic reference height using real-time techniques, which was accompanied by measurements of micrometeorological quantities. Dry deposition fluxes of NO$_{2}$ and HNO$_{3}$ were inferred using the "big leaf multiple resistance approach". Furthermore, we predict bi-directional surface-atmosphere exchange fluxes of NH$_{3}$ and HONO with the aid of a canopy compensation point model, which considers the bi-directional exchange of NH$_{3}$ between biosphere and atmosphere as a dynamic process [Sutton et al., 1998]. These approaches were based on the variation of surface parameters, which determine trace gas uptake/release by vegetative canopies and which were not experimentally determined. Dry deposition fluxes of aerosol particles were derived using established empirical parameterizations. The total annual N deposition was estimated to be 7.5 to 8.5 kgN ha$^{-1}$ yr$^{-1}$, whereof 2 to 3 kgN ha$^{-1}$ yr$^{-1}$ are attributed to dry N deposition and \sim 5.5 kgN ha$^{-1}$ yr$^{-1}$ to wet deposition. Thus, dry deposition may contribute on average 30 % to the total annual N deposition. It is expected that 2 to 6.5 kgN ha$^{-1}$ yr$^{-1}$ are (re-)emitted from the pasture site, mainly in form of gaseous NH$_{3}$, but also HONO and NO. Our results suggest that the modeled average contemporary N deposition to tropical grasslands is underestimated by a factor of two or three.
B12A-05 11:20h
Nitrogen Additions Increase the Diversity of Carbon Compounds Degraded by Fungi in Boreal Forests
Boreal forest soils in North America harbor a large reservoir of organic C, and this region is increasingly exposed to long-range atmospheric N transport from Eurasia. By examining the responses of decomposers to N deposition in these forests, we hope to improve predictions of the fate of boreal carbon pools under global change. We tested the hypothesis that the functional diversity of decomposer fungi would increase under N fertilization in boreal forests where fungal growth was otherwise N-limited, owing to a reduction in competitive exclusion of fungal groups. We collected soil and leaf litter from three Alaskan sites that represent different successional stages at 5, 17, or 80 years following severe forest fire. Each site had been exposed for two years to nitrogen and phosphorus fertilization in a factorial design, with four plots per treatment. Nutrient limitation of fungal growth varied depending on successional stage. The standing hyphal length of decomposer fungi in soil (i.e. Ascomycota and Basidiomycota) responded to neither N nor P in the 5-year old site, increased under N fertilization in the 17-year old site, and increased where N and P was added simultaneously in the 80-year old site (site x N x P interaction: P = 0.001). We used BIOLOG microplates for filamentous fungi to obtain an index of the diversity of carbon use by decomposer fungi; each of 95 wells of these plates contains a different carbon-based compound, as well as a dye that changes color upon metabolism of the compound. Saline leaf litter extracts were mixed with fungal growth medium and then added to the microplates. The number of wells displaying metabolic activity was counted following incubation for five days. We found that N fertilization raised the average number of positive wells per plate from 14 to 27 (P = 0.012), with no significant differences in responses among sites. Phosphorus additions did not alter functional diversity of fungi in any site. Since increases in functional diversity occurred even in forests where fungal growth was not limited by N, alleviation of competitive exclusion does not appear to be the mechanism underlying this response. Our findings indicate that N fertilization could potentially result in the decomposition of a wider variety of organic carbon compounds in boreal forest soil, with possible consequences for CO2 release to the atmosphere.
B12A-06 11:35h
Biodiversity Risks from Atmospheric Nitrogen Deposition in California
Atmospheric nitrogen deposition alters structure and function of terrestrial ecosystems, because nitrogen availability is often limits overall productivity. These alterations can drive losses of biodiversity, as nitrophilous species increase in abundance and outcompete species adapted to more oligotrophic conditions. California is recognized as a "biodiversity hotspot," with a high fraction of endemic taxa with narrow ranges. A state-wide risk screening includes: 1) a 36 x 36 km map of total N-deposition for 2002, developed from the Community Multiscale Air Quality Model (CMAQ); 2) identification of sensitive habitat types from literature and local expertise; 3) overlay of a statewide vegetation map (FRAP); 4) overlay of species occurrence data from the California Natural Diversity Data Base (CNDDB); and 5)species life-history and habitat requirements. The CMAQ model indicates that 55,000 km$^{2}$ (total area 405,205 km$^{2}$) are exposed to $>$5 kg-N ha $^{-1}$ year $^{-1}$, and 10,000 km$^{2}$ are exposed to $>$10 kg-N ha $^{-1}$ year $^{-1}$. Deposition hotspots include coastal urban areas (Los Angeles-San Diego, and the San Francisco Bay Area), the agricultural Central Valley, and parts of the Sierra Nevada foothills. The major known impact of N-deposition in California is increased growth and dominance of invasive annual grasses in low biomass ecosystems, such as coastal sage scrub, serpentine grassland, desert scrub, and vernal pools. For example, 800 km$^{2}$ out of a total 6300 km$^{2}$ of coastal sage scrub are exposed to more than 10 kg-N ha $^{-1}$ year $^{-1}$, primarily in Southern California. Of 225 federal and state "Threatened" and "Endangered" plant taxa, 101 are exposed on average to $>$5 kg-N ha $^{-1}$ year $^{-1}$. Of an additional 1022 plant taxa listed as "rare," 288 are exposed to $>$5 kg-N ha $^{-1}$ year $^{-1}$. Many of these highly exposed taxa are associated with sensitive habitat types and are vulnerable to annual grass invasions. This broad-scale screening outlines potential impacts on California's biodiversity, and paves the way for finer scale analyses using a 4 x 4 km CMAQ deposition map (under development), more complete local data on species occurrences, and establishment of critical N-loads for particular ecosystems.
B12A-07 11:50h
Atmospheric Nitrogen Deposition: An increasingly Important Source of "new" Nitrogen Supporting Coastal Eutrophication
Atmospheric deposition of nitrogen (AD-N) to the North Atlantic Ocean (NAO) basin arises from diverse pollution sources in North America and Western Europe; these sources have increased by 5 to10 fold since the Industrial Revolution, agricultural expansion and urbanization in the NAO airshed and continue to increase in both geographic and depositional magnitudes. Based on recent estimates, AD-N flux (11.2 Tg N per year) accounts for 46-57 per cent of the total new or externally-supplied anthropogenic N flux to the NAO. In US estuarine and coastal waters, from 10 to over 40 per cent of new N loading is attributed to AD-N; estimates for North Carolina's Albemarle-Pamlico Sound system range from 20 to over 30 per cent. In developing regions of the world, AD-N is one of the most rapidly expanding sources of new N. AD-N has been linked to eutrophication in N-sensitive coastal waters. In North Carolina, N deposition has increased since the 1960's as a result of urbanization (chiefly NOx) and more recently agricultural growth (NH4+ and organic N). In particular, rapidly-expanding livestock operations have led to increases in the generation of N-enriched wastes and manures; a substantial proportion (30- >70 per cent) of which may be emitted as NH3 gas. Recent growth and intensification of animal operations in the midwest and coastal regions (e.g., Mid-Atlantic coastal plain) have been linked to increasing amounts of NH4+ deposition, according to a 2 decadal analysis of the National Acid Deposition Program (NADP) network. The impacts of both increasing amounts and altered chemical composition of AD-N are being examined in the N-limited, eutrophying (i.e., expanding algal blooms, hypoxia and anoxia) Neuse River Estuary, Pamlico Sound and coastal waters of North Carolina. Because of its relatively large contribution to total new N loading and potential biogeochemical and ecological importance in N sensitive waters, AD-N requires attention from air/watershed nutrient budgeting and management perspectives.
B12A-08 12:05h
Development and Implementation of Critical Loads for Atmospheric Deposition: Federal Land Management Implications
Critical loads for atmospheric deposition have been widely developed and used in Europe, Canada, and other countries. Critical loads are used to influence air pollution emissions reductions, thereby protecting and restoring aquatic and terrestrial ecosystems. In the United States, federal land management agencies are adopting the critical load concept as a potentially valuable resource management tool. Certain parks and wilderness areas are currently being affected by anthropogenic nitrogen and sulfur deposition. Effects of excess deposition include acidification, nitrogen enrichment, toxicity, and changes in biotic communities. Streams in both Shenandoah and Great Smoky Mountains National Parks are experiencing chronic and episodic acidification and brook trout fisheries in Shenandoah have been affected. High elevation ecosystems in Rocky Mountain National Park are undergoing subtle changes in aquatic and terrestrial ecosystems attributable to atmospheric deposition. Natural resources in many other federal areas have been affected or are at risk from deposition. Federal land managers are refining strategies for critical loads that include working with scientists to identify resources sensitive to deposition, defining resource protection criteria that will meet management objectives, and estimating and implementing critical loads. Critical loads will be used in resource management decisions and federal land management planning. They will be used to evaluate management actions and assess progress towards meeting management goals. Federal land managers will also communicate critical loads information to air pollution regulatory agencies to inform emissions management strategies for clean air.