Effect of increased nitrogen fixation on stratospheric ozone

Nitrogen compounds are produced by biological reactions and by industrial processes from the abundant nitrogen gas (N₂) in the atmosphere. The formation of compounds from atmospheric nitrogen is called fixation. In nature, nitrogen compounds undergo many conversions, but under aerobic conditions, characterized by the presence of oxygen, they tend to be converted to the nitrate (NO ₃ ^- ) form. Under anaerobic conditions, characterized by the absence of oxygen, the nitrate is denitrified, and the nitrogen contained therein is converted into nitrogen gas (N₂) and nitrous oxide (N₂O), which escape into the atmosphere. The nitrous oxide diffuses into the stratosphere, where it decomposes to yield nitrogen gas and small amounts of nitric oxide (NO) and nitrogen dioxide (NO₂), which react with ozone (O₃) to convert it to oxygen (O₂). The ozone in the stratosphere is produced by the reaction of light with oxygen and is destroyed primarily by reactions with the nitrogen oxides.As long as the production and destruction are equal, the ozone in the stratosphere is maintained at a constant concentration. Increased nitrogen fixation will lead to increased denitrification, increased amount of nitrous oxide moving into the stratosphere, and a reduction in ozone concentration.Ozone in the stratosphere attenuates the ultraviolet light received from the sun. As the ozone concentration decreases, more ultraviolet light will reach the surface of the earth. The fear is that this additional radiation will have detrimental effects on living organisms and possibly on the climate.Because the global use of fixed nitrogen in fertilizers has increased greatly in recent years and in 1974 amounted to almost 40 million metric tons, the eventual generation of nitrous oxide from the fertilizer nitrogen after application to the soil has been cited as a potential environmental hazard. In response to this concern, this document estimates nitrogen fixation, nitrous oxide production, and ozone reduction based on two methods of calculation and on various increases in nitrogen fixation. Uncertainties and information gaps in the nitrogen cycle are pointed out.This document does not review either the projected biological effects of ozone depletion or the stratospheric chemistry of ozone. These topics are dealt with at length in other studies.World fixation of nitrogen in 1974, expressed in millions of metric tons per year (MT/yr), was estimated to be as follows.Most of the estimates given are based on inadequate data; consequently, actual amounts may be significantly different from those shown. The study of nitrogen fixed in the oceans has not progressed far enough to permit reliable estimates. However, estimates of the amount of nitrogen fixed for fertilizer and other industrial uses in 1974 are considered reliable. The trend of industrial fixation of nitrogen offers some indication of the trend in total amount of nitrogen fixed. It is estimated that 174 MT of nitrogen were fixed by all processes in 1950. Total fixation in 1850 could have been 150 MT of nitrogen.Nitrous oxide-nitrogen production on land is estimated as 5 to 10 MT/yr; published estimates of production in the ocean, however, range from less than 1 to 100 MT/yr. The higher value was based on reported supersaturation of ocean waters with nitrous oxide.Two methods of estimating the decrease in ozone concentration in the stratosphere were used. Method I is based on nitrogen fixation. It involves the assumptions that the relative increase in production of nitrous oxide is proportional to the relative increase in total nitrogen fixation and that sufficient time has elapsed for the rate of denitrification to come to equilibrium with fixation; i.e., the lag time between increased fixation and increased denitrification has passed. This method, using fixation estimated for 1950 as a base, suggests that the reduction in ozone would be 5.8 and 11.5% as a consequence of increased fixation of 50 and 100 MT of nitrogen per year, respectively.Method II is based on nitrous oxide evolution. It involves the assumption that the global rate of production of nitrous oxide is 100 MT/yr (based on supersaturation of this gas in the ocean and on changes in measured concentrations of nitrous oxide in the atmosphere). Method II leads to estimates of ozone reduction much lower than those from Method I. For example, on the assumption that global production of nitrous oxide-nitrogen is 100 MT/yr and that 5% of the nitrogen denitrified is released as nitrous oxide, the estimated ozone reduction is 1% with an increase of 100 MT/yr in nitrogen fixation. This method is forced to assume an unknown source of nitrous oxide in the ocean and an unknown sink for nitrous oxide in the troposphere.There are great uncertainties in many of the estimates that have been made for nitrogen fixation and for nitrous oxide production, and there are many information gaps that need to be filled before the question of the effects of increased nitrogen fixation on the ozone layer can be answered. Perhaps the biggest information needs are in the areas of nitrogen transformations and the quantities of nitrous oxide produced in the ocean. Other needs deal with the complexities of the nitrogen cycle on land. The lag time between fixation by various processes and denitrification must be known as a basis for estimating how soon predicted effects based on equilibrium conditions can be expected. Concentrations of nitrous oxide and their fluctuations in the troposphere (lower atmosphere) need to be monitored to provide an index to variations and increases in production. Improved models are needed to relate the ozone concentration in the stratosphere to nitrogen fixation and nitrous oxide production on earth.In spite of the uncertainties in the predictions of the effects of increased fixation of nitrogen on stratospheric ozone, the potential hazard is sufficiently serious that, in addition to research on the various phases of the global nitrogen cycle that impinge upon the nitrous oxide-ozone question, research on the efficiency of use of all fixed forms of nitrogen should be worthwhile. Editor's Note: Although the data for sources, sinks, reservoirs, and rate processes in this article are undergoing rapid revision presently, it, nonetheless, is one of the clearest statements of the physics, chemistry, and biology of the fertilizer/ozone problem available to date.This report was developed by eleven scientists (see Appendix 1 for names and affiliations) representing the subject matter areas of atmospheric chemistry, chemical engineering, environmental science and chemistry, microbiology, oceanography, plant genetics, soil biochemistry, soil physics, and soil chemistry. This task force of scientists chaired by Parker F. Pratt, met under the auspices of the Council for Agricultural Science and Technology (CAST), whose headquarters office is at the Department of Agronomy, Iowa State University, Ames, Iowa 50011, U.S.A. The task force met in Denver, Colorado from October 23 to 25, 1975, to prepare a first draft of the report. The chairman then prepared a revised version and returned it to members of the task force for review and comment. A second revision was then prepared and returned for further comment. Finally, the report was edited and reproduced for transmittal through the U.S. Congressional Committees concerned with the matter of ozone depletion. It was originally issued as a CAST Report Number 53, January, 1976, but had not been formally published heretofore.     

Searching for periodicities in the MACHO light curve of LMC X-2

Using the exceptional long-term monitoring capabilities of the MACHO project, we present here the optical history of LMC X-2 for a continuous 6-yr period. These data were used to investigate the previously claimed periodicities for this source of 8.15 h and 12.54 d: we find upper limits of 0.10 mag and 0.09 mag, respectively.     

Leatherback turtles: The menace of plastic

The leatherback, Dermochelyscoriacea, is a large sea turtle that feeds primarily on jellyfish. Floating plastic garbage could be mistaken for such prey. Autopsy records of 408 leatherback turtles, spanning 123 years (1885-2007), were studied for the presence or absence of plastic in the GI tract. Plastic was reported in 34% of these cases. If only cases from our first report (1968) of plastic were considered, the figure was 37%. Blockage of the gut by plastic was mentioned in some accounts. These findings are discussed in the context of removal of top predators from poorly understood food chains.     

An eclipse of the X-ray flux from the dwarf nova OY Carinae in quiescence

We present a phase-resolved ROSAT HRI X-ray light curve of the dwarf nova OY Car in quiescence. The X-ray flux is eclipsed at the same time as the optical eclipse of the primary, and the region of X-ray emission is comparable in size to the white dwarf. We use subsequent optical observations to update the orbital ephemeris of the system.     

Quantitative SIMS analysis of trace Au in pyrite using the infinite velocity (IV) method

The quantitative analysis of trace concentrations of Au in pyrite using a new procedure of SIMS quantification is investigated. Analysis of an Au implanted pyrite standard demonstrates the infinite velocity (IV) method as a viable procedure for evaluation of trace concentrations of Au in pyrite. Comparison of naturally occurring trace concentrations of Au in pyrite using standard-derived relative sensitivity factor (RSF) and IV quantification methods is shown as a means of evaluating the chemical environments in which Au is found in pyrite.     

A diatom-based Holocene record of human impact from a coastal environment: Tuckean Swamp, eastern Australia

Diatom-based paleolimnological studies are being increasingly used to track anthropogenic change in estuaries. Little is known, however, about the direction and nature of long-term environmental changes in Australian estuaries. In this study, shifts in diatom assemblages preserved in a ²¹⁰Pb and C¹⁴ AMS dated sediment core from Tuckean Swamp were analysed to determine environmental changes that had taken place as a result of changing land-use practices. Prior to European impact, the diatom assemblage remained relatively stable and was dominated by Actinocyclus normanii and Diploneis smithii. An increasing dominance of Cyclotella meneghiniana correlates well with changed land use activities in the catchment area and indicates an increase of freshwater influence in the swamp’s environment. A major shift in species composition began ∼1970, Eunotia flexuosa becoming dominant. The assemblage shifts recorded at this site appear to be consistent with environmental changes triggered by human activities such as vegetation clearance, drainage and the construction of a barrage. This study demonstrates the use of paleolimnoology in an estuarine environment to provide pre-impact data necessary for management of the aquatic environment.     

Misfit functionals in Laplace‐Fourier domain waveform inversion,with application to wide‐angle ocean bottom seismograph data

In seismic waveform inversion, non‐linearity and non‐uniqueness require appropriate strategies. We formulate four types of L2 normed misfit functionals for Laplace‐Fourier domain waveform inversion: i) subtraction of complex‐valued observed data from complex‐valued predicted data (the ‘conventional phase‐amplitude’ residual), ii) a ‘conventional phase‐only’ residual in which amplitude variations are normalized, iii) a ‘logarithmic phase‐amplitude’ residual and finally iv) a ‘logarithmic phase‐only’ residual in which the only imaginary part of the logarithmic residual is used. We evaluate these misfit functionals by using a wide‐angle field Ocean Bottom Seismograph (OBS) data set with a maximum offset of 55 km. The conventional phase‐amplitude approach is restricted in illumination and delineates only shallow velocity structures. In contrast, the other three misfit functionals retrieve detailed velocity structures with clear lithological boundaries down to the deeper part of the model. We also test the performance of additional phase‐amplitude inversions starting from the logarithmic phase‐only inversion result. The resulting velocity updates are prominent only in the high‐wavenumber components, sharpening the lithological boundaries. We argue that the discrepancies in the behaviours of the misfit functionals are primarily caused by the sensitivities of the model gradient to strong amplitude variations in the data. As the observed data amplitudes are dominated by the near‐offset traces, the conventional phase‐amplitude inversion primarily updates the shallow structures as a result. In contrast, the other three misfit functionals eliminate the strong dependence on amplitude variation naturally and enhance the depth of illumination. We further suggest that the phase‐only inversions are sufficient to obtain robust and reliable velocity structures and the amplitude information is of secondary importance in constraining subsurface velocity models.     

A portable experimental hillslope for frozen ground studies

Frozen ground hydrological effects on runoff, storage, and release have been observed in the field and tested in numerical models, but few physical models of frozen slopes (at scales from 1 to 15 m) exist partly because the design of such an experiment requires new engineering design for realistic whole‐slope freezing and physical model innovation. Here, we present a new freezable tilting hillslope physical model for hydrological system testing under a variety of climate conditions with the ability to perform multiple (up to 20 per year) freeze–thaw cycles. The 4 × 2 m hillslope is mobile and tiltable on the basis of a modified tri‐axle 4.88‐m (16′) dump trailer to facilitate testing multiple configurations. The system includes controllable boundary conditions on all surfaces; examples of side and baseflow boundary conditions include permeable membranes, impermeable barriers, semipermeable configurations, and constant head conditions. To simulate cold regions and to freeze the hillslope in a realistic and controlled manner, insulation and a removable freezer system are incorporated onto the top boundary of the hillslope. The freezing system is designed to expedite the freezing process by the addition of a 10,130‐KJ (9,600‐BTU) refrigeration coil to the top‐centre of the insulated ceiling. Centre placement provides radial freezing of the hillslope in a top‐down fashion, similar to what natural systems encounter in the environment. The perimeter walls are insulated with 100 mm of spray foam insulation, whereas the base of the hillslope is not insulated to simulate natural heat fluxes beneath the frozen layer of soil. Our preliminary testing shows that covers can be frozen down to ?10 °C in approximately 7 days, with subsequent thaw on a similar time frame.     

Stable Sulfur Isotopic Evidence for Historical Changes of Sulfur Cycling in Estuarine Sediments from Northern Florida

Data on abundance and isotopic composition of porewater and sedimentary sulfur species are reported for relatively uncontaminated and highly contaminated fine-grained anoxic sediments of St. Andrew Bay, Florida. A strong contrast in amount and composition of sedimentary organic matter at the two sites allows a comparative study of the historical effects of increased organic loading on sulfur cycling and sulfur isotopic fractionation. In the contaminated sediments, an increase in organic loading caused increased sedimentary carbon/sulfur ratios and resulted in higher rates of bacterial sulfate reduction, but a lower efficiency of sulfide oxidation. These differences are well reflected in the isotopic composition of dissolved sulfate, sulfide, and sedimentary pyrite. Concentration and isotopic profiles of dissolved sulfate, organic carbon, and total sulfur suggest that the anaerobic decomposition of organic matter is most active in the upper 8cm but proceeds at very slow rates below this depth. The rapid formation of more than 90% of pyrite in the uppermost 2 cm which corresponds to about 3 years of sediment deposition allows the use of pyrite isotopic composition for tracing changing diagenetic conditions. Sediment profiles of the sulfur isotopic composition of pyrite reflect present-day higher rates of bacterial sulfate reduction and lower rates of sulfide oxidation, and record a profound change in the diagenetic cycling of sulfur in the contaminated sediments coincident with urban and industrial development of the St. Andrew Bay area.