Chemosphere - Global Change Science最新文献

Context Abstract: The atmospheric budget of N₂O, a greenhouse gas and the dominant source of total reactive nitrogen to the stratosphere, remains a controversial subject. Gas-phase atmospheric chemical sources of N₂O are not well documented, but studies of atmospheric N₂O samples show a mass-independent heavy oxygen isotope enrichment which is suggestive of the existence of missing in situ sources or sinks. We have studied N₂O production from the reaction of electronically excited NO₂ with N₂. Contrary to earlier findings, our results show that this process is an insignificant source of atmospheric N₂O.

Main Abstract: Production of N₂O as a product of the collisional deactivation of electronically excited NO₂ by N₂ has been investigated with the goal of establishing the importance of this process in the atmospheric N₂O budget. The experimental approach minimizes potential interferences from heterogeneous reactions, multiphoton processes, and self-quenching. At the 95% confidence limit, we find that the quantum yield for production of N₂O from 488 nm photolysis of 60 Torr of 0.1% NO₂ in N₂ is less than 4 × 10⁻⁸, i.e., at least 10,000 times smaller than a previously published value; this result suggests that quenching of electronically excited NO₂ by N₂ is unimportant as a source of atmospheric N₂O.

The agricultural water system especially contaminated with high nitrogen was surveyed in Saitama Prefecture, Japan. The anthropogenic nitrogen, such as fertilizer, livestock wastes, etc., caused the occurrence of high nitrate-nitrogen (20–30 mgN/l) in the groundwater in this area. The concentration of dissolved N₂O in the groundwater ranged from 0 to 28.2 μgN/l. The main source of N₂O in the groundwater seemed to be nitrified N from the stockbreeding areas. As this groundwater flowed into the organic matter rich areas, such as paddy fields and a small river, denitrification actively occurred and much N₂O was produced there. Dissolved N₂O concentration in these places ranged from 10 to 400 μgN/l and N₂O gas flux from the small river was extremely high (about 500 μgN/m² · min). While nitrate-nitrogen could be removed in these areas, these areas often served as significant N₂O sources. The effective and feasible method to decrease the N₂O emission ratio while keeping the denitrification activity high should be explored.

To demonstrate the efficiency of putting sulfur into soil as a denitrification electron donor, the column and the batch experiments were carried out. When denitrification proceeded sufficiently, N₂O formation in the soil column packed with elemental sulfur was kept low. In the case of adding CaCO₃ for pH adjustment and adding elemental sulfur or iron sulfide, N₂O production was suppressed. This indicated the possibility to decrease N₂O emission by adding sulfur into the soil and proceeding sulfur denitrification.

Though estimates of the total N₂O emitted by automobiles differ widely, automobiles are believed to be a significant source of non-agricultural anthropogenic N₂O emissions. At the Third Conference of the Parties (COP-3) UN Framework Convention on Climate Change, held in Kyoto in 1997, N₂O was designated as a greenhouse gas whose release into the atmosphere must be reduced. This action increased the need for more accurate estimates of automotive N₂O emissions. The wide variation in estimates may be attributed to differences in emission test modes, types of catalysts, and levels of catalyst deactivation involved in the tests. In this study, we examined the influence of automotive catalyst deactivation on N₂O emissions from the perspective of catalyst temperature frequency distribution. Using a model gas and deactivated three-way catalysts (TWCs), we applied the exhaust emission test modes of various countries. The results indicate that the factor behind the increase of N₂O emissions following catalyst deactivation is not growth in N₂O generation, but a decline in the N₂O decomposition capability of the catalyst. It was also found that the effect of catalyst deactivation differs according to the catalyst composition and the emission test mode.

Context Abstract: Atmospheric concentrations of nitrous oxide, a greenhouse gas, are increasing due to human activities. Our analysis suggests that a third of global anthropogenic N₂O emission is from aquatic sources (rivers, estuaries, continental shelves) and the terrestrial sources comprise the remainder. Over 80% of aquatic anthropogenic N₂O emissions are from the Northern Hemisphere mid-latitudes consistent with the geographic distribution of N fertilizer use, human population and atmospheric N deposition. These N inputs to land have increased aquatic as well as terrestrial anthropogenic N₂O emissions because a substantial portion enters aquatic systems and results in increased N₂O production. Thus, wise management of N in the terrestrial environment could help reduce/control both aquatic and terrestrial N₂O emissions.

Main Abstract: The global distribution of N₂O emissions from rivers, estuaries, continental shelves, and oceans are compared to each other, and to terrestrial emissions, using existing gridded inventories. Rivers, estuaries and continental shelves (1.9 Tg N y⁻¹) account for about 35% of total aquatic N₂O emissions; oceanic emissions comprise the remainder. Oceanic N₂O emissions are approximately equally distributed between the Northern and Southern Hemispheres; however, over 90% of emissions from estuaries and rivers are in the Northern Hemisphere. N₂O emissions from rivers, estuaries, and continental shelves combined equal oceanic emissions in both the 20°–45°N and 45°–66°N latitudinal zones. Over 90% of river and estuary emissions are considered anthropogenic (1.2 Tg N y⁻¹); only 25% of continental shelf emissions are considered anthropogenic (0.1 Tg N y⁻¹); oceanic emissions are considered natural. Overall, approximately one third of both aquatic and of terrestrial emissions are anthropogenic.

Natural terrestrial emissions are highest in tropical latitudes while natural aquatic emissions are relatively evenly distributed among latitudinal zones. Over half of both the anthropogenic terrestrial and aquatic emissions occur between 20° and 66°N. Anthropogenic N inputs to the terrestrial environment drive anthropogenic N₂O emissions from both land and aquatic ecosystems, because a substantial portion of the anthropogenic N applied to watersheds enters rivers, estuaries and continental shelves.

Riparian zones, which sit at the interface between terrestrial and aquatic components of the landscape, often receive and process large amounts of excess nitrogen (N) that moves out of agricultural fields towards streams. These areas thus have the potential to be “hotspots” of nitrous oxide (N₂O) production in the landscape. However, current Intergovernmental Program on Climate Change (IPCC) methodologies for calculating national N₂O emission inventories do not explicitly account for riparian N₂O production. In this paper, we examine the nature and extent of N₂O production in riparian zones, present some new data on N₂O production in these areas, and propose a modification to the current IPCC methodology for quantifying N₂O emissions from agriculture. We also present an example of how large-scale riparian restoration efforts to achieve agricultural water quality objectives could cause significant changes in regional N₂O budgets. Although current data are inadequate to propose a quantitative emission factor for riparian N₂O emissions, they suggest that these emissions are likely to be significant in many regions. Specific data on riparian N₂O emissions should be collected in association with detailed watershed mass balance studies that allow for evaluation of several aspects of the IPCC methodology at once and provide constraints on the magnitude of fluxes that are difficult to measure, e.g. N₂O flux, N₂O:N₂ ratio. Riparian and wetland restoration projects to reduce NO₃⁻ delivery to coastal waters are being considered in many areas of the world. These projects may affect regional and global N₂O budgets, but only if they alter the N₂O:N₂ ratio during denitrification.

Nitrous oxide (N₂O) and nitric oxide (NO) emissions from soil are affected by many factors. Soil nitrogen source, especially N fertilizer input, and soil moisture might be the most important factors to control these two gases emission rate. In this study, laboratory incubation experiments were conducted to determine the effect of the urea form and the soil moisture on N₂O and NO emissions in Japanese Andosols. Results showed that there were no significant differences in the total amount of N₂O and NO emissions over 77 d between non-coated and coated urea (CU) treatments, except for NO emission at 40% wfps (water filled pore space) where it was reduced by 23% when CU was applied. As compared to easily decomposable urea (U), however, CU did reduce N₂O and NO emissions in the earlier period shortly after fertilization. The results also indicated that soil moisture had a much more significant effect on N₂O and NO emissions than the form of urea. From 40% to 100% wfps, there was a positive relationship between N₂O emission and soil water content and a negative relationship for NO. The flux ratio of NO/N₂O was governed by soil moisture with a greatest value at the lowest wfps treatments for each fertilizer treatment. Soil moisture could be the most important factor controlling N₂O and NO emissions when a rich N supply exist in soil.

In situ measurement of atmospheric nitrous oxide (N₂O) has been carried out at Hateruma monitoring station (lat 24°03^′N, long 123°48^′E) since March 1996 by the National Institute for Environmental Studies (NIES). A fully automated gas chromatograph equipped with an electron capture detector (ECD) measures the N₂O concentrations at a frequency of 3 air samples per hour. Details of the experimental methods and procedures are presented in this paper. The N₂O concentrations observed from March 1996 to February 1999 increased at an average rate of 0.64 ppb/yr. The observed data also suggest that there is a weak annual cycle of N₂O concentration, increasing in autumn and winter and decreasing in spring and summer, with a peak-to-peak amplitude of at most 0.3 ppb. The N₂O mixing ratios, smoothed with the 24-h running average, clearly showed short-term variability with synoptic timescales and had peak-to-peak amplitudes of about 1 ppb or less. These short-term variations correlated positively with the short-term variations of CO₂ during the period from winter to early spring when the air masses arriving at Hateruma are dominantly transported from the Asian continent. The ΔN₂O/ΔCO₂ ratios could be used to constrain the relative strengths of these fluxes on a regional scale.

In most soils, biogenic formation of N₂O is enhanced by an increase in available mineral N through increased nitrification and denitrification. N-fertilization, therefore, directly results in additional N₂O formation. In addition, these inputs may lead to indirect formation of N₂O after N leaching or runoff, or following deposition of NO_y and NH_x from gaseous losses of NO_x and NH₃. Anthropogenic N input into agricultural systems includes N from synthetic fertilizer, animal wastes, increased biological N-fixation, mineralization of crop residue returned to the field and cultivation of organic soils through enhanced organic matter mineralization. Nitrous oxide may be emitted (1) directly to the atmosphere from agricultural fields, (2) from animal confinements or pastoral systems, or (3) from N applied to agricultural systems which is transported into ground and surface waters through atmospheric deposition, sewage and surface runoff and eventually into surface water (rivers and oceans) where additional N₂O is produced. Eventually, all N that moves through the soil system will be either terminally sequestered in soil or buried sediments or denitrified in aquatic systems.

Using Food and Agricultural Organization of the United Nations (FAO) databases for fertilizer input, crop and animal production, and human population and the IPCC (1997) methodology for estimating N₂O from soil, we first estimated the N input into food production and then calculated N₂O emissions derived from N input into food production systems from 1500 until the year 2020. Using these estimates for N₂O emissions (∼6 Tg N in 1990 and ∼9 Tg N in 2020) as input to a simple atmospheric box model we estimated global atmospheric N₂O concentrations over time. During the 20th century, a fast expansion of agricultural land coupled with intensification of land use probably caused about 80% of the net increase in atmospheric N₂O, from ∼275 ppbv in 1900 to ∼294 ppbv in 1970, to projected concentrations of ∼317 in 2000 and ∼345 in 2020. With the increasing amount of fertilizer N application needed to feed an additional 1.5 billion people in the next 20 years, an accelerated rate of N₂O accumulation in the atmosphere is calculated for the coming decades. This is in contrast with the observed trends during the past decade, which indicate a linear increase in atmospheric N₂O, but is in line with observed trends during the whole 20th century, which show a non-linear increase in atmospheric N₂O.