The Significance of Diffusion Limitation for Oxygen Isotope Fractionation in Soil Respiration
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Natural abundances of stable isotopes of nitrogen and carbon (${\delta}^{15}N$ and ${\delta}^{13}C$ ) are being widely used to study N and C cycle processes in plant and soil systems. Variations in ${\delta}^{15}N$ of the soil and the plant reflect the potentially variable isotope signature of the external N sources and the isotope fractionation during the N cycle process. $N_2$ fixation and N fertilizer supply the nitrogen, whose ${\delta}^{15}N$ is close to 0%o, whereas the compost as. an organic input generally provides the nitrogen enriched in $^{15}N$ compared to the atmospheric $N_2$ . The isotope fractionation during the N cycle process decreases the ${\delta}^{15}N$ of the substrate and increases the ${\delta}^{15}N$ of the product. N transformations such as N mineralization, nitrification, denitrification, assimilation, and the $NH_3$ volatilization have a specific isotope fractionation factor (${\alpha}$ ) for each N process. Variation in the ${\delta}^{13}C$ of plants reflects the photosynthetic type of plant, which affects the isotope fractionation during photosynthesis. The ${\delta}^{13}C$ of C3 plant is significantly lower than, whereas the ${\delta}^{13}C$ of C4 plant is similar to that of the atmospheric $CO_2$ . Variation in the isotope fractionation of carbon and nitrogen can be observed under different environmental conditions. The effect of environmental factors on the stomatal conductance and the carboxylation rate affects the carbon isotope fractionation during photosynthesis. Changes in the environmental factors such as temperature and salt concentration affect the nitrogen isotope fractionation during the N cycle processes; however, the mechanism of variation in the nitrogen isotope fractionation has not been studied as much as that in the carbon isotope fractionation. Isotope fractionation factors of carbon and nitrogen could be the integrated factors for interpreting the effects of the environmental factors on plants and soils.
Isotopes of nitrogen
Nitrogen Cycle
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Atmospheric oxygen is about 23 ‰ higher in 18 O than ocean surface water, whereas oxygen in isotopic equilibrium with ocean water would be only 6 ‰ higher in 18 O. The fractionation of 18 O during respiration has been measured on natural populations in unfiltered marine surface water samples. The decrease of dissolved oxygen concentration and the increase in δ 18 O due to respiration was measured as a function of time. An average enrichment factor of 21 ‰ was calculated for the removal of oxygen in a closed system. The results indicate that the enrichment of 18 O in the atmosphere, and possibly the present oxygen concentration, may be controlled by biogenic processes.
Equilibrium fractionation
Oxygen-18
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Soil respiration
Soil carbon
Variation (astronomy)
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Nitrogen Assimilation
Assimilation (phonology)
Nitrogen Cycle
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Mass-independent oxygen isotope fractionation provides new insights to the research on global changes.Based on an introduction to mass-independent isotope fractionation,this paper discusses the definition of oxygen isotope anomaly [Δ(17O)] and the production mechanisms of massindependent oxygen isotope fractionation,particularly the application of massindependent oxygen isotope fractionation to earth sciences.The productivity assessed with Δ(17O) is total biosphere productivity.It removes the limitation of only evaluating terrestrial or oceanic productivity individually and establishes a basis for the productivity estimates in a more broad temporal and spatial scale.In particular,using Δ(17O) to quantify effectively the relative contribution of homogenous and heterogeneous reaction pathways of aerosol sulfate and nitrate opens a new way for investigating the interaction between climate and aerosol.The combination of Δ(17O) and S isotope in the ice core not only traces the source and transport of sulfate and nitrate but also provides detailed information on their oxidation processes.The discovery of sulfate and nitrate Δ(17O) in some arid areas can reasonably reduce the great uncertainty of identifying the sources and genesis of some sediments.This demonstrates that mass-independent oxygen isotope fractionation will play a more important role in the research on(ancient) atmospheric ozone activity,chemistry in volcanic plumes and O,S and N biogeochemical cycle.
Equilibrium fractionation
Biogeochemical Cycle
Mass-independent fractionation
Sulfate aerosol
Ice core
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Degradation
Carbon fibers
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Chemical fractionation
Hydrogen molecule
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The stable isotopic composition of dissolved oxygen(δ18ODO) can provide important information about the oxygen cycling in the aquatic systems. Different biogeochemical processes,including atmospheric O2 dissolution,biological respiration and photosynthesis,play an important role on the isotopic composition of DO. In this paper,we re-view recent progress of δ18ODO in marine systems,including the oxygen isotopic fractionation during the different biogeochemical processes,the analysis techniques and the applications in various dissolved oxygen studies. Key directions for future research include an analytical advance,especially an effective pre-treatment method,theoretical studies of some unknown aspects and a future understanding of the δ18ODO dynamics under the influence of key physical processes.
Biogeochemical Cycle
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