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Oxygen exchange between (de) nitrification intermediates and H2O and its implications for source determination of NO3- and N2O: a review
No full textStable isotope analysis of oxygen (O) is increasingly used to determine the origin of nitrate (NO3- and nitrous oxide (N2O) in the environment. The assumption underlying these studies is that the O-18 signature of NO3- and N2O provides information on the different O sources (O-2 and H2O) during the production of these compounds by various biochemical pathways. However, exchange of O atoms between H2O and intermediates of the (de)nitrification pathways may change the isotopic signal and thereby bias its interpretation for source determination. Chemical exchange of O between H2O and various nitrogenous oxides has been reported, but the probability and extent of its occurrence in terrestrial ecosystems remain unclear. Biochemical O exchange between H2O and nitrogenous oxides, NO2- in particular, has been reported for monocultures of many nitrifiers and denitrifiers that are abundant in nature, with exchange rates of up to 100%. Therefore, biochemical O exchange is likely to be important in most soil ecosystems, and should be taken into account in source determination studies. Failing to do so might lead to Q) an overestimation of nitrification as NO3- source, and (ii) an overestimation of nitrifier denitrification and nitrification-coupled denitrification as N2O production pathways. A method to quantify the rate and controls of biochemical O exchange in ecosystems is needed, and we argue this can only be done reliably with artificially enriched O-18 compounds. We conclude that in N source determination studies, the O isotopic signature of especially N2O should only be used with extreme caution. Copyright (C) 2007 John Wiley & Sons, Ltd
Source Determination of Nitrous Oxide Based on Nitrogen and Oxygen Isotope Tracing: Dealing with Oxygen exchange
No full textSource determination of nitrous oxide (N2O) from soils has so far been complicated by methodological constraints: the frequently used 15N tracer method could not differentiate between pathways related to nitrification, that is, nitrifier nitrification (NN), nitrifier denitrification (ND), and nitrification-coupled denitrification (NCD). To overcome this problem, a dual isotope method using both 15N and 18O was proposed. However, O exchange between nitrogen oxides and water has been found to disturb such a method. We here explain in detail a novel dual isotope method that allows to quantify O exchange in denitrification and to differentiate N2O production from NN, ND, NCD, and fertilizer denitrification (FD). The method has already been applied to a range of soils with good success. Potential of and scope for further improvement of the method are discusse
Oxygen exchange between nitrogen oxides and H2O can occur during nitrifier pathways
No full textInterpretation of the oxygen isotopic signature of soil-derived N2O may be flawed when it is based on reaction stoichiometry and fractionation alone. In fact, oxygen (O) exchange between H2O and intermediates of N2O production pathways may largely determine this O isotopic signature. Although in our previous work we conclusively proved the occurrence of O exchange during N2O production by denitrification of NO3¿, its occurrence in N2O production pathways by nitrifiers remains unclear. The aim of this study was to examine the likeliness of O exchange during various stages of N2O production in soil via nitrification, nitrifier denitrification and denitrification. We evaluated a set of scenarios on the presence of such exchange using data from a series of 18O and 15N tracing experiments. The measured actual O incorporation from H2O into N2O (AOI) was compared with the theoretical maximum O incorporation (MOI) from various scenarios that differed in their assumptions on the presence of O exchange. We found that scenarios where O exchange was assumed to occur exclusively during denitrification could not explain the observed AOI, as it exceeded the MOI for 9 out of 10 soils. This demonstrates that additional O exchange must have occurred in N2O production through nitrifier pathways. It remains to be determined in which steps of these pathways O exchange can take place. We conclude that O exchange is likely to be mediated by ammonia oxidizers during NO2¿ reduction (nitrifier denitrification), and that it could possibly occur during NO2¿ oxidation to NO3¿ by nitrite oxidizers as wel
Nitrifier denitrification as a distinct and significant source of nitrous oxide from soil
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The 18O signature of biogenic nitrous oxide is determined by O exchange with water
No full textTo effectively mitigate emissions of the greenhouse gas nitrous oxide (N2O) it is essential to understand the biochemical pathways by which it is produced. The 18O signature of N2O is increasingly used to characterize these processes. However, assumptions on the origin of the O atom and resultant isotopic composition of N2O that are based on reaction stoichiometry may be questioned. In particular, our deficient knowledge on O exchange between H2O and nitrogen oxides during N2O production complicates the interpretation of the 18O signature of N2O. Here we studied O exchange during N2O formation in soil, using a novel combination of 18O and 15N tracing. Twelve soils were studied, covering soil and land-use variability across Europe. All soils demonstrated the significant presence of O exchange, as incorporation of O from 18O-enriched H2O into N2O exceeded their maxima achievable through reaction stoichiometry. Based on the retention of the enrichment ratio of 18O and 15N of NO into N2O, we quantified O exchange during denitrification. Up to 97% (median 85%) of the N2O-O originated from H2O instead of from the denitrification substrate NO. We conclude that in soil, the main source of atmospheric N2O, the 18O signature of N2O is mainly determined by H2O due to O exchange between nitrogen oxides and H2O. This also challenges the assumption that the O of N2O originates from O2 and NO, in ratios reflecting reaction stoichiometr
Oxygen exchange with water alters the oxygen isotopic signature of nitrate in soil ecosystems
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92166.pdf (Publisher’s version ) (Closed access)6 p
Nitrifier denitrification can be a source of N2O from soil: a revised approach to the dual-isotope labelling method
No full textNitrifier denitrification (i.e. nitrite reduction by ammonia oxidizers) is one of the biochemical pathways of nitrous oxide (N2O) production. It is increasingly suggested that this pathway may contribute substantially to N2O production in soil, the major source of this greenhouse gas. However, although monoculture studies recognize its potential, methodological drawbacks prohibit conclusive proof that nitrifier denitrification occurs in actual soils. Here we suggest and apply a new isotopic approach to identify its presence in soil. In incubation experiments with 12 soils, N2O production was studied using oxygen (O) and nitrogen (N) isotope tracing, accounting for O exchange. Microbial biomass C and N and phospholipid fatty acid (PLFA) patterns were analysed to explain potential differences in N2O production pathways. We found that in at least five of the soils nitrifier denitrification must have contributed to N2O production. Moreover, it may even have been responsible for all NH4+-derived N2O in most soils. In contrast, N2O as a by-product of ammonia oxidation contributed very little to total production. Microbial biomass C and N and PLFA-distinguished microbial community composition were not indicative of differences in N2O production pathways. Overall, we show that combined O and N isotope tracing may still provide a powerful tool to understand N2O production pathways, provided that O exchange is accounted for. We conclude that nitrifier denitrification can indeed occur in soils, and may in fact be responsible for the greater proportion of total nitrifier-induced N2O production
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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