1,721,115 research outputs found
Biosphere-atmosphere-exchange of C and N trace gases and microbial N turnover processes in irrigated agricultural systems of the Aral Sea Basin, Uzbekistan
No full textLand-use and agricultural practices affect the soil microbial carbon (C) and nitrogen (N) turnover and hence the biosphere-atmosphere exchange of greenhouse gasses (GHG), namely N2O, CH4 and CO2. In view of the global importance of irrigated agriculture, it is crucial to understand how and to which extent this land-use system interferes with the terrestrial N and C cycles and contributes to the global source strength of atmospheric GHG. Up to now, knowledge of trace gas exchange and N turnover from irrigated agriculture in arid and semiarid regions is much less developed than in other climate zones. Therefore, this study aims at providing more detailed insights into the biosphere-atmosphere exchange of trace gases and the underlying soil microbial transformation processes of the irrigated agricultural systems in the Aral Sea Basin (ASB), Uzbekistan. A two-year field study was carried out to quantify and compare emissions of N2O and CH4 in various annual and perennial land-use systems dominating in the study region Khorezm in western Uzbekistan: irrigated cotton, winter wheat and rice crops, a poplar plantation as well as a natural Tugai (floodplain) forest. Irrigated agricultural production in the ASB was shown to be a relevant source of GHG especially due to high emissions of N2O during the annual cropping of wheat and cotton. Average N2O emissions ranged from 10 to 150 µg N2O N m 2h-1 with highest N2O emissions in the cotton fields, covering a similar range described in previous studies of other irrigated cropping systems. Seasonal variations in N2O emissions were principally controlled by fertilization and irrigation management. Very high N2O emissions of up to 3000 µg N2O-N m-2 h-1 were measured in periods directly following N fertilizer application in combination with irrigation events. These “emission pulses” accounted for 80-95% of the total N2O emissions over the cropping season for cotton and wheat. Cumulated emissions over one season varied from 0.5 to 6.5 kg N2O-N ha-1. The unfertilized poplar plantation showed high N2O emissions over the entire study period (30µg N2O N m 2h-1), whereas only negligible fluxes of N2O (2O N m 2h-1) occurred in the natural Tugai forest. Observations of significant CH4 fluxes were restricted to the flooded rice fields, with mean flux rates of 32 mg CH4 m 2d-1 and a seasonal total of 35.2 kg CH4 ha-1. The global warming potential (GWP) of the N2O and CH4 fluxes was highest under rice and cotton, with seasonal changes between 500 and 3000 kg CO2 eq.ha-1. The biennial cotton-wheat-rice crop rotation commonly practiced in the region averaged a GWP of 2500 kg CO2 eq.ha 1 year-1. In addition, laboratory incubation studies were conducted to assess the aggregated gaseous N losses composed of NO, N2O, and N2 from fertilized and irrigated agricultural fields in the ASB. NO3- fertilizer and irrigation water were applied to the incubation vessels to assess its influence on the gaseous N emissions. Under the soil conditions, naturally found after concomitant irrigation and fertilization, denitrification was the dominant process and N2 the main gaseous product of denitrification. Based on the results of these laboratory incubation studies, the magnitude of N2 emissions for the different field research sites of irrigated cotton could be estimated to be in the range of 24±9 to 175±65 kg-N ha-1season-1, while emissions of NO were only of minor importance (between 0.1 and 0.7 kg-N ha-1 season-1). The findings demonstrate that under the current agricultural practices in the irrigated dryland soils of the ASB, denitrification is a major pathway of N losses and that beside N2O extensive amounts of N fertilizer are lost as N2 to the atmosphere. Moreover, the experimental design of this study allows assessing the potential for reducing GHG emissions from these land-use systems. It is argued that there is wide scope for reducing the GWP of this agroecosystem by (i) optimization of fertilization and irrigation practices and (ii) conversion of annual cropping systems into perennial forest plantations, especially on less profitable, marginal lands.Biosphäre-Atmosphäre Austausch von C/N Spurengasen und mikrobielle N Umsetzungsprozesse in bewässerten, landwirtschaftlichen Produktions-systemen des Aralseebeckens, Usbekistan Die mikrobiellen Umsetzungsprozesse von Kohlenstoff (C) und Stickstoff (N) in Böden und der damit verbundene Austausch von Treibhausgasen zwischen Biosphäre und Atmosphäre werden maßgeblich von der Landnutzung und den landwirtschaftlichen Methoden beeinflusst. Angesichts der weltweiten Bedeutung von bewässerter Landwirtschaft ist es äußerst wichtig zu verstehen, in wie weit diese landwirtschaftlichen Systeme die globalen N und C Kreisläufe beeinflussen und zu den globalen Treibhausgasemissionen beitragen. Im Gegensatz zu den landwirtschaftlichen Systemen der temperaten Klimazonen ist über N und C Spurengasemissionen aus bewässerter Landwirtschaft in ariden und semiariden Gebieten nur sehr wenig bekannt. Um einen wesentlichen Beitrag zur Schließung dieser Forschungsdefizite zu leisten, konzentrierte sich diese Studie auf den Austausch von strahlungsaktiven Spurengasen zwischen Biosphäre und Atmosphäre und die hiermit assoziierten mikrobiellen N Umsetzungsprozesse in den Böden der bewässerten landwirtschaftlichen Systeme im Aralsee-Becken (ASB) von Usbekistan. Dafür wurde über einen Zeitraum von zwei Jahren in verschiedenen einjährigen und mehrjährigen Landnutzungssystemen die Emissionen der Treibhausgase Lachgas (N2O) und Methan (CH4) untersucht. Ausgewählt wurden Landnutzungsysteme die typisch für das Untersuchungsgebiet Khorezm, in West-Usbekistan, sind: bewässerter Baumwoll-, Winter Weizen- und Reisanbau sowie eine Pappel-Plantage und der natürliche „Tugai“ Auenwald entlang des Amu Darya Flusses. Es konnte festgestellt werden, dass der bewässerte Landbau im ASB insbesondere aufgrund von hohen N2O Emissionen aus dem Baumwoll- und Weizenanbau eine maßgebliche Quelle von Treibhausgasen darstellt. In den einjährigen Anbausystemen wurden mittlere N2O Emissionsraten zwischen 10 und 150 µg N2O N m-2h-1 festgestellt, wobei die höchsten Emissionen in Baumwollfeldern gemessen wurden. Über die gesamte Saison wurden die N2O Emissionen hauptsächlich von Düngung und Bewässerung beeinflusst. Dabei traten extrem hohe N2O Emissionen (bis zu 3000 µg N2O-N m-2 h-1) auf, wenn mineralischer N-Dünger direkt vor der Bewässerung appliziert wurde. Diese „Emissionsspitzen“ hatten einen Anteil von 80-95% an den Gesamtemissionen von N2O bezogen auf die Vegetationsperiode von Baumwolle und Weizen. Insgesamt variierten die N2O Emissionen über eine Saison von 0,5 bis 6,5 kg N2O-N ha-1. In der ungedüngten Pappel-Plantage wurden über den gesamten Messzeitraum hohe N2O Emissionen (30 µg N2O N m-2h-1) gemessen, wohingegen in dem Tugai Wald lediglich äußerst kleine Flüsse von N2O (2O N m-2h-1) festgestellt wurden. Bedeutende CH4 Emissionen traten nur in den gefluteten Reisfeldern auf, mit einer durchschnittlichen Flussrate von 32 mg CH4 m-2d-1 und einer Gesamtemission über die Vegetationsperiode von 35,2 kg CH4 ha-1. Das Treibhauspotenzial der N2O und CH4 Flüsse, dargestellt als CO2-Äquivalent, war am höchsten für den Reis- und Baumwollanbau, wobei auf den verschiedenen Messflächen die Gesamtemission einer Saison von 500 bis zu 3000 kg CO2 eq.ha-1 variierte. Für eine zweijährige Rotation von Baumwolle-Weizen und Reis, wie sie typisch für das Untersuchungsgebiet ist, konnte ein durchschnittliches Treibhauspotenzial von 2500 kg CO2 eq.ha-1 Jahr-1ermittelt werden. Zusätzlich wurden im Labor Inkubationsversuche an intakten Bodensäulen durchgeführt um die gasförmigen Stickstoffverluste, bestehend aus NO, N2O, und N2, der gedüngten und bewässerten Anbausysteme des ASB zu erfassen. Ammoniumnitrat Dünger wurde zusammen mit Wasser auf die Bodensäulen appliziert, um den Einfluss von gleichzeitiger Düngung und Bewässerung zu simulieren. Es konnte gezeigt werden, dass nach synchroner Düngung und Bewässerung Denitrifikation der vorherrschende Prozess in den Böden ist, und dass der größte Teil des Nitrats vollständig zu molekularem Stickstoff (N2) denitrifiziert wird. Aufgrund dieser Ergebnisse war es möglich für Baumwolle die Größenordnung der gasförmigen N Verluste von den verschiedenen Messflächen abzuschätzen. Demnach wurden von den einzelnen Baumwollfeldern zwischen 24±9 und 175±65 kg-N ha-1Saison-1 als N2 emittiert, während nur geringe Mengen von NO freigesetzt wurden (zwischen 0,1 und 0,7 kg-N ha-1 Saison-1). Diese Studie konnte somit zeigen, dass unter den gegenwärtigen landwirtschaftlichen Methoden im ASB, erhebliche Mengen von Stickstoff durch Denitrifikation als N2 an die Atmosphäre abgegeben werden. Ferner erlaubte das experimentelle Design dieser Studie Möglichkeiten einer Reduktion des Ausstoßes von Treibhausgasen aus diesen Anbausystemen abzuschätzen. Abschließend kann festgestellt werden, dass durch (i) eine Optimierung der Dünge- und Bewässerungsmethoden und (ii) einen Wechsel von einjährigen Feldfrüchten auf mehrjährige Baumplantagen, insbesondere auf unrentablen, marginalen Boden, das Treibhauspotential dieses landwirtschaftlichen Produktionssystems wesentlich reduziert werden kann.</p
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe 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
Nitrous oxide emissions from fertilized, irrigated cotton (Gossypium hirsutum L.) in the Aral Sea Basin, Uzbekistan : influence of nitrogen applications and irrigation practices
No full textNitrous oxide emissions were monitored at three sites over a 2-year period in irrigated cotton fields in Khorezm, Uzbekistan, a region located in the arid deserts of the Aral Sea Basin. The fields were managed using different fertilizer management strategies and irrigation water regimes. N2O emissions varied widely between years, within 1 year throughout the vegetation season, and between the sites. The amount of irrigation water applied, the amount and type of N fertilizer used, and topsoil temperature had the greatest effect on these emissions.\ud
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Very high N2O emissions of up to 3000 μg N2O-N m−2 h−1 were measured in periods following N-fertilizer application in combination with irrigation events. These “emission pulses” accounted for 80–95% of the total N2O emissions between April and September and varied from 0.9 to 6.5 kg N2O-N ha−1.. Emission factors (EF), uncorrected for background emission, ranged from 0.4% to 2.6% of total N applied, corresponding to an average EF of 1.48% of applied N fertilizer lost as N2O-N. This is in line with the default global average value of 1.25% of applied N used in calculations of N2O emissions by the Intergovernmental Panel on Climate Change.\ud
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During the emission pulses, which were triggered by high soil moisture and high availability of mineral N, a clear diurnal pattern of N2O emissions was observed, driven by daily changes in topsoil temperature. For these periods, air sampling from 8:00 to 10:00 and from 18:00 to 20:00 was found to best represent the mean daily N2O flux rates. The wet topsoil conditions caused by irrigation favored the production of N2O from NO3− fertilizers, but not from NH4+ fertilizers, thus indicating that denitrification was the main process causing N2O emissions. It is therefore argued that there is scope for reducing N2O emission from irrigated cotton production; i.e. through the exclusive use of NH4+ fertilizers. Advanced application and irrigation techniques such as subsurface fertilizer application, drip irrigation and fertigation may also minimize N2O emission from this regionally dominant agro-ecosystem
Variations on the Author
No full text“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
Appropriate Similarity Measures for Author Cocitation Analysis
No full textWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Supplement to Letter on High N2O Emissions from Rice
No full textThis document comprises supplementary information to a Letter to the Editor published by Proceedings of the National Academy of Sciences of the United States of America (PNAS). The letter and the supplement have been written by 17 authors from the International Rice Research Institute as well as other research institutes and academe. The rationale of the letter and this supplement is to clarify the author’s position on a recent PNAS article that reports very high emissions of nitrous oxide from rice fields. Collectively, these two documents elaborate on several concerns regarding some generalizations of this PNAS article that are deemed unfounded and speculative
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