1,720,988 research outputs found
Seasonal variation of radial oxygen loss in Vallisneria spiralis L.: An adaptive response to sediment redox?
No full textIn temperate shallow aquatic bodies large seasonal variations in water temperature result in a wide range of benthic respiration rates which are coupled to changes of pore water redox. To cope with such sediment modifications, we hypothesize that rooted macrophytes vary the oxygen amount released by roots. To this purpose, we reinterpreted published data on seasonal oxygen and inorganic carbon fluxes measured in vegetated (Vallisneria spiralis L.) sediments by combining them with the outcomes from laboratory incubations of apical tips and intact plants.Results suggest that V. spiralis transfers progressively higher amounts of oxygen to roots in the shift winter-summer. Maximum radial oxygen loss occurs in early autumn and probably overlaps with the lowest sediment redox. At the end of the summer, the exhaustion of energy yielding electron acceptor pools is in fact coupled to input of labile organic matter from senescent meadows, further exacerbating the demand of oxidized compounds to support degradation processes. The oxygen released by roots measured in hydroponic conditions corresponds to ∼7% of the plant gross production in the light; a small amount of oxygen is leaked also in darkness. We speculate that the oxygen injected in the pore water by a V. spiralis meadow can significantly affect the sediment biogeochemistry of eutrophic sites, representing up to ∼20% of the daily benthic oxygen consumption. © 2012 Elsevier B.V
Seasonal regulation of nitrification in a rooted macrophyte (Vallisneria spiralis L.) meadow under eutrophic conditions
No full textVariable oxygen release from the root of macrophytes growing in ammonium-rich organic substrates can stimulate the process of nitrification. To verify this hypothesis, we performed seasonal measurements of potential nitrification activity in sediments with and without the perennial submersed plant Vallisneria spiralis L. (Hydrocharitaceae). Pore water and sediment features were simultaneously considered in order to provide insights into the regulation of the process. Results demonstrated a significant effect of season and plant presence on potential nitrification activity, with higher rates in winter and lower rates in summer. Vegetated sediment displayed lower pore water ammonium, but always higher potential nitrification activity compared to the unvegetated substrate, regardless the season. Nitrification activity was strongly correlated with pore water redox status, which were affected by both season and plant presence. Along its annual cycle V. spiralis promoted more oxidized condit..
Effects of increasing organic matter loads on pore water features of vegetated (Vallisneria spiralis L.) and plant-free sediments
No full textThe effects of organic enrichment on pore water chemistry of bare and Vallisneria spiralis L. colonized sediments were investigated. Substrates of three organic levels were created by adding different amounts of powdered fish feed (0, 5 and 10g/l of sediment, respectively) to homogenized sediment and incubated with and without plants. Redox potential (Eh), reduced compounds (CH 4, Fe 2+, Mn 2+) and nutrients (PO 43-, NH 4+) were analyzed at time zero and after 6, 10, 13 and 17 days. In control microcosms V. spiralis sediments displayed significantly higher Eh and lower CH 4, Fe 2+, Mn 2+, PO 43- and NH 4+ concentrations than bare ones. In organic enriched microcosms methanogenesis became the main degradation pathway when other electron acceptor pools were depleted. However, lower levels of interstitial Fe 2+, Mn 2+ and PO 43- were found in vegetated sediments compared to bare ones and this difference was maintained during the whole experimental time. Root oxygen release in the rizosphere seemed to be the main responsible of this outcome, as also suggested by the nitrification potential assay, indicating the maintenance of oxic microniches. V. spiralis can act as an engineer species in urban, organic impacted sediments due to its high tolerance against reduced conditions, which makes this macrophyte an interesting option in aquatic ecosystems restoration programs. © 2012 Elsevier B.V
Upscaling nitrogen removal processes in fluvial wetlands and irrigation canals in a patchy agricultural watershed
No full textThe evaluation of nitrogen (N) removal in aquatic ecosystems within human exploited watersheds may allow the setting of upper limits of N use in agriculture. However, such an evaluation is complex owing to the small scale heterogeneity of aquatic ecosystems, including dominant vegetation and inherent N-related processes. In this work, microbial denitrification and primary producer assimilation were estimated in fluvial wetlands and irrigation canals of the Mincio River watershed (~ 850 km2, Italy), and scaled-up to the whole watershed using GIS and remote-sensing techniques. Denitrification in the fluvial wetland area which covered only ~ 3% of the watershed was estimated to permanently remove 80% of the excess N, defined as N soil surplus (~ 5500 t N y−1) minus N river export (~ 1300 t N y−1). Adding the estimated N uptake by primary producers in the wetland system, approximately all the N excess produced in adjacent agricultural land of the Mincio watershed can be retained or removed by internal processes. In contrast, the canal network had a linear extent of over 1770 km but its estimated denitrification capacity was comparatively much smaller (up to 7% of the excess N). Combinations of N-budgeting at the whole basin scale, experimental data, GIS and remote sensing techniques offer the possibility to analyze N retention capacity in heterogeneous aquatic environments. The application of such approach to the Mincio watershed stresses the functional relevance of even small wetland areas in agricultural settings
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe 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
Soil Denitrification, the Missing Piece in the Puzzle of Nitrogen Budget in Lowland Agricultural Basins
Full text linkDenitrification is a key process buffering the environmental impacts of agricultural nitrate loads but, at present, remains the least understood and poorly quantified sink in nitrogen budgets at the watershed scale. The present work deals with a comprehensive and detailed analysis of nitrogen sources and sinks in the Burana–Volano–Navigabile basin, the southernmost portion of the Po River valley (Northern Italy), an intensively cultivated (> 85% of basin surface) low-lying landscape. Agricultural census data, extensive monitoring of surface–groundwater interactions, and laboratory experiments targeting N fluxes and pools were combined to provide reliable estimates of soil denitrification at the basin scale. In the agricultural soils of the basin, nitrogen inputs exceeded outputs by nearly 40% (~ 80 kg N ha−1 year−1), but this condition of potential N excess did not translate into widespread nitrate pollution. The general scarcity of inorganic nitrogen species in groundwater and soils indicated limited leakage and storage. Multiple pieces of evidence supported that soil denitrification was the process that needed to be introduced in the budget to explain the fate of the missing nitrogen. Denitrification was likely boosted in the soils of the studied basin, prone to waterlogged conditions and consequently oxygen-limited, owing to peculiar features such as fine texture, low hydraulic conductivity, and shallow water table. The present study highlighted the substantial contribution of soil denitrification to balancing nitrogen inputs and outputs in agricultural lowland basins, a paramount ecosystem function preventing eutrophication phenomena
Souces, transformations and sinks of nitrogen in a heavily impacted watershed (Oglio River, Northern Italy).
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