[PDF] Nitrogen Cycling In Permeable Sediments eBook

Author : Morvarid Azizian
Publisher :
Page : 285 pages
File Size : 41,85 MB
Release : 2017
Category :
ISBN : 9780355414042

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Bioavailable forms of nitrogen, such as nitrate, are necessary for aquatic ecosystem productivity. Excess nitrate in aquatic systems, however, can adversely affect ecosystems and degrade both surface water and groundwater. Some of this excess nitrate can be removed in the sediments that line the bottom of rivers and coastal waters, through the exchange of water between surface water and groundwater (known as hyporheic exchange).Several process-based models have been proposed for estimating nitrate removal in aquatic systems but these (1) do not consider the multiscale nature of hyporheic exchange flows; (2) rely on simplified conceptualizations of mixing within streambed sediments (e.g., a well-mixed box); (3) neglect important steps in the N-cycle (e.g., nitrification and ammonification); and/or (4) adopt pseudo-first-order kinetic descriptions of denitrification. On the other hand, a number of empirical correlations have been published based on in-stream measurements of nitrate uptake using reach-scale stable isotope tracer experiments. While these correlations are noteworthy in many respects, they do not account for physical processes known to play an important role in nutrient processing, such as the exchange of water between a stream, sediments, and groundwater.In this thesis, I develop and test a simple and scalable process-based model for estimating the nitrate uptake velocity that addresses the limitations identified above. In particular, my model accounts for: (1) hyporheic exchange at multiple scales together with ambient groundwater flow; (2) the broad residence time distributions characteristic of hyporheic exchange; (3) key biogeochemical reactions associated with N-cycling, including respiration, ammonification, nitrification, and denitrification; and (4) the nonlinear nature of the pertinent biogeochemical reaction rates, including Monod kinetics for aerobic respiration and denitrification, and second-order kinetics for nitrification. Using this modeling framework I systematically evaluate primary controls on stream N-cycling and evaluate how multi-scale and regional factors are likely to affect this process. I also demonstrate how my model predictions compare with previously published reach-scale measurements of nitrate removal, develop scaling relationships by which my process-based model can be applicable to larger scales (i.e., watershed and regional scales), and provide some mechanistic explanations for previous observations.

Author : Alexandra Mina Fernandes Rao
Publisher :
Page : 249 pages
File Size : 45,66 MB
Release : 2006
Category :
ISBN :

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The results obtained reveal that the benthic and pelagic sub-systems on continental shelves are more dynamically coupled than previously thought because of the rapid biogeochemical reactions and porewater transport rates that characterize most of the shelf seafloor. These findings reveal that a better mechanistic understanding of benthic carbon and nutrient cycling at continental margins is required to advance understanding of marine ecosystems and global biogeochemistry.

Author : T. Henry Blackburn
Publisher :
Page : 488 pages
File Size : 49,42 MB
Release : 1988-03-21
Category : Science
ISBN :

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Nitrogen discharge to the coastal environment has been increasing, posing the threat of accelerated eutrofication. Considerable research has been conducted in recent years to examine the impact of the nitrogen loading in coastal ecosystems. Based on proceedings from a SCOPE symposium held at the University of Aarhus in July of 1985, this volume covers a variety of up-to-date developments in research on nitrogen cycling in coastal marine environments. Topics include the role of nitrogen in algal productivity, regeneration of nutrients in the water column and the sediments, and the flow of nitrogen in coastal ecosystems.

Author : Angelos K. Hannides
Publisher :
Page : 878 pages
File Size : 21,64 MB
Release : 2008
Category : Biogeochemical cycles
ISBN :

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This dissertation explores two recently recognized pathways of organic matter supply to marine sediments and their impact on sedimentary biogeochemistry: The entrapment and decomposition of particulate organic matter in permeable sediments and the resulting nutrient dynamics were investigated with a specially designed experimental laboratory microcosm that allows permeable sediment incubation under controlled physical forcing. Microcosm generated enhanced solute transport rates were roughly proportional to sediment column permeability. Comparison with field observations revealed that the enhanced transport rates induced by the experimental conditions were lower than those observed in the field, and this was also reflected in the enhanced buildup of pore water nutrients relative to concentrations in field sediments. Particulate organic matter and nutrient enrichment experiments, conducted with the microcosm, demonstrated the rapid uptake of both particles and solutes by the permeable sediment column under physical forcing, the rapid decomposition of the removed particles, and the lack of regenerated nutrient build-up. Nutrient enrichment experiments with sediment plug chambers demonstrated the rapid uptake and retention of nutrients by surface permeable sediments, as well as the loss of nitrogen though benthic denitrification in sediment grain microzones. The processing of large organic matter packages on the deepsea floor by deep-sea communities was investigated using natural and experimental whale, kelp and wood falls in the California Borderland Basins region, focusing on their quality and its impact on sedimentary organic enrichment and redox shifts. Labile organic material generated by the processing of whale falls and kelp falls generated substantial pore water sulfide levels in impacted sediment, despite a small organic enrichment signal. In contrast, wood derived material input did not result in sulfidic conditions, despite generating very high organic enrichment in impacted sediments, due to its refractory nature. A constructed model incorporating metazoan processing and dispersal of fragmented material revealed that whale fall processing is characterized by higher rates of tissue removal, organic carbon release, and carbon sedimentation rates than wood fall processing. However, organic carbon release rates, when normalized to fall weight, are similar between the two types of falls, due to differences in their carbon content and in absorption rates during metazoan ingestion.

Author : Robert W. Howarth
Publisher : Springer Science & Business Media
Page : 300 pages
File Size : 50,42 MB
Release : 2012-12-06
Category : Science
ISBN : 9400917767

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Human activity has dramatically altered the global nitrogen cycle in recent decades. These changes are not evenly distributed around the world; rather, they are greatest in regions of significant industrial and agricultural activity, as the synthesis and use of inorganic fertilizers, cultivation of legumes, burning of fossil fuels, and the simple act of concentrating humans and animals in dense populations all lead to the release of excess, reactive forms of nitrogen into the environment. In part because reactive nitrogen is frequently a limiting nutrient in many terrestrial and aquatic systems, an excess can lead to a variety of adverse effects on both environmental and human health. The North Atlantic Ocean and its contributing watersheds constitute a region which has seen perhaps the greatest increase in anthropogenically-derived nitrogen. In May of 1994, the International Scope Nitrogen Project, with funding from the Andrew Mellon Foundation, the United Nations Environment Program, and the World Meteorological Organization, sponsored a workshop held on Block Island, RI, USA, entitled `Nitrogen Dynamics of the North Atlantic Basin'. More than 50 scientists from 12 different countries convened with a unique set of goals: an integrated and comprehensive estimate of the current nitrogen cycle of the ocean, coastal systems, and contributing watersheds of the North Atlantic region; an analysis of human-induced changes to those cycles; and an assessment of the current and future effects of human-induced changes to nitrogen cycling throughout the globe.