10,233 research outputs found
Stable Crank-Nicolson Discretisation for Incompressible Miscible Displacement Problems of Low Regularity
In this article we study the numerical approximation of incompressible
miscible displacement problems with a linearised Crank-Nicolson time
discretisation, combined with a mixed finite element and discontinuous Galerkin
method. At the heart of the analysis is the proof of convergence under low
regularity requirements. Numerical experiments demonstrate that the proposed
method exhibits second-order convergence for smooth and robustness for rough
problems.Comment: Enumath 200
Estimating ConsumersĂŻÂŸâ Valuation of Organic and Cosmetically Damaged Apples
A mixed probit model was applied to survey data to analyze consumersĂŻÂŸâ willingness to buy apples with cosmetic damage caused by the sooty blotch and flyspeck (SBFS) disease complex. The analysis finds consumers will pay a premium for organic production methods and for apples with low amounts of SBFS damage. Behavioral variables such as experience in growing fruit significantly affect the willingness to buy apples of different damage levels. ConsumersĂŻÂŸâ tolerance of very blemished apples is limited and they trade off production technology attributes for cosmetic appearance. Better understanding of this trade-off is important to organic producersĂŻÂŸâ decisions about disease control.
Estimating Consumers\u27 Valuation of Organic and Cosmetically Damaged Apples
The sooty blotch and flyspeck (SBFS) disease complex causes cosmetic damage but does not affect the safety or eating quality of apples. Treatment for disease is more difficult and costly for organic producers, and consumers\u27 willingness to pay for organic apples needs to be considered in growers\u27 choice of production technologies. A mixed probit model was applied to survey data to evaluate consumers\u27 willingness to buy apples. The results show consumers will pay a premium for organic production methods and for apples with low amounts of SBFS damage. Behavioral variables such as experience growing fruit significantly affect the willingness to buy apples of different damage levels. Consumers have limited tolerance of very blemished apples and trade off production technology attributes for cosmetic appearance. Better understanding of this tradeoff can improve organic producers\u27 decisions about disease control
Ultrastructure and complex polar architecture of the human pathogen Campylobacter jejuni
Campylobacter jejuni is one of the most successful food-borne human pathogens. Here we use electron cryotomography to explore the ultrastructure of C. jejuni cells in logarithmically growing cultures. This provides the first look at this pathogen in a near-native state at macromolecular resolution (âŒ5 nm). We find a surprisingly complex polar architecture that includes ribosome exclusion zones, polyphosphate storage granules, extensive collar-shaped chemoreceptor arrays, and elaborate flagellar motors
Unrecognized controls on microbial functioning in Blue Carbon ecosystems: The role of mineral enzyme stabilization and allochthonous substrate supply
Tidal wetlands are effective carbon sinks, mitigating climate change through the longâterm removal of atmospheric CO2. Studies along surfaceâelevation and thus floodingâfrequency gradients in tidal wetlands are often used to understand the effects of accelerated seaâlevel rise on carbon sequestration, a process that is primarily determined by the balance of primary production and microbial decomposition. It has often been hypothesized that rates of microbial decomposition would increase with elevation and associated increases in soil oxygen availability; however, previous studies yield a wide range of outcomes and equivocal results. Our mechanistic understanding of the elevationâdecomposition relationship is limited because most effort has been devoted to understanding the terminal steps of the decomposition process. A few studies assessed microbial exoâenzyme activities (EEAs) as initial and rateâlimiting steps that often reveal important insight into microbial energy and nutrient constraints. The present study assessed EEAs and microbial abundance along a coastal ecotone stretching a flooding gradient from tidal flat to high marsh in the European Wadden Sea. We found that stabilization of exoâenzymes to mineral sediments leads to high specific EEAs at low substrate concentrations in frequently flooded, sedimentârich zones of the studied ecotone. We argue that the high background activity of a mineralâassociated enzyme pool provides a stable decomposition matrix in highly dynamic, frequently flooded zones. Furthermore, we demonstrate that microbial communities are less nutrient limited in frequently flooded zones, where inputs of nutrientârich marine organic matter are higher. This was reflected in both increasing exoâenzymatic carbon versus nutrient acquisition and decreasing fungal versus bacterial abundance with increasing flooding frequency. Our findings thereby suggest two previously unrecognized mechanisms that may contribute to stimulated microbial activity despite decreasing oxygen availability in response to accelerated seaâlevel rise
Assessing the longâterm carbonâsequestration potential of the semiânatural salt marshes in the European Wadden Sea
Salt marshes and other blue carbon ecosystems have been increasingly recognized for their carbon (C)âsink function. Yet, an improved assessment of organic carbon (OC) stocks and Câsequestration rates is still required to include blue C in Câcrediting programs. Particularly, factors inducing variability in the permanence of sequestration and allochthonous contributions to soil OC stocks require an improved understanding. This study evaluates the potential for longâterm C sequestration in the semiânatural salt marshes of the European Wadden Sea (WS), conducting deep (1.3 m) downâcore OCâdensity assessments in sites with known site histories and accretion records. Because these young marshes have developed from tidalâflat ecosystems and have undergone rapid succession during the last 80â120 yr, the identification of different ecosystem stages downâcore was crucial to interpret possible changes in OC density. This was conducted based on the downâcore distribution of different foraminiferal taxa and grain sizes. Comparisons of historic and recent accretion rates were conducted to understand possible effects of accretion rate on downâcore changes in OC density. ÎŽ13C in OC was used to assess the origin of accumulated OC (autochthonous vs. allochthonous sources). We show that large amounts of shortâterm accumulated OC are lost downâcore in the wellâaerated marsh soils of the WS region and thus emphasize the importance of deep sampling to avoid overestimation of C sequestration. Despite steep declines in OCâdensity downâcore, minimum values of OC density in the saltâmarsh soils were considerably higher than those of the former tidalâflat sediments that the marshes were converted from, illustrating the greater Câsequestration potential of the vegetated ecosystem. However, our data also suggest that marineâderived allochthonous OC makes up a large fraction of the effectively, longâterm preserved OC stock, whereas atmospheric CO2 removal by marsh vegetation contributes relatively little. The implication of this finding for Câcrediting approaches in blue C ecosystems has yet to be clarified
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