2,533 research outputs found
Plant communities affect arbuscular mycorrhizal fungal diversity and community composition in grassland microcosms
The diversity of arbuscular mycorrhizal (AM) fungi was investigated in an unfertilized limestone grassland soil supporting different synthesized vascular plant assemblages that had developed for 3 yr. The experimental treatments comprised: bare soil; monocultures of the nonmycotrophic sedge Carex flacca; monocultures of the mycotrophic grass Festuca ovina; and a species-rich mixture of four forbs, four grasses and four sedges. The diversity of AM fungi was analysed in roots of Plantago lanceolata bioassay seedlings using terminal-restriction fragment length polymorphism (T-RFLP). The extent of AM colonization, shoot biomass and nitrogen and phosphorus concentrations were also measured. The AM diversity was affected significantly by the floristic composition of the microcosms and shoot phosphorus concentration was positively correlated with AM diversity. The diversity of AM fungi in P. lanceolata decreased in the order: bare soil > C. flacca > 12 species > F. ovina. The unexpectedly high diversity in the bare soil and sedge monoculture likely reflects differences in the modes of colonization and sources of inoculum in these treatments compared with the assemblages containing established AM-compatible plants
The soil microbial community alters patterns of selection on flowering time and fitness‐related traits in Ipomoea purpurea
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154384/1/ajb21426.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154384/2/ajb21426_am.pd
Effects of growing conditions and source habitat on plant traits and functional group definition
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72499/1/j.1365-2435.2001.00487.x.pd
On the idempotents of Hecke algebras
We give a new construction of primitive idempotents of the Hecke algebras
associated with the symmetric groups. The idempotents are found as evaluated
products of certain rational functions thus providing a new version of the
fusion procedure for the Hecke algebras. We show that the normalization factors
which occur in the procedure are related to the Ocneanu--Markov trace of the
idempotents.Comment: 11 page
Shearless bifurcations in particle transport for reversed shear tokamaks
Some internal transport barriers in tokamaks have been related to the
vicinity of extrema of the plasma equilibrium profiles. This effect is
numerically investigated by considering the guiding-center trajectories of
plasma particles undergoing ExB drift motion, considering that the electric
field has a stationary nonmonotonic radial profile and an electrostatic
fluctuation. In addition, the equilibrium configuration has a nonmonotonic
safety factor profile. The numerical integration of the equations of motion
yields a symplectic map with shearless barriers. By changing the parameters of
the safety factor profile, the appearance, and breakup of these shearless
curves are observed. The successive shearless curves breakup and recovering is
explained using concepts from bifurcation theory. We also present bifurcation
sequences associated to the creation of multiple shearless curves. Physical
consequences of scenarios with multiple shearless curves are discussed.Comment: 18 pages, 9 figures. Replacement improved the tex
Do Alterations in the Rate of Gastric Emptying after Injection Sclerotherapy for Oesophageal Varices Play any Role in the Development of Portal Hypertensive Gastropathy?
Bleeding from portal hypertensive gastropathy
(PHG) has been estimated to account for upto 30%
of all upper gastrointestinal haemorrhage in patients
with cirrhosis and portal hypertension. Although
portal hypertension seems to be an essential prerequisite,
the precise mechanisms responsible for
the development of PHG are unknown. The aim of
this study was to examine the role of injection sclerotherapy
of oesophageal varices in the development
of PHG. Gastric emptying was studied using a
radionuclide test meal with the emptying characteristics
of a slow liquid in 57 patients with cirrhosis
and/or portal hypertension (median age 53 yrs), of
whom 34 had received injection sclerotherapy for
their oesophageal varices and 20 normal healthy
volunteers (median age 42 yrs). As vagal damage is
associated with more rapid emptying of liquids,
despite hold up of solids, this technique might be
expected to demonstrate such damage if gastric
emptying was accelerated. The results indicated that
there was no difference in the rate of gastric
emptying between normal healthy volunteers and
portal hypertensive patients. However, patients who
had received injection sclerotherapy emptied their
stomachs faster than those who had not (p<0.05).
Furthermore, the speed of gastric emptying correlated
directly with the number of injections (r=0.41;
p=0.02) and the volume of sclerosant injected
(r=0.39; p=0.03). These observations suggest that
injection sclerotherapy for oesophageal varices
results in disturbances of gastric emptying that
may contribute to the pathogenesis of portal hypertensive
gastropathy
ExB drift particle transport in tokamaks
In tokamaks, modification of the plasma profiles can reduce plasma transport,
improving particle confinement. However, this improvement is still not
completely understood. In this work, we consider a drift wave test particle
model to investigate the influence of the electric and magnetic field profiles
on plasma transport. Test particle orbits subjected to ExB drift are
numerically integrated and their transport coefficient is obtained. We conclude
that sheared profiles reduce particle transport, even for high amplitude
perturbations. In particular, nonmonotonic electric and magnetic fields produce
shearless transport barriers, which are particularly resistant to perturbations
and reduce even more the transport coefficient.Comment: 10 pages, 9 figures, 1 table. Published in Brazilian Journal of
Physic
Management practices influence the competitive potential of weed communities and their value to biodiversity in South African vineyards.
Weeds have negative impacts on crop production but also play a role in sustaining biodiversity in agricultural landscapes. This trade‐off raises the question of whether it is possible to promote weed communities with low competitive potential but high value to biodiversity. Here, we explored how weed communities respond to different vineyard management practices in South Africa's Western Cape, aiming to identify whether any specific practices are associated with more beneficial weed communities. Eight weed community characteristics representative of abundance, diversity and functional composition were used as indicators of competitive potential and biodiversity value. We explored how these responded to farm management strategy (organic, low input or conventional) and weed management practices (herbicides, tillage, mowing or combinations of these) using ordination and mixed models. Mown sites were associated with weed communities of high biodiversity value, with higher weed cover in both winter and summer, higher diversity and more native weeds. Mowing also promoted shorter weeds than either tillage or herbicides, considered to be less competitive with grapevines. However, high summer weed cover may be problematic where competition for water is critical, in which case tillage offers a method to limit summer weed cover that did not adversely affect diversity or native weeds. In contrast, herbicide‐treated sites had characteristics indicative of a lower biodiversity value and higher potential for competitiveness with few native weeds, lower diversity and relatively tall, small‐seeded weeds. Mowing in winter combined with tillage in spring may thus optimise the biodiversity benefits and production costs of Western Cape vineyard weeds
Quantitative localized proton-promoted dissolution kinetics of calcite using scanning electrochemical microscopy (SECM)
Scanning electrochemical microscopy (SECM) has been used to determine quantitatively the kinetics of proton-promoted dissolution of the calcite (101̅4) cleavage surface (from natural “Iceland Spar”) at the microscopic scale. By working under conditions where the probe size is much less than the characteristic dislocation spacing (as revealed from etching), it has been possible to measure kinetics mainly in regions of the surface which are free from dislocations, for the first time. To clearly reveal the locations of measurements, studies focused on cleaved “mirror” surfaces, where one of the two faces produced by cleavage was etched freely to reveal defects intersecting the surface, while the other (mirror) face was etched locally (and quantitatively) using SECM to generate high proton fluxes with a 25 μm diameter Pt disk ultramicroelectrode (UME) positioned at a defined (known) distance from a crystal surface. The etch pits formed at various etch times were measured using white light interferometry to ascertain pit dimensions. To determine quantitative dissolution kinetics, a moving boundary finite element model was formulated in which experimental time-dependent pit expansion data formed the input for simulations, from which solution and interfacial concentrations of key chemical species, and interfacial fluxes, could then be determined and visualized. This novel analysis allowed the rate constant for proton attack on calcite, and the order of the reaction with respect to the interfacial proton concentration, to be determined unambiguously. The process was found to be first order in terms of interfacial proton concentration with a rate constant k = 6.3 (± 1.3) × 10–4 m s–1. Significantly, this value is similar to previous macroscopic rate measurements of calcite dissolution which averaged over large areas and many dislocation sites, and where such sites provided a continuous source of steps for dissolution. Since the local measurements reported herein are mainly made in regions without dislocations, this study demonstrates that dislocations and steps that arise from such sites are not needed for fast proton-promoted calcite dissolution. Other sites, such as point defects, which are naturally abundant in calcite, are likely to be key reaction sites
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