2,670 research outputs found
Nitrogen deposition does not enhance Sphagnum decomposition
Long-term additions of nitrogen (N) to peatlands have altered bryophyte growth, species dominance, N content in peat and peat water, and often resulted in enhanced Sphagnum decomposition rate. However, these results have mainly been derived from experiments in which N was applied as ammonium nitrate (NH4NO3), neglecting the fact that in polluted areas, wet deposition may be dominated either by NO3- or NH4+. We studied effects of elevated wet deposition of NO3- vs. NH4+ alone (8 or 56 kg N ha(-1) yr(-1) over and above the background of 8 kg N ha(-1) yr(-1) for 5 to 11 years) or combined with phosphorus (P) and potassium (K) on Sphagnum quality for decomposers, mass loss, and associated changes in hummock pore water in an ombrotrophic bog (Whim). Adding N, especially as NH4+, increased N concentration in Sphagnum, but did not enhance mass loss from Sphagnum. Mass loss seemed to depend mainly on moss species and climatic factors. Only high applications of N affected hummock pore water chemistry, which varied considerably over time. Overall, C and N cycling in this N treated bog appeared to be decoupled. We conclude that moss species, seasonal and annual variation in climatic factors, direct negative effects of N (NH4+ toxicity) on Sphagnum production, and indirect effects (increase in pH and changes in plant species dominance under elevated NO3- alone and with PK) drive Sphagnum decomposition and hummock C and N dynamics at Whim. (C) 2016 Elsevier B.V. All rights reserved.Peer reviewe
Reconstructing a model of quintessential inflation
We present an explicit cosmological model where inflation and dark energy
both could arise from the dynamics of the same scalar field. We present our
discussion in the framework where the inflaton field attains a nearly
constant velocity (where
is the e-folding time) during inflation. We show that the model
with and can easily satisfy inflationary constraints,
including the spectral index of scalar fluctuations (),
tensor-to-scalar ratio () and also the bound imposed on
during the nucleosynthesis epoch (). In our
construction, the scalar field potential always scales proportionally to the
square of the Hubble expansion rate. One may thereby account for the two vastly
different energy scales associated with the Hubble parameters at early and late
epochs. The inflaton energy could also produce an observationally significant
effective dark energy at a late epoch without violating local gravity tests.Comment: 18 pages, 7 figures; added refs, published versio
Decentralised Learning MACs for Collision-free Access in WLANs
By combining the features of CSMA and TDMA, fully decentralised WLAN MAC
schemes have recently been proposed that converge to collision-free schedules.
In this paper we describe a MAC with optimal long-run throughput that is almost
decentralised. We then design two \changed{schemes} that are practically
realisable, decentralised approximations of this optimal scheme and operate
with different amounts of sensing information. We achieve this by (1)
introducing learning algorithms that can substantially speed up convergence to
collision free operation; (2) developing a decentralised schedule length
adaptation scheme that provides long-run fair (uniform) access to the medium
while maintaining collision-free access for arbitrary numbers of stations
Stress corrosion in titanium alloys and other metallic materials
Multiple physical and chemical techniques including mass spectroscopy, atomic absorption spectroscopy, gas chromatography, electron microscopy, optical microscopy, electronic spectroscopy for chemical analysis (ESCA), infrared spectroscopy, nuclear magnetic resonance (NMR), X-ray analysis, conductivity, and isotopic labeling were used in investigating the atomic interactions between organic environments and titanium and titanium oxide surfaces. Key anhydrous environments studied included alcohols, which contain hydrogen; carbon tetrachloride, which does not contain hydrogen; and mixtures of alcohols and halocarbons. Effects of dissolved salts in alcohols were also studied. This program emphasized experiments designed to delineate the conditions necessary rather than sufficient for initiation processes and for propagation processes in Ti SCC
Modeling Kelvin wave cascades in superfluid helium
We study two different types of simplified models for Kelvin wave turbulence on quantized vortex lines in superfluids near zero temperature. Our first model is obtained from a truncated expansion of the Local Induction Approximation (Truncated-LIA) and it is shown to possess the same scalings and the essential behaviour as the full Biot-Savart model, being much simpler than the later and, therefore, more amenable to theoretical and numerical investigations. The Truncated-LIA model supports six-wave interactions and dual cascades, which are clearly demonstrated via the direct numerical simulation of this model in the present paper. In particular, our simulations confirm presence of the weak turbulence regime and the theoretically predicted spectra for the direct energy cascade and the inverse wave action cascade. The second type of model we study, the Differential Approximation Model (DAM), takes a further drastic simplification by assuming locality of interactions in k-space via using a differential closure that preserves the main scalings of the Kelvin wave dynamics. DAMs are even more amenable to study and they form a useful tool by providing simple analytical solutions in the cases when extra physical effects are present, e.g. forcing by reconnections, friction dissipation and phonon radiation. We study these models numerically and test their theoretical predictions, in particular the formation of the stationary spectra, and closeness of numerics for the higher-order DAM to the analytical predictions for the lower-order DAM
Does enhanced nitrogen deposition represent a threat to Sphagnum and thus the sustainability of Scottish peatlands?
Nutrient limited ombrotrophic bogs and peatlands support high conservation valued ecosystems, potentially susceptible to current elevated levels of reactive nitrogen (N) deposition. Here, we present the effects and consequences of different N forms, wet, dry, reduced and oxidised N on the functioning of a bog moss, Sphagnum capillifolium. Sphagnum mosses maintain the acid, low nutrient conditions, crucial for the sustainability of peat lands, where productivity must exceed decomposition. Dry deposited ammonia substantially elevated shoot N status, which led to tissue breakdown loss of function and death in S. capillifolium. Wet deposited nitrate and ammonium also negatively affected S. capillifolium, significantly reducing shoot extension and cover and significantly elevating N status. These effects occurred over 5 years and were significant even at the lowest reduced N dose, 8 kg N ha-1 y1 (background = 8-10 kg N ha-1 y-1), highlighting the threat N poses for the effective functioning of bog ecosystems
On compatibility of string effective action with an accelerating universe
In this paper, we fully investigate the cosmological effects of the moduli
dependent one-loop corrections to the gravitational couplings of the string
effective action to explain the cosmic acceleration problem in early (and/or
late) universe. These corrections comprise a Gauss-Bonnet (GB) invariant
multiplied by universal non-trivial functions of the common modulus
and the dilaton . The model exhibits several features of cosmological
interest, including the transition between deceleration and acceleration
phases. By considering some phenomenologically motivated ansatzs for one of the
scalars and/or the scale factor (of the universe), we also construct a number
of interesting inflationary potentials. In all examples under consideration, we
find that the model leads only to a standard inflation () when the
numerical coefficient associated with modulus-GB coupling is positive,
while the model can lead also to a non-standard inflation (), if
is negative. In the absence of (or trivial) coupling between the GB term and
the scalars, there is no crossing between the phases, while
this is possible with non-trivial GB couplings, even for constant dilaton phase
of the standard picture. Within our model, after a sufficient amount of e-folds
of expansion, the rolling of both fields and can be small. In
turn, any possible violation of equivalence principle or deviations from the
standard general relativity may be small enough to easily satisfy all
astrophysical and cosmological constraints.Comment: 30 pages, 8 figures; v2 significant changes in notations, appendix
and refs added; v3 significant revisions, refs added; v4 appendix extended,
new refs, published versio
Shot Noise in Digital Holography
We discuss on noise in heterodyne holography in an off-axis configuration. We
show that, for a weak signal, the noise is dominated by the shot noise on the
reference beam. This noise corresponds to an equivalent noise on the signal
beam of one photoelectron per pixel, for the whole sequence of images used to
build the digital hologram
Attributing scientific and technical progress: the case of holography
Holography, the three-dimensional imaging technology, was portrayed widely as a paradigm
of progress during its decade of explosive expansion 1964–73, and during its subsequent
consolidation for commercial and artistic uses up to the mid 1980s. An unusually
seductive and prolific subject, holography successively spawned scientific insights, putative
applications and new constituencies of practitioners and consumers. Waves of forecasts,
associated with different sponsors and user communities, cast holography as a field on the
verge of success—but with the dimensions of success repeatedly refashioned. This retargeting
of the subject represented a degree of cynical marketeering, but was underpinned by
implicit confidence in philosophical positivism and faith in technological progressivism.
Each of its communities defined success in terms of expansion, and anticipated continual
progressive increase. This paper discusses the contrasting definitions of progress in holography,
and how they were fashioned in changing contexts. Focusing equally on reputed ‘failures’ of some aspects of the subject, it explores the varied attributes by which success and failure were linked with progress by different technical communities. This important case illuminates the peculiar post-World War II environment that melded the military, commercial and popular engagement with scientific and technological subjects, and the
competing criteria by which they assessed the products of science
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