1,262 research outputs found
Phosphorus Transformations in an Oxisol under contrasting land-use systems: The role of the soil microbial biomass
It is generally assumed that phosphorus (P) availability for plant growth on highly weathered and P-deficient tropical soils may depend more on biologically mediated organic P (Po) turnover processes than on the release of adsorbed inorganic P (Pi). However, experimental evidence showing the linkages between Po, microbial activity, P cycling and soil P availability is scarce. To test whether land-use systems with higher soil Po are characterized by greater soil biological activity and increased P mineralization, we analyzed the partitioning of P among various organic and inorganic P fractions in soils of contrasting agricultural land-use systems and related it to biological soil properties. Isotopic labeling was used to obtain information on the turnover of P held in the microbial biomass. Soil samples were taken from grass-legume pasture (GL), continuous rice (CR) and native savanna (SAV) which served as reference. In agreement with estimated P budgets (+277, +70 and 0 kg P ha−1 for CR, GL and SAV, respectively), available P estimated using Bray-2 and resin extraction declined in the order CR > GL > SAV. Increases in Bray-2 and resin Pi were greater in CR than GL relative to total soil P increase. Organic P fractions were significantly less affected by P inputs than inorganic fractions, but were a more important sink in GL than CR soils. Extractable microbial P (Pchl) was slightly higher in GL (6.6 mg P kg−1) than SAV soils (5.4 mg P kg−1), and significantly lowest in CR (2.6 mg P kg−1). Two days after labeling the soil with carrier free 33P, 25, 10 and 2% of the added 33P were found in Pchl in GL, SAV and CR soils, respectively, suggesting a high and rapid microbial P turnover that was highest in GL soils. Indicators of P mineralization were higher in GL than CR soils, suggesting a greater transformation potential to render Po available. Legume-based pastures (GL) can be considered as an important land-use option as they stimulate P cycling. However, it remains to be investigated whether crops planted in pasture-crop rotations could benefit from the enhanced Po cycling in grass-legume soils. Furthermore, there is need to develop and test a direct method to quantify Po mineralization in these system
SrCu_2(BO_3)_2 - a Two Dimensional Spin Liquid
We study an extended Shastry-Sutherland model for SrCu_2(BO_3)_2 and analyze
the low lying parts of the energy spectrum by means of a perturbative unitary
transformation based on flow equations. The derivation of the 1-magnon
dispersion (elementary triplets) is discussed. Additionally, we give a
quantitative description (symmetries and energies) of bound states made from
two elementary triplets. Our high order results allow to fix the model
parameters for SrCu_2(BO_3)_2 precisely: J_1=6.16(10)meV, x:=J_2/J_1=0.603(3),
J_\perp=1.3(2)meV. To our knowledge this is the first quantitative treatment of
bound states in a true 2d model.Comment: 4 pages, 3 figures, Proceeding paper of the HFM2000 conference in
Waterloo, Canada, Jun 200
Thermodynamic Properties of Spin Ladders with Cyclic Exchange
By high temperature series expansion and exact complete diagonalization the
magnetic susceptibility chi(T) and the specific heat C(T) of a two-leg S=1/2
ladder with cyclic (4-spin) exchange are computed. Both methods yield
convincing results for not too small temperatures. We find that a small amount
of cyclic exchange influences the thermodynamical properties significantly. Our
results can serve as reliable basis for an efficient analysis of experimental
dataComment: 6 pages, 7 figure
Precision determination of the dpi -> NN transition strength at threshold
An unusual but effective way to determine at threshold the dpi -> NN
transition strength is to exploit the hadronic ground-state broadening in
pionic deuterium, accessible by x-ray spectroscopy. The broadening is dominated
by the true absorption channel dpi- -> nn, which is related to s-wave pion
production pp -> dpi+ by charge symmetry and detailed balance. Using the exotic
atom circumvents the problem of Coulomb corrections to the cross section as
necessary in the production experiments. Our dedicated measurement finds
(1171+23/-49) meV for the broadening yielding (252+5/-11) \mub.Comment: 4 pages, 2 figures, 1 tabl
Spatially continuous snow depth mapping by aeroplane photogrammetry for annual peak of winter from 2017 to 2021 in open areas
Information on snow depth and its spatial distribution is important for
numerous applications, including natural hazard management, snow water
equivalent estimation for hydropower, the study of the distribution and
evolution of flora and fauna, and the validation of snow hydrological
models. Due to its heterogeneity and complexity, specific remote sensing
tools are required to accurately map the snow depth distribution in Alpine
terrain. To cover large areas (>100 km2),
airborne laser scanning (ALS) or aerial photogrammetry with large-format
cameras is needed. While both systems require piloted aircraft for data
acquisition, ALS is typically more expensive than photogrammetry but yields
better results in forested terrain. While photogrammetry is slightly
cheaper, it is limited due to its dependency on favourable acquisition
conditions (weather, light conditions). In this study, we present
photogrammetrically processed high-spatial-resolution (0.5 m) annual snow
depth maps, recorded during the peak of winter over a 5-year period under
different acquisition conditions over a study area around Davos,
Switzerland. Compared to previously carried out studies, using the Vexcel
UltraCam Eagle Mark 3 (M3) sensor improves the average ground sampling distance to
0.1 m at similar flight altitudes above ground. This allows for very
detailed snow depth maps in open areas, calculated by subtracting a snow-off
digital terrain model (DTM, acquired with ALS) from the snow-on digital
surface models (DSMs) processed from the airborne imagery. Despite
challenging acquisition conditions during the recording of the UltraCam
images (clouds, shaded areas and fresh snow), 99 % of unforested areas
were successfully photogrammetrically reconstructed. We applied masks (high
vegetation, settlements, water, glaciers) to increase the reliability of the
snow depth calculations. An extensive accuracy assessment was carried out
using check points, the comparison to DSMs derived from unpiloted aerial
systems and the comparison of snow-free DSM pixels to the ALS DTM. The
results show a root mean square error of approximately 0.25 m for the
UltraCam X and 0.15 m for the successor, the UltraCam Eagle M3. We developed
a consistent and reliable photogrammetric workflow for accurate snow depth
distribution mapping over large regions, capable of analysing snow
distribution in complex terrain. This enables more detailed investigations
on seasonal snow dynamics and can be used for numerous applications related
to snow depth distribution, as well as serving as a ground reference for new
modelling approaches and satellite-based snow depth mapping.</p
The barrel DIRC of PANDA
Cooled antiproton beams of unprecedented intensities in the momentum range of 1.5-15 GeV/c will be used for the PANDA experiment at FAIR to perform high precision experiments in the charmed quark sector. The PANDA detector will investigate antiproton annihilations with beams in the momentum range of 1.5 GeV/c to 15 GeV/c on a fixed target. An almost 4π acceptance double spectrometer is divided in a forward spectrometer and a target spectrometer. The charged particle identification in the latter is performed by ring imaging Cherenkov counters employing the DIRC principle
An investigation into the perspectives of providers and learners on MOOC accessibility
An effective open eLearning environment should consider the target learner’s abilities, learning goals, where learning takes place, and which specific device(s) the learner uses. MOOC platforms struggle to take these factors into account and typically are not accessible, inhibiting access to environments that are intended to be open to all. A series of research initiatives are described that are intended to benefit MOOC providers in achieving greater accessibility and disabled learners to improve their lifelong learning and re-skilling. In this paper, we first outline the rationale, the research questions, and the methodology. The research approach includes interviews, online surveys and a MOOC accessibility audit; we also include factors such the risk management of the research programme and ethical considerations when conducting research with vulnerable learners. Preliminary results are presented from interviews with providers and experts and from analysis of surveys of learners. Finally, we outline the future research opportunities. This paper is framed within the context of the Doctoral Consortium organised at the TEEM'17 conference
Orbital order in the low-dimensional quantum spin system TiOCl probed by ESR
We present electron spin resonance data of Ti (3) ions in single
crystals of the novel layered quantum spin magnet TiOCl. The analysis of the g
tensor yields direct evidence that the d_{xy} orbital from the t_{2g} set is
predominantly occupied and owing to the occurrence of orbital order a linear
spin chain forms along the crystallographic b axis. This result corroborates
recent theoretical LDA+U calculations of the band structure. The temperature
dependence of the parameters of the resonance signal suggests a strong coupling
between spin and lattice degrees of freedom and gives evidence for a transition
to a nonmagnetic ground state at 67 K.Comment: revised version, accepted for publication in Phys. Rev. B, Rapid Com
The barrel DIRC of PANDA
Cooled antiproton beams of unprecedented intensities in the momentum range of 1.5-15 GeV/c will be used for the PANDA experiment at FAIR to perform high precision experiments in the charmed quark sector. The PANDA detector will investigate antiproton annihilations with beams in the momentum range of 1.5 GeV/c to 15 GeV/c on a fixed target. An almost 4π acceptance double spectrometer is divided in a forward spectrometer and a target spectrometer. The charged particle identification in the latter is performed by ring imaging Cherenkov counters employing the DIRC principle
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