494 research outputs found
The role of tectonic uplift, climate, and vegetation in the long-term terrestrial phosphorous cycle
Phosphorus (P) is a crucial element for life and therefore for maintaining ecosystem productivity. Its local availability to the terrestrial biosphere results from the interaction between climate, tectonic uplift, atmospheric transport, and biotic cycling. Here we present a mathematical model that describes the terrestrial P-cycle in a simple but comprehensive way. The resulting dynamical system can be solved analytically for steady-state conditions, allowing us to test the sensitivity of the P-availability to the key parameters and processes. Given constant inputs, we find that humid ecosystems exhibit lower P availability due to higher runoff and losses, and that tectonic uplift is a fundamental constraint. In particular, we find that in humid ecosystems the biotic cycling seem essential to maintain long-term P-availability. The time-dependent P dynamics for the Franz Josef and Hawaii chronosequences show how tectonic uplift is an important constraint on ecosystem productivity, while hydroclimatic conditions control the P-losses and speed towards steady-state. The model also helps describe how, with limited uplift and atmospheric input, as in the case of the Amazon Basin, ecosystems must rely on mechanisms that enhance P-availability and retention. Our novel model has a limited number of parameters and can be easily integrated into global climate models to provide a representation of the response of the terrestrial biosphere to global change
Differences in muscle enzyme profile and metabolic response to fighting in bulls from one to three years old
The metabolic response to exercise is partially linked to muscle enzyme profile, and both factors might be influenced by ageing. This research
aims: 1) To analyze the differences in muscle characteristics of fighting bulls of different ages; and 2) To assess whether the muscle enzyme profile
is affected by ageing in the same way it affects other bovine breeds. Muscle biopsies from gluteus medius and semitendinosus muscles were
withdrawn immediately after bullfighting, at depths of 3 and 5 cm, from a total of 24 animals, belonging to three age groups: A (1-year-olds; n=6),
B (2-year-olds; n=6) and C (3-year-olds; n=12). Muscle concentrations of glycogen, lactate, adenosin triphosphate and glucose-6-P were determined.
In addition, activities of citrate synthase, 3-OH-acyl-dehydrogenase, glycogen phosphorylase, lactate dehydrogenase and hexokinase were
measured. No significant differences were found between the two analyzed muscles and neither between depths for any of the muscle parameters.
Group A showed lower muscle lactate accumulation, with positive correlations between oxidative and glycolytic enzymes, reflecting a balance
between both metabolic pathways. Group B was the most oxidative, whereas group C was the most glycolytic. Therefore, muscle oxidative potential
achieved its maximum at 2 years of age, whereas the glycolytic capacity increased progressively with ageing. These results are different from those
showed by other bovine breeds, with a progressive reduction in oxidative potential from birth. In conclusion, age significantly influences metabolic
response to fighting as well as muscle enzyme profile in bulls
Singlet vs. triplet interelectronic repulsion in confined atoms
Hund’s multiplicity rule invariably holds for the ground configurations of few-electron atoms as well as those of multi-electron quantum dots. However, the ordering of the corresponding interelectronic repulsions exhibits a reversal in the former but not in the latter system, upon varying the system parameters. Here, we investigate the transition between these two types of behaviour by studying few-electron atoms confined in spherical cavities. “Counter-intuitive” ordering of the interelectronic repulsions is confirmed when the nuclear charge is low enough and the cavity radius is large enough
Microstructure and velocity of field-driven solid-on-solid interfaces moving under stochastic dynamics with local energy barriers
We study the microscopic structure and the stationary propagation velocity of
(1+1)-dimensional solid-on-solid interfaces in an Ising lattice-gas model,
which are driven far from equilibrium by an applied force, such as a magnetic
field or a difference in (electro)chemical potential. We use an analytic
nonlinear-response approximation [P.A. Rikvold and M. Kolesik, J. Stat. Phys.
100, 377 (2000)] together with kinetic Monte Carlo simulations. Here we
consider interfaces that move under Arrhenius dynamics, which include a
microscopic energy barrier between the allowed Ising/lattice-gas states. Two
different dynamics are studied: the standard one-step dynamic (OSD) [H.C. Kang
and W. Weinberg, J. Chem. Phys. 90, 2824 (1992)] and the two-step
transition-dynamics approximation (TDA) [T. Ala-Nissila, J. Kjoll, and S.C.
Ying, Phys. Rev. B 46, 846 (1992)]. In the OSD the effects of the applied force
and the interaction energies in the model factorize in the transition rates (a
soft dynamic), while in the TDA such factorization is not possible (a hard
dynamic). In full agreement with previous general theoretical results we find
that the local interface width under the TDA increases dramatically with the
applied force. In contrast, the interface structure with the OSD is only weakly
influenced by the force, in qualitative agreement with the theoretical
expectations. Results are also obtained for the force-dependence and anisotropy
of the interface velocity, which also show differences in good agreement with
the theoretical expectations for the differences between soft and hard
dynamics. Our results confirm that different stochastic interface dynamics that
all obey detailed balance and the same conservation laws nevertheless can lead
to radically different interface responses to an applied force.Comment: 18 pages RevTex. Minor revisions. Phys. Rev. B, in pres
Numerical Study of a Mixed Ising Ferrimagnetic System
We present a study of a classical ferrimagnetic model on a square lattice in
which the two interpenetrating square sublattices have spins one-half and one.
This model is relevant for understanding bimetallic molecular ferrimagnets that
are currently being synthesized by several experimental groups. We perform
exact ground-state calculations for the model and employ Monte Carlo and
numerical transfer-matrix techniques to obtain the finite-temperature phase
diagram for both the transition and compensation temperatures. When only
nearest-neighbor interactions are included, our nonperturbative results
indicate no compensation point or tricritical point at finite temperature,
which contradicts earlier results obtained with mean-field analysis.Comment: Figures can be obtained by request to [email protected] or
[email protected]
Parameterized optimized effective potential for the ground state of the atoms He through Xe
Parameterized orbitals expressed in Slater-type basis obtained within the
optimized effective potential framework as well as the parameterization of the
potential are reported for the ground state of the atoms He through Xe. The
total, kinetic, exchange and single particle energies are given for each atom.Comment: 47 pages, 1 figur
Decay of metastable phases in a model for the catalytic oxidation of CO
We study by kinetic Monte Carlo simulations the dynamic behavior of a
Ziff-Gulari-Barshad model with CO desorption for the reaction CO + O
CO on a catalytic surface. Finite-size scaling analysis of the fluctuations
and the fourth-order order-parameter cumulant show that below a critical CO
desorption rate, the model exhibits a nonequilibrium first-order phase
transition between low and high CO coverage phases. We calculate several points
on the coexistence curve. We also measure the metastable lifetimes associated
with the transition from the low CO coverage phase to the high CO coverage
phase, and {\it vice versa}. Our results indicate that the transition process
follows a mechanism very similar to the decay of metastable phases associated
with {\it equilibrium} first-order phase transitions and can be described by
the classic Kolmogorov-Johnson-Mehl-Avrami theory of phase transformation by
nucleation and growth. In the present case, the desorption parameter plays the
role of temperature, and the distance to the coexistence curve plays the role
of an external field or supersaturation. We identify two distinct regimes,
depending on whether the system is far from or close to the coexistence curve,
in which the statistical properties and the system-size dependence of the
lifetimes are different, corresponding to multidroplet or single-droplet decay,
respectively. The crossover between the two regimes approaches the coexistence
curve logarithmically with system size, analogous to the behavior of the
crossover between multidroplet and single-droplet metastable decay near an
equilibrium first-order phase transition.Comment: 27 pages, 22 figures, accepted by Physical Review
Analysis of Undergraduate Dermatology Syllabi at Spanish Universities: Does the Weight of Theoretical Content Match the Skin Conditions Seen in Primary Care and General Dermatology Practices?
Introduction: Undergraduate dermatology courses vary in the nearly 50 Spanish medical faculties
that teach the subject. This study aimed to describe the characteristics of these courses
and to analyze whether the weight assigned to dermatology topics reflects the caseloads of
primary care physicians and general dermatologists in the Spanish national health system.
Material and methods: Cross-sectional study of syllabi used in Spanish medical faculties during
the 2021---2022 academic year. We determined the number of teaching hours in public and
private university curricula and compared the weight of dermatology topics covered to the
dermatology caseloads of primary care physicians and general dermatologists as reported in
published studies.
Results: Most medical faculties taught dermatology for one semester. The median number of
credits offered was 4.5. On average, lectures covered 24 theoretical topics, and seminars and
workshops covered 9 topics. We identified a clear disparity between the percentage of time
devoted to dermatology topics in course lectures and the skin conditions usually managed in
primary care and general dermatology practices
- …