bluuebunny/crossref_metadata_2025_split huggingface dataset:
Controls on the large‐scale spatial variations of dune field properties in the barchanoid portion of White Sands dune field, New Mexico
Authors: Pelletier Jon D. 
Published: January 2015

AbstractPrevious studies have shown that sediment fluxes and dune sizes are a maximum near the upwind margin of the White Sands dune field and decrease, to first order, with increasing distance downwind. These patterns have alternatively been attributed to a shear‐stress overshoot associated with a roughness transition localized at the upwind margin and to the influence of long‐wavelength topography on the hydrology and hence erodibility of dune field sediments. I point out an issue that compromises the shear‐stress overshoot model and further test the hypothesis that long‐wavelength topographic variations, acting in concert with feedbacks among aerodynamic, granulometric, and geomorphic variables, control dune field properties at White Sands. Building upon the existing literature, I document that the mean and variability of grain sizes, sand dryness, aerodynamic roughness lengths, bed shear stresses, sediment fluxes, and ripple and dune heights all achieve local maxima at the crests of the two most prominent scarps in the dune field, one coincident with the upwind margin and the other located 6–7 km downwind. Computational fluid dynamics (CFD) modeling predicts that bed shear stresses, erosion rates, and the supply of relatively coarse, poorly sorted sediments are localized at the two scarps due to flow line convergence, hydrology, and the spatially distributed adjustment of the boundary layer to variations in dune size. As a result, the crests of the scarps have larger ripples due to the granulometric control of ripple size. Larger grain sizes and/or larger ripples lead to larger dunes and hence larger values of bed shear stress in a positive feedback.
The role of local‐scale heterogeneities in terrestrial ecosystem modeling
Authors: Pappas Christoforos , Fatichi Simone , Rimkus Stefan , Burlando Paolo , Huber Markus O. 
Published: January 2015

AbstractThe coarse‐grained spatial representation of many terrestrial ecosystem models hampers the importance of local‐scale heterogeneities. To address this issue, we combine a range of observations (forest inventories, eddy flux tower data, and remote sensing products) and modeling approaches with contrasting degrees of abstraction. The following models are selected: (i) Lund‐Potsdam‐Jena (LPJ), a well‐established, area‐based, dynamic global vegetation model (DGVM); (ii) LPJ‐General Ecosystem Simulator, a hybrid, individual‐based approach that additionally considers plant population dynamics in greater detail; and (iii) distributed in space‐LPJ, a spatially explicit version of LPJ, operating at a fine spatial resolution (100 m × 100 m), which uses an enhanced hydrological representation accounting for lateral connectivity of surface and subsurface water fluxes. By comparing model simulations with a multivariate data set available at the catchment scale, we argue that (i) local environmental and topographic attributes that are often ignored or crudely represented in DGVM applications exert a strong control on terrestrial ecosystem response; (ii) the assumption of steady state vegetation and soil carbon pools at the beginning of simulation studies (e.g., under “current conditions”), as embedded in many DGVM applications, is in contradiction with the current state of many forests that are often out of equilibrium; and (iii) model evaluation against vegetation carbon fluxes does not imply an accurate simulation of vegetation carbon stocks. Having gained insights about the magnitude of aggregation‐induced biases due to smoothing of spatial variability at the catchment scale, we discuss the implications of our findings with respect to the global‐scale modeling studies of carbon cycle and we illustrate alternative ways forward.
Nutrient uptake dynamics across a gradient of nutrient concentrations and ratios at the landscape scale
Authors: Gibson Catherine A. , O'Reilly Catherine M. , Conine Andrea L. , Lipshutz Sondra M. 
Published: January 2015

AbstractUnderstanding interactions between nutrient cycles is essential for recognizing and remediating human impacts on water quality, yet multielemental approaches to studying nutrient cycling in streams are currently rare. Here we utilized a relatively new approach (tracer additions for spiraling curve characterization) to examine uptake dynamics for three essential nutrients across a landscape that varied in absolute and relative nutrient availability. We measured nutrient uptake for soluble reactive phosphorous, ammonium‐nitrogen, and nitrate‐nitrogen in 16 headwater streams in the Catskill Mountains, New York. Across the landscape, ammonium‐nitrogen and soluble reactive phosphorus had shorter uptake lengths and higher uptake velocities than nitrate‐nitrogen. Ammonium‐nitrogen and soluble reactive phosphorus uptake velocities were tightly correlated, and the slope of the relationship did not differ from one, suggesting strong demand for both nutrients despite the high ambient water column dissolved inorganic nitrogen: soluble reactive phosphorus ratios. Ammonium‐nitrogen appeared to be the preferred form of nitrogen despite much higher nitrate‐nitrogen concentrations. The uptake rate of nitrate‐nitrogen was positively correlated with ambient soluble reactive phosphorus concentration and soluble reactive phosphorus areal uptake rate, suggesting that higher soluble reactive phosphorus concentrations alleviate phosphorus limitation and facilitate nitrate‐nitrogen uptake. In addition, these streams retained a large proportion of soluble reactive phosphorus, ammonium‐nitrogen, and nitrate‐nitrogen supplied by the watershed, demonstrating that these streams are important landscape filters for nutrients. Together, these results (1) indicated phosphorus limitation across the landscape but similarly high demand for ammonium‐nitrogen and (2) suggested that nitrate‐nitrogen uptake was influenced by variability in soluble reactive phosphorus availability and preference for ammonium‐nitrogen.
The relative importance of methanogenesis in the decomposition of organic matter in northern peatlands
Authors: Corbett J. Elizabeth , Tfaily Malak M. , Burdige David J. , Glaser Paul H. , Chanton Jeffrey P. 
Published: January 2015

AbstractUsing an isotope‐mass balance approach and assuming the equimolar production of CO2 and CH4 from methanogenesis (e.g., anaerobic decomposition of cellulose), we calculate that the proportion of total CO2 production from methanogenesis varies from 37 to 83% across a variety of northern peatlands. In a relative sense, methanogenesis was a more important pathway for decomposition in bogs (80 ± 13% of CO2 production) than in fens (64 ± 5.7% of CO2 production), but because fens contain more labile substrates they may support higher CH4 production overall. The concentration of CO2 produced from methanogenesis (CO2‐meth) can be considered equivalent to CH4 concentration before loss due to ebullition, plant‐mediated transport, or diffusion. Bogs produced slightly less CO2‐meth than fens (2.9 ± 1.3 and 3.7 ± 1.4 mmol/L, respectively). Comparing the quantity of CH4 present to CO2‐meth, fens lost slightly more CH4 than bogs (89 ± 2.8% and 82 ± 5.3%, respectively) likely due to the presence of vascular plant roots. In collapsed permafrost wetlands, bog moats produced half the amount of CO2‐meth (0.8 ± 0.2 mmol/L) relative to midbogs (1.6 ± 0.6 mmol/L) and methanogenesis was less important (42 ± 6.6% of total CO2 production relative to 55 ± 8.1%). We hypothesize that the lower methane production potential in collapsed permafrost wetlands occurs because recently thawed organic substrates are being first exposed to the initial phases of anaerobic decomposition following collapse and flooding. Bog moats lost a comparable amount of CH4 as midbogs (63 ± 7.0% and 64 ± 9.3%).
The role of water tracks in altering biotic and abiotic soil properties and processes in a polar desert in Antarctica
Authors: Ball Becky A. , Levy Joseph 
Published: January 2015

AbstractGroundwater discharge via water tracks is a largely unexplored passageway routing salts and moisture from high elevations to valley floors in the McMurdo Dry Valleys (MDV) of Antarctica. Given the influence that water tracks have on the distribution of liquid water in seasonally thawed Antarctic soils, it is surprising how little is known about their role in structuring biotic and abiotic processes this cold desert ecosystem. Particularly, it is unclear how soil biota will respond to the activation of new water tracks resulting from enhanced active layer thickening or enhanced regional snowmelt. In the MDV, water tracks are both wetter and more saline than the surrounding soils, constituting a change in soil habitat suitability for soil biology and therefore the ecological processes they carry out. To investigate the net impact that water tracks have on Dry Valley soil biology, and therefore the ecosystem processes for which they are responsible, we analyzed microbial biomass and activity in soils inside and outside of three water tracks and relate this to the physical soil characteristics. Overall, our results suggest that water tracks can significantly influence soil properties, which can further impact biological biovolume and both biotic and abiotic fluxes of CO2. However, the nature of its impact differs with water track, further suggesting that not all water tracks can be regarded the same.
Temporal responses of groundwater‐surface water exchange to successive storm events
Authors: Dudley‐Southern Marina , Binley Andrew 
Published: January 2015

AbstractGroundwater‐surface water exchange within the hyporheic zone is widely recognized as a key mechanism controlling the fate of nutrients within catchments. In gaining river systems, groundwater‐surface water interactions are constrained by upwelling groundwater but there is increasing evidence that a rapid rise in river stage during storm events can result in a temporary reversal of vertical hydraulic gradients, leading to surface water infiltration into the subsurface and supply of surface‐borne reactive solutes to this biogeochemically active interface. At a UK study site, using logged hydraulic heads in the surface water, riverbed, and riverbanks and logged electrical conductivity at multiple depths in the riverbed we show that storm events can lead to a temporary reversal of vertical hydraulic gradient with mixing evident up to 30 cm beneath the riverbed. Cross‐channel variability is evident, with the center of the channel consistently having shorter reversals of hydraulic gradient, compared to the channel margins. The direction of shallow subsurface riverbank flow at the site is also reactive to storm events, temporarily aligning with the surface flow direction and then reverting back to preevent conditions. Such a transition of flow paths during events is also likely to lead to expansion of lateral hyporheic exchange. This study provides evidence that storm events can be a key driver of enhanced hyporheic exchange in gaining river systems, which may support nutrient reactions beyond the duration of event‐driven change. Our observations demonstrate the dynamic nature of the hyporheic zone, which should be considered when evaluating its biogeochemical function.
Cerebral Cortex
Authors: Majewska Ania K 
Published: January 2015

Abstract

The cerebral cortex is a telencephalic structure present in some vertebrate species located at the surface of the cerebral hemispheres. It is the brain area most recently acquired in evolution. The cerebral cortex resembles a six layer sheet of neurons that in many animals, including humans, is folded to fit into the confines of the skull. Each cerebral cortex hemisphere is made of four anatomically distinct lobes: frontal, parietal, occipital and temporal. Within these lobes, different cortical functions are carried out by functionally and anatomically distinct cortical areas. Although structurally similar, the two hemispheres of the cortex are not functionally equivalent. The cerebral cortex is involved in many higher‐level functions such as sensory perception, cognition, language, memory, decision making, motor planning and control.

Key Concepts

The cerebral cortex is well‐organised anatomical structure with six neuronal layers and subareas that are anatomically and functionally distinct.

The cerebral cortex is composed of different neuronal subtypes that are organised into networks that connect neurons within and between distinct functional areas.

A large part of the cerebral cortex is devoted to processing sensory stimuli from the environment.

The remainder of the cortex is devoted to motor planning and control, as well as integration of multiple sensory cues and cognitive processing.

The two hemispheres of the brain are similar in organisation but distinct in some functions.

The complex networks of the cerebral cortex are specified in development using intrinsic molecular cues and spontaneous neuronal activity, although sensory‐driven activity can also play a role.
Neurotransmitter Release from Presynaptic Terminals
Authors: Sullivan Jane M , Schweizer Felix E 
Published: January 2015

Abstract

Neurotransmitter release from presynaptic nerve terminals is a central component of synaptic transmission, the main process by which neurons communicate with each other and with target cells. Neurotransmitter is packaged inside small vesicles in the presynaptic nerve terminal. When an action potential invades the nerve terminal, voltage‐gated calcium channels open and calcium flows into the cell. The influx of calcium triggers the fusion of the neurotransmitter‐filled synaptic vesicles with the cell membrane, thereby releasing their contents into the synaptic cleft. In addition to action potential‐evoked release, neurotransmitter can also be released ‘spontaneously’ in the absence of action potential firing. Although evoked and spontaneous release share many features, there is growing evidence that they may serve different functions and might be modulated through different signalling pathways, adding richness to the synaptic repertoire.

Key Concepts

Neurotransmitter release is the main mechanism by which neurons communicate with one another and with muscles—without neurotransmission, we could not sense our environment, think or act.

Neurotransmitter release is tightly regulated, and is very rapidly triggered by the firing of an action potential, which depolarises the presynaptic terminal and causes the influx of calcium ions.

Neurotransmitter release is probabilistic; only a fraction of action potentials succeed in triggering release of neurotransmitter from any particular release site.

Calcium ions are the critical signal that coordinates the molecular interactions responsible for fusion of neurotransmitter‐filled synaptic vesicles with the plasma membrane.

Because synaptic vesicles are roughly the same size and contain about the same amount of neurotransmitter, neurotransmitter release can be characterised as ‘quantal’, that is, occurring in discrete packets.

Neurotransmitter release can occur through three different mechanisms: fast synchronous, slower asynchronous and spontaneous. Synchronous and asynchronous releases are triggered by action potentials, whereas spontaneous release occurs in the absence of action potential firing.

Although synchronous, asynchronous and spontaneous neurotransmitter release share many common features, current evidence suggests that different molecules, signalling pathways and/or vesicle pools might be responsible for mediating and modulating these different forms of neurotransmitter release.
Biotin
Authors: Waldrop Grover L 
Published: January 2015

AbstractBiotin is a vitamin that is a cofactor for enzymes involved in carbon dioxide metabolism. Biotin‐dependent enzymes catalyse carboxylations, decarboxylations and transcarboxylations. These reactions are involved in a variety of metabolic functions from fatty acid synthesis to gluconeogenesis and are found in all animals, plants and bacteria. The common feature of biotin‐dependent enzymes is they are multifunctional. They contain two separate active sites that catalyse distinct chemical reactions. The biotin moiety, which is covalently attached to the enzyme via the amino acid lysine, alternates between the two distinct active sites. In one active site, biotin is carboxylated, whereas, in the other active site, the carboxyl group is transferred to an acceptor molecule. Avidin and streptavidin are proteins that bind biotin very tightly. The high affinity between biotin and avidin/streptavidin is utilised for a variety of biotechnological techniques such as protein purification and immunoassays such as Western blotting.Key ConceptsBiotin is a vitamin.Biotin is a cofactor for enzymes involved in CO2metabolism.Biotin‐dependent enzymes catalyse carboxylation, decarboxylation and transcarboxylation reactions.Biotin‐dependent enzymes are multifunctional and contain two distinct active sites.Biotin, which is covalently attached to a protein, moves between each of the two active sites.The two active sites and the biotin carrier protein can comprise three separate proteins that form a complex or they can form domains on a single polypeptide.The biotin‐dependent enzyme acetyl‐CoA carboxylase is an emerging target for antibiotic development.The proteins avidin and streptavidin bind biotin very tightly and with a high degree of specificity.The very high affinity and specificity of the biotin avidin/streptavidin interaction is used for many biotechnology techniques.
Amino Acid Degradation
Authors: Sawers R Gary 
Published: January 2015

Abstract

Amino acids are valuable metabolic fuels, providing a supply of both nitrogen and carbon for intermediary metabolism and energy for growth. Controlled degradation of amino acids is important in the maintenance of the carbon–nitrogen balance. It is becoming increasingly apparent that imbalance in amino acid degradation can have important consequences for both development and disease. Generally, the first step in degradation of amino acids results in the amino group either being incorporated into other nitrogenous compounds or being excreted as ammonia or urea, while the carbon skeleton is catabolised to one of a few common metabolic intermediates. Thus, an understanding of amino acid degradation provides knowledge of the interrelationships between metabolic pathways and helps explain some of the clinical features when deficiencies in amino acid metabolism occur.

Key Concepts

Amino acids are important growth substrates for microorganisms.

Tight control of amino acid degradation and cycling maintains the C–N balance.

Glutamate is a key central amino acid in maintenance of the C–N balance.

The first step in amino acid degradation is removal of the α‐amino group.

Key steps in amino acid degradation include deamination, catalysed by pyridoxal‐phosphate‐dependent transaminases, oxidoreductases or carbon–oxygen lyases, decarboxylase reactions and carbon skeleton rearrangements catalysed by isomerases.

Carbon skeletons arising from amino acid breakdown are channelled into central metabolism.

Production and excretion of urea and uric acid by animals and birds and reptiles, respectively, avoids the accumulation of toxic levels of ammonia in blood and tissues.

Metabolic products derived from
l
‐serine are essential for cell proliferation and a functional nervous system.

Absence of key enzymes, or imbalance in amino acid degradation, leads to severe disease states, such as phenylketonuria and methylmalonic aciduria.
Acquired Bleeding Disorders
Authors: O'Sullivan Jennifer , Hunt Beverley J 
Published: January 2015

Abstract

Acquired bleeding disorders include disseminated intravascular coagulation (
DIC
), vitamin K deficiency, massive transfusion, renal and liver disease and hyperfibrinolysis. A history of a bleeding tendency should be followed by appropriate laboratory investigations.
DIC
is characterised by consumption of platelets and coagulation factors and increased fibrinolytic activity. The treatment of the underlying condition is key to
DIC
management. In major bleeding, serial coagulation tests should be performed to guide use of blood components. Vitamin K deficiency occurs in neonates and malabsorptive states. Abnormalities in liver disease involve coagulation and fibrinolytic defects and platelet dysfunction, whereas in renal disease, abnormal platelet function and adhesion is the primary abnormality. Hyperfibrinolysis involves excessive fibrinolytic activity causing disproportionately elevated
FDPs
(fibrinogen degradation products) (D‐dimer levels) and should be considered in patients with liver disease and disseminated cancers. Acquired coagulation inhibitors result from immune‐mediated inhibition of a coagulation factor in patients with inherited bleeding disorder or previously normal haemostasis.

Key Concepts

Key to management of DIC is treatment of the underlying condition.

Only replace coagulation proteins and platelets in bleeding patient or for planned interventions.

Early use of fresh frozen plasma to prevent coagulopathy development is currently recommended.

Tranexamic acid should be given in all patients at risk of or with significant bleeding.

Vitamin K prophylaxis should be offered to all neonates at birth.

Prothrombin complex concentrate is recommended when rapid reversal of vitamin K antagonist therapy is required.

Vitamin K given either orally or intravenously will reverse the effects of VKAs in 6 h.

In liver disease, asymptomatic patients with abnormal coagulation tests do not routinely require replacement therapy.

Abnormalities of platelet function and adhesion are the major causes of bleeding in renal disease.

Acquired coagulation inhibitors can occur in patients with congenital coagulation factor deficiencies or in patients with previously normal haemostasis.
Competition
Authors: Keddy Paul A 
Published: January 2015

AbstractCompetition is the all‐pervasive interaction between organisms in which each reduces the performance of the other, either by depleting mutually required resources or by directly inflicting damage. Competition is an important factor that controls the distribution and abundance of many if not most plants and animals. More generally, it has been a potent force driving natural selection and is a major factor (along with stress, disturbance, predation and mutualism) in shaping the structure of biological communities. Too often, competition is assumed to be occurring without sufficient evidence. Carefully designed experiments are one of the best sources of evidence for competition. Different kinds of experiments uncover different views of competition. In general, plants are organised into competitive hierarchies in which light is often the key resource. Mathematical models allow us to explore how competition might lead to either coexistence or exclusion.Key ConceptsThe contest between organisms for access to essential resources is called competition.Competition is an important factor controlling the distribution and abundance of many if not most plants and animals.Kinds of competition can be described by the mechanism of interaction, the type of organisms involved and the relative impacts of the competitors.Mathematical models, based on assumptions about organism interactions, are sometimes used to explore the consequences of competition.Except in relatively trivial cases of aggression, competition between organisms can be detected and measured only through carefully designed experiments.Competition can be either symmetrical (competitors equally matched) or asymmetrical (a clear winner and loser) depending upon the species and the habitat.Competition may lead to resource partitioning (particularly among motile animals) or may be counterbalanced by natural disturbances (in plants and among sessile animals).