19 research outputs found
LATTICE MODEL FOR A BINARY MIXTURE OF HARD RODS AND HARD CUBES. APPLICATION TO SOLUTE INDUCED NEMATIC → ISOTROPIC TRANSITIONS
Par un traitement de mécanique statistique, on étudie les propriétés d'un mélange binaire composé de bâtons durs Lx 1x 1 (L = 5,10) et de cubes durs Dx Dx D (1 ⩽ 2 ⩽ 2) placés sur un réseau cubique simple. Le nombre sans dimension Φ = Pν0/kT, où ν0 est le volume d'une maille du réseau, est choisi pour que le système soit anisotrope lorsque les bâtons durs sont seuls présents. A Φ constant, on peut induire une transition du premier ordre du mélange partiellement anisotrope vers une phase isotrope en augmentant la concentration x des cubes. On trouve une petite zone où deux phases coexistent. On étudie cette transition en fonction de Φ, x, L et D et on compare nos prédictions théoriques aux résultats expérimentaux récents obtenus pour les mélanges de CCl4 dans une phase nématique. Notre modèle prévoit avec succès l'existence, la position et la largeur de la région à deux phases, et donne l'ordre de grandeur observé pour la dépression de température à la transition nématique-isotrope induite par un soluté. On trouve que le paramètre d'ordre des bâtons à la transition est indépendant de x et D, résultat vérifié expérimentalement. On discute le rôle des forces répulsives et les limitations d'une approximation de champ moyen pour traiter les mélanges nématiques.A statistical mechanical treatment for a two component mixture of hard rods of dimensions Lx 1x 1 (L = 5.10) and hard cubes of dimensions Dx Dx D (1 ⩽ D ⩽ 2), placed on a simple cubic lattice, is described. The dimensionless pressure-to-temperature ratio Φ = Pν0/kT (where ν0 is the volume of a lattice site) is chosen so that the system is anisotropic when only rods are present. At constant Φ the partially aligned anisotropic mixture can be induced to undergo a first-order transition to the isotropic phase by increasing the concentration x of the cubes. A small two phase region is found. The dependence of this transition on Φ, x, L, and D is described. Recent experimental results for mixtures of nematics with CCl4 are cited and compared with the findings of the lattice calculation. The model successfully predicts the existence, the general position and the extent of the observed two phase region, as well as the correct magnitude of the solute induced nematic — isotropic transition temperature depression. In agreement with experiments, the transition order parameter of the rods is found to be independent of the concentration or size of the cubes. The role of repulsive forces and the limitations of this and other mean field treatments of nematic mixtures are discussed
Vegetation and Moisture Controls on Soil C Mineralization in Semi-Arid Environments
Mechanisms of vegetation control on C mineralization in semiarid ecosystems are not well understood. We developed a series of model predictions for beneath the native shrub Wyoming big sagebrush [Artemisia tridentata (Nutt.) ssp. wyomingensis], the invasive annual grass cheatgrass (Bromus tectorum L.), and the exotic introduced perennial grass crested wheatgrass [Agropyron desertorum (L.) Gaertn.]. Soil samples (0–10 cm) collected biweekly for two growing seasons were analyzed in the laboratory for: water content, CO2 from intact soil cores and CO2 from soils sieved and wetted to 23%, total organic C, total N, and microbial biomass C. Our results suggest that different vegetation types in the Great Basin affect C mineralization primarily through modification of soil moisture and, secondarily, the amount of labile C. Soils beneath cheatgrass and sagebrush canopy retained more water after high‐and moderate‐intensity rainfalls than soils beneath crested wheatgrass and sagebrush interspace. Sagebrush canopy probably intercepts more incoming precipitation without significant throughfall to the soil surface below than cheatgrass or crested wheatgrass. At the same time, soils beneath cheatgrass had 8% more labile C and 36% higher C mineralization rates than sagebrush. Regression analysis showed that soil water content alone explained nearly 84% of the variation, and adding information on labile C accounted for nearly 88% of the variation in soil C mineralization rates. With increasing variability of precipitation in this region, the continuously increasing presence of cheatgrass in the semiarid and arid western United States may significantly impact the CO2 contributions to overall greenhouse gas emissions
Global pattern of leaf litter nitrogen and phosphorus in woody plants
Forest ecosystems exert an important influence on global biogeochemical cycles.
A global dataset of nitrogen (N) and phosphorus (P) concentrations in leaf-
litter of woody plants was compiled from the literature. Among the 677 data
sets, 482 included P concentrations and the N:P ratio. At a global scale, the
mean leaf-litter N and P and N:P ratio were 10.9 mg g-1, 0.85 mg g-1 and 18.3,
respectively. Leaf-litter N and P were significantly correlated. When the data
was grouped by continents, the highest mean N was found in Africa (19.5 mg g-1),
and the lowest in North America (8.18 mg g-1). P was significantly smaller in
the Asian Islands (Japan and Malaysia, 0.44 mg g-1) than on the Asian mainland.
For the global dataset, leaf-litter N increased linearly with mean annual
temperature and annual precipitation and decreased with latitude. Although leaf-
litter P showed no significant relationship with temperature, it declined
linearly with precipitation and there was a convex quadratic relationship with
latitude. For the global dataset and also for different functional groups (e.g.
shrubs, evergreen broadleaf, deciduous broadleaf, and conifers) the leaf-litter
N:P ratio generally followed a positive linear relationship with temperature and
precipitation, and showed a concave quadratic response with latitude. The
differences in leaf-litter N:P ratio among functional groups and among
continents should be taken into account when modeling biogeochemical cycles in
different regions as well as on a global scale
Estimating net primary production of boreal forests in Finland and Sweden from field data and remote sensing
Heather-grass competition on Scottish moorlands: interacting effects of nutrient enrichment and grazing regime
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