Nitrogen dynamics in European forest ecosystems : considerations regarding anthropogenic nitrogen depositions.

This study deals with the nutrient cycle of forest ecosystems over large geographic regions in Europe as affected by nitrogen deposition. The view is taken that the nitrogen cycle of a forest ecosystem has a maximum capacity for circulating nitrogen. Two different cases are defined: case (1) in which the nutrient cycle functions below its maximum capacity, and case (2) in which the circulation operates at the maximum level

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

When microbes and consumers determine the limiting nutrient of autotrophs: a theoretical analysis

Ecological stoichiometry postulates that differential nutrient recycling of elements such as nitrogen and phosphorus by consumers can shift the element that limits plant growth. However, this hypothesis has so far considered the effect of consumers, mostly herbivores, out of their food-web context. Microbial decomposers are important components of food webs, and might prove as important as consumers in changing the availability of elements for plants. In this theoretical study, we investigate how decomposers determine the nutrient that limits plants, both by feeding on nutrients and organic carbon released by plants and consumers, and by being fed upon by omnivorous consumers. We show that decomposers can greatly alter the relative availability of nutrients for plants. The type of limiting nutrient promoted by decomposers depends on their own elemental composition and, when applicable, on their ingestion by consumers. Our results highlight the limitations of previous stoichiometric theories of plant nutrient limitation control, which often ignored trophic levels other than plants and herbivores. They also suggest that detrital chains play an important role in determining plant nutrient limitation in many ecosystems

Measuring and modeling continuous quality distributions of soil organic matter

An understanding of the dynamics of soil organic matter (SOM) is important for our ability to develop management practices that preserve soil quality and sequester carbon. Most SOM decomposition models represent the heterogeneity of organic matter by a few discrete compartments with different turnover rates, while other models employ a continuous quality distribution. To make the multi-compartment models more mechanistic in nature, it has been argued that the compartments should be related to soil fractions actually occurring and having a functional role in the soil. In this paper, we make the case that fractionation methods that can measure continuous quality distributions should be developed, and that the temporal development of these distributions should be incorporated into SOM models. The measured continuous SOM quality distributions should hold valuable information not only for model development, but also for direct interpretation. Measuring continuous distributions requires that the measurements along the quality variable are so frequent that the distribution approaches the underlying continuum. Continuous distributions lead to possible simplifications of the model formulations, which considerably reduce the number of parameters needed to describe SOM turnover. A general framework for SOM models representing SOM across measurable quality distributions is presented and simplifications for specific situations are discussed. Finally, methods that have been used or have the potential to be used to measure continuous quality SOM distributions are reviewed. Generally, existing fractionation methods will have to be modified to allow measurement of distributions or new fractionation techniques will have to be developed. Developing the distributional models in concert with the fractionation methods to measure the distributions will be a major task. We hope the current paper will help generate the interest needed to accommodate this

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