Anisotropic elastic theory of preloaded granular media

A macroscopic elastic description of stresses in static, preloaded granular media is derived systematically from the microscopic elasticity of individual inter-grain contacts. The assumed preloaded state and friction at contacts ensure that the network of inter-grain contacts is not altered by small perturbations. The texture of this network, set by the preparation of the system, is encoded in second and fourth order fabric tensors. A small perturbation generates both normal and tangential inter-grain forces, the latter causing grains to reorient. This reorientation response and the incremental stress are expressed in terms of the macroscopic strain.Comment: 7 pages, 2 figures. Accepted version. [email protected] [email protected]

Methane fluxes from tropical coastal lagoons surrounded by mangroves, Yucatán, Mexico

Methane concentrations in the water column and emissions to the atmosphere were determined for three tropical coastal lagoons surrounded by mangrove forests on the Yucatán Peninsula, Mexico. Surface water dissolved methane was sampled at different seasons over a period of 2 years in areas representing a wide range of salinities and anthropogenic impacts. The highest surface water methane concentrations (up to 8378 nM) were measured in a polluted canal associated with Terminos Lagoon. In Chelem Lagoon, methane concentrations were typically lower, except in the polluted harbor area (1796 nM). In the relatively pristine Celestún Lagoon, surface water methane concentrations ranged from 41 to 2551 nM. Methane concentrations were negatively correlated with salinity in Celestún, while in Chelem and Terminos high methane concentrations were associated with areas of known pollution inputs, irrespective of salinity. The diffusive methane flux from surface lagoon water to the atmosphere ranged from 0.0023 to 15 mmol CH4 m-2 d-1. Flux chamber measurements revealed that direct methane release as ebullition was up to 3 orders of magnitude greater than measured diffusive flux. Coastal mangrove lagoons may therefore be an important natural source of methane to the atmosphere despite their relatively high salinity. Pollution inputs are likely to substantially enhance this flux. Additional statistically rigorous data collected globally are needed to better consider methane fluxes from mangrove-surrounded coastal areas in response to sea level changes and anthropogenic pollution in order to refine projections of future atmospheric methane budgets

Nitrogen assimilation efficiency in maize genotypes under ammonia stress.

Crescimento de plantas, amonia livre, acucares soluveis, atividade da glutamina sintetase (GS) nitrate reductase (NR) nas folhas e pH de rizosfera em quatro genotipos de milho foram comparados na presenca de NH4+e NO3- como forma de N em cultura hidroponica com areia: vermiculita. Na presenca de nitrato, todos os genotipos de milho testados exibiram semelhantes valores para todos os parametros analisados, exceto a atividade da NR. Esta enzima teve uma tendencia inversa ao crescimento da planta, com alta atividade foliar no Pionner 3230 que exibiu um menor crescimento de planta, indicando nao ser este um bom parametro de eficiencia na assimilacao de nitrogenio. Na presenca de amonio como unica fonte de N, os genotipos de milho podem ser facilmente separados em termos de crescimento da planta e resposta de assimilacao de nitrogenio. A atividade da GS em folhas de milho tratado com NH4+, foi positivamente correlacionada com o crescimento da planta e a acumulacao de amonia livre decresceu a medida que a atividade de GS aumentou, indicando estes dois parametros fisiologicos como fatores chave na eficiencia de assimilacao de N. O teor de acucares soluveis no tecido da parte aerea foi significativamente reduzido em plantas supridas com NH4+ devido ao alto requerimento de esqueleto de carbono para incorporacao de NH4+ em amino acido

The Λ\LambdaCDM growth rate of structure revisited

We re-examine the growth index of the concordance $\Lambda$ cosmology in the light of the latest 6dF and {\em WiggleZ} data. In particular, we investigate five different models for the growth index $\gamma$, by comparing their cosmological evolution using observational data of the growth rate of structure formation at different redshifts. Performing a joint likelihood analysis of the recent supernovae type Ia data, the Cosmic Microwave Background shift parameter, Baryonic Acoustic Oscillations and the growth rate data, we determine the free parameters of the $\gamma(z)$ parametrizations and we statistically quantify their ability to represent the observations. We find that the addition of the 6dF and {\em WiggleZ} growth data in the likelihood analysis improves significantly the statistical results. As an example, considering a constant growth index we find $\Omega_{m0}=0.273\pm 0.011$ and $\gamma=0.586^{+0.079}_{-0.074}$.Comment: 8 pages, 5 figures, Accepted for publication by International J. of Modern Physics D (IJMPD). arXiv admin note: substantial text overlap with arXiv:1203.672

Avaliação silvicultural de Rapanea ferruginea e Citharexylum myrianthum plantadas em pastagens abandonadas.

A utilização de resíduos gerados nas indústrias de celulose e papel como insumo agrícola e florestal é uma alternativa para o problema de descarte, além de contribuir para a redução do custo da produção agrícola e florestal. No entanto, para o uso adequado e seguro desses resíduos, torna-se necessário conhecer seus efeitos em atributos químicos do solo. Foi avaliado o efeito de doses de carbonato de cálcio, cinza de biomassa florestal, lama de cal e lodos celulósicos em atributos químicos de um Neossolo Regolítico Distrófico húmico. O pH e os teores de Ca, Mg e P aumentaram com o aumento das doses testadas, enquanto o teor de Al e sua saturação reduziram com a aplicação de todos os materiais avaliados. Os teores de K aumentaram principalmente pela aplicação de cinzas de madeira. A relação Ca/Mg foi elevada pela aplicação de carbonato de cálcio, lama de cal e lodo celulósico a valores que podem ser prejudiciais. As cinzas foram importantes fontes de K, Ca, Mg e P.Nota científica

Dynamics of Counterion Condensation

Using a generalization of the Poisson-Boltzmann equation, dynamics of counterion condensation is studied. For a single charged plate in the presence of counterions, it is shown that the approach to equilibrium is diffusive. In the far from equilibrium case of a moving charged plate, a dynamical counterion condensation transition occurs at a critical velocity. The complex dynamic behavior of the counterion cloud is shown to lead to a novel nonlinear force-velocity relation for the moving plate.Comment: 5 pages, 1 ps figure included using eps