Rhizodeposition and the enhanced mineralization of 2,4-dichlorophenoxyacetic acid in soil from the Trifolium pratense rhizosphere

Enhanced biodegradation of organic xenobiotic compounds in the rhizosphere is frequently recorded although the specific mechanisms are poorly understood. We have shown that the mineralization of 2,4-dichlorophenoxyacetic acid (2,4-D) is enhanced in soil collected from the rhizosphere of Trifolium pratense[e.g. maximum mineralization rate = 7.9 days(-1) and time at maximum rate (t(1)) = 16.7 days for 12-day-old T. pratense soil in comparison with 4.7 days(-1) and 25.4 days, respectively, for non-planted controls). The purpose of this study was to gain a better understanding of the plant-microbe interactions involved in rhizosphere-enhanced biodegradation by narrowing down the identity of the T. pratense rhizodeposit responsible for stimulating the microbial mineralization of 2,4-D. Specifically, we investigated the distribution of the stimulatory component(s) among rhizodeposit fractions (exudates or root debris) and the influence of soil properties and plant species on its production. Production of the stimulatory rhizodeposit was dependent on soil pH (e.g. t(1) for roots grown at pH 6.5 was significantly lower than for those grown at pH 4.4) but independent of soil inorganic N concentration. Most strikingly, the stimulatory rhizodeposit was only produced by T. pratense grown in non-sterile soil and was present in both exudates and root debris. Comparison of the effect of root debris from plant species (three each) from the classes monocotyledon, dicotyledon (non-legume) and dicotyledon (legume) revealed that legumes had by far the greatest positive impact on 2,4-D mineralization kinetics. We discuss the significance of these findings with respect to legume-rhizobia interactions in the rhizosphere.

Aeration for plant root respiration in a tidal marsh

This paper investigates the tidal effects on aeration conditions for plant root respiration in a tidal marsh. We extend the work of Ursino et al. ( 2004) by using a two-phase model for air and water flows in the marsh. Simulations have been conducted to examine directly the link between the airflow dynamics and the aeration condition in the marsh soil. The results show that the effects of entrapped air on water movement in the vadose zone are significant in certain circumstances. Single-phase models based on Richards' equation, which neglect such effects, may not be adequate for quantifying the aeration condition in tidal marsh. The optimal aeration condition, represented by the maximum of the integral magnitude of tidally advected air mass ( TAAM) flux, is found to occur near the tidal creek for the four soil textures simulated. This may explain the observation that some salt marsh plant species grow better near tidal creeks than in the inner marsh areas. Our analyses, based on the two-phase model and predicted TAAM flux magnitude, provide further insight into the positive feedback'' mechanism proposed by Ursino et al. ( 2004). That is, pioneer plants may grow successfully near the creek where the root aeration condition is optimal. The roots of the pioneer plants can soften and loosen the rhizosphere soil, which increases the evapotranspiration rate, the soil porosity, and absolute permeability and weakens the capillary effects. These, in turn, improve further the root aeration conditions and may lead to colonization by plants less resistant to anaerobic conditions.

Near-bottom depletion of zooplankton over coral reefs: III: vertical gradient of predation pressure

Many diurnal planktivorous fish in coral reefs efficiently consume zooplankton drifting in the overlying water column. Our survey, carried out at two coral reefs in the Red Sea, showed that most of the diurnal planktivorous fish foraged near the bottom, close to the shelters from piscivores. The planktivorous fish were order of magnitude more abundant near (

The effect of copper toxicity on the growth and root morphology of Rhodes grass (Chloris gayana Knuth.) in resin buffered solution culture

A solution culture experiment was conducted to examine the effect of Cu toxicity on Rhodes grass (Chloris gayana Knuth.), a pasture species used in mine-site rehabilitation. The experiment used dilute, solution culture to achieve external nutrient concentrations, which were representative of the soil solution, and an ion exchange resin to maintain stable concentrations of Cu in solution. Copper toxicity was damaging to plant roots, with symptoms ranging from disruption of the root cuticle and reduced root hair proliferation, to severe deformation of root structure. A reduction in root growth was observed at an external Cu concentration of < 1 mu M, with damage evident from an external concentration of 0.2 mu M. Critical to the success of this experiment, in quantitatively examining the relationship between external Cu concentration and plant response, was the use of ion exchange resin to buffer the concentration of Cu in solution. After some initial difficulty with pH control, stable concentrations of Cu in solution were maintained for the major period of plant growth. The development of this technique will facilitate future investigations of the effect of heavy metals on plants.

Hydraulics of skimming flows on stepped chutes: The effects of inflow conditions?

Modern stepped spillways are typically designed for large discharge capacities corresponding to a skimming flow regime for which flow resistance is predominantly form drag. The writer demonstrates that the inflow conditions have some effect on the skimming flow properties. Boundary layer calculations show that the flow properties at inception of free-surface aeration are substantially different with pressurized intake. The re-analysis of experimental results highlights that the equivalent Darcy friction factor is f similar to 0.2 in average on uncontrolled stepped Chute and f similar to 0.1 on stepped chute with pressurized intake. A simple design chart is presented to estimate the residual flow velocity, and the agreement of the calculations with experimental results is deemed satisfactory for preliminary design.

Dam break wave of thixotropic fluid

Thixotropy is the characteristic of a fluid to form a gelled structure over time when it is not subjected to shearing, and to liquefy when agitated. Thixotropic fluids are commonly used in the construction industry (e.g., liquid concrete and drilling fluids), and related applications include some forms of mud flows and debris flows. This paper describes a basic study of dam break wave with thixotropic fluid. Theoretical considerations were developed based upon a kinematic wave approximation of the Saint-Venant equations down a prismatic sloping channel. A very simple thixotropic model, which predicts the basic theological trends of such fluids, was used. It describes the instantaneous state of fluid structure by a single parameter. The analytical solution of the basic flow motion and theology equations predicts three basic flow regimes depending upon the fluid properties and flow conditions, including the initial degree of jamming of the fluid (related to its time of restructuration at rest). These findings were successfully compared with systematic bentonite suspension experiments. The present work is the first theoretical analysis combining the basic principles of unsteady flow motion with a thixotropic fluid model and systematic laboratory experiments.

Advances in distributed parameter approach to the dynamics and control of activated sludge processes for wastewater treatment

This paper presents a review of modelling and control of biological nutrient removal (BNR)-activated sludge processes for wastewater treatment using distributed parameter models described by partial differential equations (PDE). Numerical methods for solution to the BNR-activated sludge process dynamics are reviewed and these include method of lines, global orthogonal collocation and orthogonal collocation on finite elements. Fundamental techniques and conceptual advances of the distributed parameter approach to the dynamics and control of activated sludge processes are briefly described. A critical analysis on the advantages of the distributed parameter approach over the conventional modelling strategy in this paper shows that the activated sludge process is more adequately described by the former and the method is recommended for application to the wastewater industry (c) 2006 Elsevier Ltd. All rights reserved.

An advanced regularization methodology for use in watershed model calibration

A calibration methodology based on an efficient and stable mathematical regularization scheme is described. This scheme is a variant of so-called Tikhonov regularization in which the parameter estimation process is formulated as a constrained minimization problem. Use of the methodology eliminates the need for a modeler to formulate a parsimonious inverse problem in which a handful of parameters are designated for estimation prior to initiating the calibration process. Instead, the level of parameter parsimony required to achieve a stable solution to the inverse problem is determined by the inversion algorithm itself. Where parameters, or combinations of parameters, cannot be uniquely estimated, they are provided with values, or assigned relationships with other parameters, that are decreed to be realistic by the modeler. Conversely, where the information content of a calibration dataset is sufficient to allow estimates to be made of the values of many parameters, the making of such estimates is not precluded by preemptive parsimonizing ahead of the calibration process. White Tikhonov schemes are very attractive and hence widely used, problems with numerical stability can sometimes arise because the strength with which regularization constraints are applied throughout the regularized inversion process cannot be guaranteed to exactly complement inadequacies in the information content of a given calibration dataset. A new technique overcomes this problem by allowing relative regularization weights to be estimated as parameters through the calibration process itself. The technique is applied to the simultaneous calibration of five subwatershed models, and it is demonstrated that the new scheme results in a more efficient inversion, and better enforcement of regularization constraints than traditional Tikhonov regularization methodologies. Moreover, it is argued that a joint calibration exercise of this type results in a more meaningful set of parameters than can be achieved by individual subwatershed model calibration. (c) 2005 Elsevier B.V. All rights reserved.

Eco-efficiency and dairy processing

The dairy industry is a global industry that provides significant nutritional benefit to many cultures. in australia the industry is especially important economically, being a large export earner, as well as a vital domestic sector. in recent years the sector has come under increased competitive pressure and has restructured to cope with the changes. the industry recently undertook an eco-efficiency project to investigate where business and environmental improvements might be found. the project involved collecting and collating previous project data and surveying 38 companies in different dairy operations, from market milk to dried products. after the survey, 10 sites in two states were visited to discuss eco-efficiency issues in detail with key players. From the surveys, visits and data compilation, a comprehensive manual was prepared to help interested companies find relevant eco-efficiency data easily and assist them in the implementation process. ten fact sheets were also produced covering the topics of water management, water recycling and re-use, refrigeration optimisation, boiler optimisation, biogas, the use of treated wastewater, yield optimisation and product recovery, optimisation of ciP systems, chemical use and membranes the project highlighted the large amount of technical and engineering expertise within the sector that could result in eco-efficiency outcomes and also identified the opportunities that exist for changes to occur in some operations to save energy, input raw materials and water.