Phosphate availability in the soil-root system : integration of oxide surface chemistry, transport and uptake

A study is presented on the adsorption of phosphate on goethite, the interaction of phosphate with other adsorbing ions at the goethite surface, and the resulting availability of phosphate to plants. The plant-availability of sorbed phosphate was determined from phosphorus uptake of plants growing on an artificial substrate containing goethite with phosphate. Uptake can be predicted from zero sink behaviour of a growing root system, diffusion and mass flow in soil, and measured non linear adsorption of phosphate on goethite. With high phosphate loading of goethite, the equilibrium phosphate concentration in solution increased, which resulted in larger phosphate availability. Competitive interaction between phosphate and sulphate on goethite caused only a small decrease in phosphate adsorption at low pH, where sulphate adsorption is strongest, but a considerable increase in the phosphate concentration in solution.Experiments showed that phosphorus uptake by plants growing on suspensions of goethite in the presence of sulphate was seven times larger at pH 3.7 than at pH 5.5. Citrate competes more strongly with phosphate than sulphate, and shows strongest interaction at pH 4.5-5. On account of the non-linear adsorption behaviour of phosphate, the relative increase in phosphate in solution upon competition is much larger at low than at high phosphate loading of goethite. Therefore, competition results in an apparent lower affinity of phosphate sorption on goethite.Adsorption of the individual anions and competitive adsorption was described with the CD-MUSIC ion adsorption model, which is based on a detailed description of the adsorbing surface and the use of surface complexes identified in spectroscopic studies. The combination of the ion adsorption model with the uptake model is a powerful tool to predict the phosphate availability to plants. This was illustrated with a simulation study in which the effect of citrate exudation from roots on the uptake of phosphate was predicted.</p

Electricity-Assisted Biological Hydrogen Production from Acetate by Geobacter sulfurreducens

Geobacter sulfurreducens is a well-known current-producing microorganism in microbial fuel cells, and is able to use acetate and hydrogen as electron donor. We studied the functionality of G. sulfurreducens as biocatalyst for hydrogen formation at the cathode of a microbial electrolysis cell (MEC). Geobacter sulfurreducens was grown in the bioelectrode compartment of a MFC with acetate as the substrate and reduction of complexed Fe(III) at the counter electrode. After depletion of the acetate the electrode potential of the bioelectrode was decreased stepwise to -1.0 V vs Ag/AgCl reference. Production of negative current was observed, which increased in time, indicating that the bioelectrode was now acting as biocathode. Headspace analyses carried out at electrode potentials ranging from -0.8 to -1.0 V showed that hydrogen was produced, with higher rates at more negative cathode potentials. Subsequently, the metabolic properties of G. sulfurreducens for acetate oxidation at the anode and hydrogen production at the cathode were combined in one-compartment membraneless MECs operated at applied voltages of 0.8 and 0.65 V. After two days, current densities were 0.44 A m(-2) at 0.8 V applied voltage and 0.22 A m(-2) at 0.65 V, using flat-surface carbon electrodes for both anode and cathode. The cathodic hydrogen recovery ranged from 23% at 0.5 V applied voltage to 43% at 0.9

Electricity-mediated biological hydrogen production

Anaerobic bacteria have the ability to produce electricity from the oxidation of organic substrates. They also may use electricity to support chemical reactions that are energetically unfavorable. In the fermentation of sugars, hydrogen can be formed as one of the main products. However, a yield of only four hydrogen per molecule of glucose can be achieved. Potentially, eight additional hydrogen molecules could be produced when the other main fermentation product acetate is converted further, which however is energetically not possible. By the input of electricity, acetate can be oxidized further to form hydrogen. This paper reviews the scarce knowledge of how electricity can be used to produce hydrogen in the microbial oxidation of acetate or other substrates. The technological design concepts and their performance are presented, and the biochemical mechanisms of electron transfer are discusse

Effects of sulphate and pH on the plant-availability of phosphate adsorbed on goethite.

The adsorption of phosphate on metal (hydr)oxides may be influenced by the pH and by the adsorption of other ions. In this study, the influence of sulphate and pH on phosphate adsorption on goethite and the availability to plants of adsorbed phosphate was examined. Maize plants were grown on suspensions of goethite with adsorbed phosphate, containing the same total amount of phosphate and either 0.11 mM or 2.01 mM sulphate at pH 3.7, 4.6 or 5.5. The uptake of phosphorus by the plants increased with the larger sulphate concentration and decreasing pH. Mean P uptake in the treatment with 2.01 mM sulphate and pH 3.7 was 55 ?mol plant^-1, whereas in the treatment with 0.11 mM sulphate and pH 5.5 it was 2 ?mol plant^-1. Batch adsorption experiments using ^32P and speciation modelling of ion adsorption showed that in the presence of sulphate, the phosphate concentration in solution strongly increased with decreasing pH, due to competitive adsorption between sulphate and phosphate on goethite. Modelled phosphate concentrations in solution in the uptake experiment were all below 0.6 ?M and correlated well with the observed P uptake. This correlation indicates that the strong influence of the sulphate concentration and pH on the plant-availability of adsorbed phosphate results from the competition between sulphate and phosphate for adsorption on goethite

Simulation of the effect of citrate exudation from roots on the plant availability of phosphate adsorbed on goethite

Rhizosphere processes strongly influence the availability of phosphorus (P) to plants. Organic ligands that are exuded from the root surface mobilize phosphorus by dissolution of P minerals or by desorption of adsorbed phosphate. We developed a mechanistic model to study the mobilization of phosphate sorbed on goethite by the exudation of citrate and consequent uptake of phosphate by the root. The use of a model allows the effects of the organic anion and pH on P desorption to be separated. The model is also used to predict concentration profiles developing around the root for phosphate, citrate (with or without accounting for degradation) and pH, providing insight into the processes that occur in the rhizosphere. Results of model calculations show that with larger rates of citrate exudation, greater P availability is predicted. Exudation at a rate of 0.5 mol citrate m1 root day1, which is in the range found for P-deficient plants, increased P availability almost 2-fold at fairly large phosphate loading of goethite (1.9 mol m2) and almost 30-fold at small phosphate loading (1.3 mol m2). Competitive adsorption causes a much greater relative increase in the phosphate concentration in solution at small than at large phosphate loading, which explains this result. Simultaneous acidification of the rhizosphere results in a smaller P mobilization than at a fixed pH of 5, as a result of the pH dependence of phosphate adsorption in the presence of citrate. Sorption of citrate increases its persistence against microbial decay, and hence has a positive effect on the mobilization of adsorbed phosphate