Multi-surface models are widely used to assess the potential ecotoxicological risk in metal-contaminated soils. Their accuracy in predicting metal speciation in soils with low metal levels was not yet tested. Now highly sensitive analytical techniques are available to experimentally validate such models at low concentration levels. The objective of this study was to test the accuracy of a multi-surface model to predict the Zn(2+) concentration and to improve our understanding of Zn bioavailability in low-Zn soils. High-Zn soils were included as controls. Model parameters were determined independently on the basis of earlier peer-reviewed publications. Model output was validated against free Zn(2+) concentrations determined with the soil column Donnan membrane technique in a range of soils varying in potentially available Zn, organic matter, clay silicate, and iron (hydr)oxide contents and pH. Deviations between predicted Zn(2+) concentrations and experimentally determined values over the whole Zn concentration range were less or equal to the experimental standard error, except for one low-Zn soil. The Zn(2+) concentration was mainly controlled by adsorption, where organic matter was predicted to be the dominant soil sorbent. The predicted Zn(2+) concentration depends more sensitively upon changes of the reactive Zn pool (application of 0.6, 1.2, 2.4, and 3.6 mg of Zn kg(-1) of soil) and organic matter content (± 0.2 and 0.4%) than pH changes (± 0.5 and 1 pH unit).

Abstract. Land-use change in the mountainous parts of northern Thailand is reflected by an increased application of agrochemicals, which may be lost to surface and groundwater. The close relation between flow paths and contaminant transport within hydrological systems requires recognizing and understanding the dominant hydrological processes. To date, the vast majority of studies on runoff generation have been conducted in temperate regions. Tropical regions suffer from a general lack of data, and little is known about runoff generation processes. To fill this knowledge gap, a three-component hydrograph separation based on geochemical tracers was carried out in a steep, remote and monsoondominated study site (7 km 2 ) in northern Thailand. Silica and electrical conductivity (EC) were identified as useful tracers and were applied to calculate the fractions of groundwater (similar to pre-event water), shallow subsurface flow and surface runoff on stormflow. K + was a useful indicator for surface runoff dynamics, and Ca 2+ provided insights into groundwater behaviour. Nevertheless, neither measure was applicable for the quantification of runoff components. Cl − and further parameters (e.g. Na + , K + , and Mg 2+ ) were also not helpful for flow path identification, nor were their concentrations distinguishable among the components.Groundwater contributed the largest fractions to stormflow (62-80 %) throughout all events, followed by shallow subsurface flow (17-36 %) and surface runoff (2-13 %). Our results provide important insights into the dynamics of the runoff processes in the study area and may be used to assess the transport pattern of contaminants (i.e. agrochemicals) here.

Aims Zinc (Zn) and phosphorus (P) deficiency often occurs at the same time and limits crop production in many soils. It has been suggested that citrate root exudation is a response of plants to both deficiencies. We used white lupin (Lupinus albus L.) as a model plant to clarify if citrate exuded by roots could increase the bioavailability of Zn and P in calcareous soils. Methods White lupin was grown in nutrient solution and in two calcareous soils in a rhizobox. Rhizosphere soil solution was sampled to determine citrate, metals and P. Based on the measured citrate concentrations, a soil extraction experiment with citrate as extractant was done. Results Absence of Zn triggered neither cluster root formation nor citrate exudation of white lupin grown in nutrient solution, whereas low P supply did. The maximum citrate concentration (∼1.5 mM) found in the cluster rhizosphere soil solution of one soil mobilized P, but not Zn. In the other soil the highest citrate concentration (∼0.5 mM) mobilized both elements. Conclusions White lupin does not respond to low Zn bioavailability by increasing citrate exudation. Such a response was observed at low P supply only. Whether Zn and P can be mobilized by citrate is soil-dependent and the possible controlling mechanisms are discussed.

No abstract

Environmental context The free Zn ion concentration in environmental aqueous systems is an important factor in determining Zn deficiency or toxicity to organisms as this species is directly bioavailable. The permeation liquid membrane technique, a tool to measure either free or bioavailable metal concentrations in solution depending on its setup, was evaluated for the first time for Zn speciation in simplified plant nutrient solutions. The technique is low-cost and applicable to a broad range of aqueous samples. Abstract The bioavailability of Zn in environmental water phases strongly depends on its speciation. One important species in studies on Zn deficiency or toxicity to organisms is the free ion. The permeation liquid membrane (PLM) technique is a tool to measure free metal concentrations with a short analysis time of 1 h and at low cost. However, so far it has only been validated for Cd, Cu, Ni and Pb. In this study we tested the effect of carrier concentrations and pH on Zn transport across the organic PLM membrane and the ability of the technique to measure free Zn in synthetic plant nutrient solution. We found that Zn membrane transport is dependent on the concentration of the carrier molecule lauric acid (LA), whereas variations in the concentration of the other carrier molecule, the crown ether Kryptofix 22DD, showed no effect, suggesting that Zn is not transported by the ‘classical’ PLM transport mechanism by binding to the crown ether. Zn preconcentration increased with increasing pH and decreased with increasing ligand concentrations. Using 0.05 M LA, Zn membrane transport is expected to be rate limiting (permeability criterion <<1) and the free Zn concentration can be measured. Under these conditions, PLM measurements agreed well with speciation calculations and with Donnan membrane technique (DMT) measurements in the presence of ligands forming negatively charged Zn complexes (ethylenediaminetetraacetate or citrate). In the presence of L-histidine higher free Zn concentrations than calculated were measured by PLM and DMT, suggesting that positively charged complexes contributed to cross-membrane transport in both methods.