Sara Preston

Predicting the activity of Cd2+ and Zn2+ in soil pore water from the radio-labile metal fraction

Abstract Cadmium and zinc were added at 3 and 300 mg kg −1 , respectively, to 23 soils and incubated at 16°C and 80% field capacity for 818 d. Following addition of metal, changes in the radio-labile concentrations of both elements were examined on seven separate sampling occasions over 818 d. At each sample time, soil pore water was extracted using Rhizon soil solution samplers, and concentrations of Cd, Zn, dissolved organic carbon, and major cations and anions were determined. The chemical speciation program WHAM 6 was used to determine free metal ion activity, (M 2+ ). Similar measurements were made on a set of historically contaminated soils from old mining areas, sewage sludge disposal facilities, and industrial sources. The two data sets were combined to give a range of values for p (Cd 2+ ) and p (Zn 2+ ) that covered 5 and 4 log 10 units, respectively. A pH-dependent Freundlich model was used to predict Zn 2+ and Cd 2+ ion activity in soil pore water. Total and radio-labile metal ion concentration in the solid phase was assumed to be adsorbed on the “whole soil,” humus, or free iron oxides to provide alternative model formats. The most successful models assumed that solubility was controlled by adsorption on soil humus. Inclusion of ionic strength as a model variable provided small improvements in model fit. Considering competition with Ca 2+ and between Zn 2+ and Cd 2+ produced no apparent improvement in model fit. Surprisingly, there was little difference between the use of total and labile adsorbed metal as a model determinant. However, this may have been due to a strong correlation between metal lability and pH in the data set used. Values of residual standard deviation for the parameterized models using labile metal adsorbed on humus were 0.26 and 0.28 for prediction of p (Cd 2+ ) and p (Zn 2+ ), respectively. Solubility control by pure Zn and Cd minerals was not indicated from saturation indices. However there may have been fixation of metals to non-radio-labile forms in CaCO 3 and Ca-phosphate compounds in the soils in the higher pH range. Independent validation of the Cd model was carried out using an unpublished data set that included measurements of isotopically exchangeable Cd. There was good agreement with the parameterized model.

Soil solution extraction techniques for microbialecotoxicity testing: a comparative evaluation

The suitability of two different techniques (centrifugation and Rhizon sampler) for obtaining the interstitial pore water of soil (soil solution), integral to the ecotoxicity assessment of metal contaminated soil, were investigated by combining chemical analyses and a luminescence-based microbial biosensor. Two different techniques, centrifugation and Rhizon sampler, were used to extract the soil solution from Insch (a loamy sand) and Boyndie (a sandy loam) soils, which had been amended with different concentrations of Zn and Cd. The concentrations of dissolved organic carbon (DOC), major anions (F- , CI-, NO3, SO4(2-)) and major cations (K+, Mg2+, Ca2+) in the soil solutions varied depending on the extraction technique used. Overall, the concentrations of Zn and Cd were significantly higher in the soil solution extracted using the centrifugation technique compared with that extracted using the Rhizon sampler technique. Furthermore, the differences observed between the two extraction techniques depended on the type of soil from which the solution was being extracted. The luminescence-based biosensor Escherichia coli HB101 pUCD607 was shown to respond to the free metal concentrations in the soil solutions and showed that different toxicities were associated with each soil, depending on the technique used to extract the soil solution. This study highlights the need to characterise the type of extraction technique used to obtain the soil solution for ecotoxicity testing in order that a representative ecotoxicity assessment can be carried out.

Use of bacterial biosensors to interpret the toxicity and mixture toxicity of herbicides in freshwater.

The dose response relationship between seven commonly used herbicides and four luminescence-based bacterial biosensors was characterised. As herbicide concentration increased the light emitted by the test organism declined in a concentration dependent manner. These dose responses were used to compare the predicted vs. observed response of a biosensor in the presence of multiple contaminants. For the majority of herbicide interactions, the relationship was not additive but primarily antagonistic and sometimes synergistic. These biosensors provide a sensitive test and are able to screen a large volume and wide range of samples with relative rapidity and ease of interpretation. In this study biosensor technology has been successfully applied to interpret the interactive effects of herbicides in freshwater environments.

Response of a Rhizobium-based luminescence biosensor to Zn and Cu in soil solutions from sewage sludge treated soils

Abstract A luminescence based biosensor (Rhizotox-C) was used as an indicator of heavy metal pollution of soils. The response of the biosensor to increasing concentrations of total soil Zn, soil solution Zn, soil solution free Zn 2+ , total soil Cu and total soil solution Cu from soils of a long-term sewage sludge field experiment was investigated. The bioluminescence response of the Rhizotox-C biosensor declined as total soil Zn, soil solution Zn and free soil solution Zn 2+ concentrations increased. The EC 25 values for the biosensor for total soil Zn, soluble soil solution Zn and free soil solution Zn 2+ were 164±43 mg kg −1 soil, 4±0.7 and 2±0.3 mg l −1 , respectively. The EC 50 values were 403±57 mg kg −1 soil, 16±3 and 6±1.0 mg l −1 , respectively. The largest soil solution concentration of Cu was about 620 μg l −1 , but this had no significant effect on luminescence. This corresponded to a total Cu concentration in bulk soil of about 349 mg kg −1 .

The role played by microorganisms in the biogenesis of soil cracks : importance of substrate quantity and quality

The development of cracks in soils has a significant effect on important soil processes such as gaseous diffusion, water flow and root development. It is the heterogeneity and tortuosity of the resulting cracks which are important as they influence and control such processes. The microbial contribution to crack formation was therefore assessed by quantifying the heterogeneity and connectivity of cracks which developed following the addition of substrates differing in quantity and quality to a sandy loam soil. The heterogeneity and connectivity of the cracks was assessed using probability and Monte Carlo techniques respectively, which provided an estimate of how the occurrence of cracks varied with sample area and the nature of the route taken by a random walker in such a space. Increasing the amount of glucose added resulted in significantly (P<0.05) less heterogeneous and less tortuous cracks on subsequent drying of the soil, after the soil had been incubated for 2 weeks at 15°C in a slurried state. The addition of glucose or different forms of cellulose, such as a native pulverised cellulose and a highly crystalline cellulose, generally resulted in significantly (P<0.05) less heterogeneous and less tortuous cracks than those formed in the absence of a substrate addition. However, the addition of carboxymethylcellulose to soil initially prevented crack formation and after a 5 or 15 day incubation period resulted in the formation of cracks which were significantly (P<0.05) more heterogeneous than those generated by unamended soil after equivalent incubation periods. Soil amended with glucose, a water soluble source of carbon, resulted in the formation of cracks which were significantly (P<0.05) less heterogeneous and less tortuous than those generated in soil amended with either pulverised native cellulose or highly crystalline cellulose, both of which are water insoluble sources of carbon. The extent of the heterogeneity or connectivity of the cracks was shown to depend on the recalcitrance of the substrate and the incubation period before the soil slurries were allowed to dry and crack. Microorganisms were thus shown to contribute to the formation of cracks, as well as hindering crack formation, highlighting a dual role as potential degraders, as well as producers, of soil binding agents.

Comparison of different microbial bioassays to assess metal‐contaminated soils

These experiments compared the sensitivity of four different types of bioassay over time after five metals were added to a wide range of soils at the maximum concentrations in the European Union Sewage Sludge Directive. Three were chronic assays (most probable number of Rhizobium leguminosarum, soil microbial C and Biolog substrate utilization). The fourth bioassay, an acute biosensor, employed a lux-marked luminescent bacterium (Escherichia coli) in the soil pore water. Five metals were added to 23 different soils as a mixture at Zn = 300, Cd = 3, Pb = 300, Cu = 135, and Ni = 75 mg/kg as nitrate salts and compared with unamended controls. Zinc and Cu were the metals most likely to be toxic at the concentrations used here. In the case of Rhizobium, the number of cells in soil was not affected after 11 d; however, by 818 d the numbers had decreased by four orders of magnitude with increasing concentrations of Zn and Cu in soil solution. Microbial biomass also was not affected after 11 d, but significantly decreased with increased Zn (p < 0.001) and Cu (p < 0.01) in soil solution after 818 d. Toxicity to the soil microbial biomass increased with time, whereas the toxicity to the biosensor remained the same. Biolog substrate utilization profiles were not responsive to the concentrations used here.

Links between substrate additions, native microbes, and the structural complexity and stability of soils

Soil pore space influences and controls a vast array of soil processes, physical, chemical and biological. The geometry and dimensions of the pore space define the rates at which such processes occur. Using relatively simple structures, generated from the desiccation of soil slurries, we investigated the action of adding a range of substrates to the soil (e.g. glucose or ammonium nitrate), in relation to emerged cracking patterns and soil stability. Using probability and Monte Carlo techniques, we quantified the heterogeneity and connectivity of the cracking patterns. We hypothesise that the addition of substrates directly acts to alter microbial activity which then alters the cracking patterns of dried soil slurries. In addition, we show that the addition of substrates acted to decrease crack heterogeneity (1.30–1.47), and increase crack connectivity (1.15–1.27) and density (10–16%), (P<0.05). The addition of glucose decreased the number of aggregates created during desiccation and decreased the stability of cracks (P<0.05). The mechanisms controlling these effects are discussed.