Bioavailability Of Zn In Soil Research Articles

Isolation and Screening of Zn (Zn) Solubilizing Rhizosphere Bacteria from Different Vegetations for Their Ability to Improve Growth, Zn Uptake, and Expression of Zn Transporter Genes in Tomato.

Zinc-solubilizing bacteria (ZSB) can convert insoluble zinc to an accessible form and increase Zn bioavailability in soil, which helps mitigate Zn deficiency in crops. In this study, different bacterial strains were screened for different Zn solubilization and plant growth promotion traits. Two bacterial strains, Acinetobacter pittii DJ55 and Stenotrophomonas maltophilia DJ24, were tested for their Zn-solubilizing potential on plate media, and both showed variable levels of Zn solubilization. The results showed that the bacterial strains applied to the plants in the pot experiment caused improvements in growth parameters compared to control conditions. DJ55, when applied with an insoluble source, enhanced plant height, leaf number, and leaf area compared to DJ24 and control conditions, while the maximum fruit weight was noticed in plants treated with ZnSO4. An increase in chlorophyll contents was noted in plants treated with ZnSO4, while maximum carotenoid contents were observed in plants treated with DJ55 + ZnO when compared with their controls. Plants supplemented with ZnO and DJ55 showed higher zinc content and iron content as compared to their respective controls. The expression patterns of the SLZIP5 and SLZIP4 genes were changed in the root and shoot. Application of ZnO stimulates both gene expression and protein synthesis in tomato roots and shoots. Inoculation of tomato plants with ZSB and insoluble ZnO reduced the expression of the SLZIP5 and SLZIP4 genes in the root and shoot. In conclusion, both strains can be considered as potential zinc-solubilizing bioinoculants to promote the growth and production yield of tomato.

Metabolomics reveals the impact of nitrogen combined with the zinc supply on zinc availability in calcareous soil via root exudates of winter wheat (Triticum aestivum)

A possible mechanism for the improved availability of zinc (Zn) in soil by combining nitrogen (N) with Zn supply was investigated based on the root exudates of winter wheat. N, Zn supply as well as their combination significantly regulated nine root exudates in winter wheat; in which, the secretion of cis-aconitic acid involving in the TCA cycle, C5-branched dibasic acid metabolism, glyoxylate and dicarboxylate metabolism and 2-oxocarboxylic acid metabolism was upregulated by N, Zn supply as well as their combination. N–Zn combination induced the activities of citrate synthase and cis-aconitase in roots and shoots of winter wheat thus to increase the concentrations of citric and aconitic acid; the decrease of isocitric acid concentrations in shoots indicated the inhibited conversion of aconitic acid to isocitric acid by N–Zn combination. It revealed a possible reason for the enhanced secretion of cis-aconitic acid by N–Zn combination. Exogenous addition of 10 μ plant−1 cis-aconitate significantly increased available Zn concentrations in soil and Zn concentrations in winter wheat under N–Zn combination. Thus, the N–Zn combination regulated the metabolism of cis-aconitic acid in winter wheat, thus enhancing the secretion of cis-aconitic acid to increase the bioavailability of Zn in soil.

Bioactive Nutrient Fortified Fertilizer: A Novel Hybrid Approach for the Enrichment of Wheat Grains With Zinc.

Zinc (Zn) is a critical micronutrient that synergizes nutrient use efficiency, and improves plant growth and human health. Low Zn bioavailability in soils affects produce quality and agricultural productivity worldwide ultimately inducing deficiency in humans and animals. Zn deficiency is a leading cause of malnutrition in underdeveloped countries where a widespread population depends upon staple cereals for daily intake of calories. Modern cereal cultivars are inherently low in Zn, eventually, plants need to be enriched with soil application of ZnSO4, but due to higher fixation losses, it becomes an inefficient source. Rhizosphere microbiome contains Zn-solubilizing bacteria (ZSB) that improve Zn bioavailability, thus increase the root function, Zn uptake, and plant growth. Niha Corp developed a hybrid process of bioactive nutrient fortified fertilizer (BNFF), which has been used to formulate Zabardast Urea (ZU) by coating bioactive Zn (BAZ) and ZSB on urea. Data obtained for 15 wheat varieties from 119 farmer field demonstration plots and eight replicated trials on 42 locations across multi-environment conditions conclude that ZU significantly improved the plant biomass and yield by 12% over non-Zn control and produced grains with 57 μg/g Zn contents, which can meet a major part of the recommended dietary allowance (RDA) of humans. The study recommends that this microbe-mediated hybrid invention (ZU) is a feasible approach to boost Zn bioavailability and Zn use efficiency, with enhanced yield and quality that may contribute to improve human health. To the best of our knowledge, this is the first wide-scale field testing of Zn enrichment in the grains of bread wheat using an innovative BNFF Urea Z technology.

Ecological risk of copper and zinc and their different bioavailability change in soil-rice system as affected by biowaste application.

A large amount of organic fertilizer application could be accompanied by soil contamination caused by trace heavy metals. A field experiment was carried out in this study to examine the accumulation and availability of copper (Cu) and zinc (Zn) in soil, and their uptake by rice under continuous application of chicken manure, pig manure and sewage sludge. Results showed that after four years of chicken manure, pig manure and sewage sludge application, the soil Cu accumulation rates were 0.15-1.17mgkg-1 yr-1, 1.01-4.22mgkg-1 yr-1 and 0.13-1.15mgkg-1 yr-1, respectively; Zn accumulation rates were 0.54-5.46mgkg-1 yr-1, 1.51-9.65mgkg-1 yr-1 and 1.13-10.47mgkg-1 yr-1, respectively. Compared to the control, the chicken- and pig manure treatments significantly decreased the DTPA-extractable Cu, but increased the DTPA-extractable Zn in soils; thus decreased the Cu contents in rice grain by 2.2-40.6% and increased the grain Zn by 2.6-30.9%, respectively, with increasing application rates and number of years. The addition of sewage sludge significantly increased bioavailability of Zn in soil and its accumulation in rice, while had limited effect on Cu bioavailability. Results suggested that the continuous application of organic fertilizer with elevated Cu and Zn contents at high application rates can induce their accumulation in soil and affect their bioavailability differently.

Opposite effects of the earthworm Eisenia fetida on the bioavailability of Zn in soils amended with ZnO and ZnS nanoparticles

The increasing release of metallic nanoparticles (NPs) or their sulfidized forms into soils have raised concerns about their potential risks to soil ecosystems. Hence, there is a need for novel strategies to remediate metallic NPs pollution in soils. In this study, to explore the feasibility of using earthworm Eisenia fetida to manage soils contaminated with metallic NPs, we simultaneously investigated the chronic soil toxicities of ZnO NPs and ZnS NPs to E. fetida, and the effects of E. fetida on Zn extractability in soils amended with ZnO NPs and ZnS NPs. After a 28 d exposure, survival rate and weight loss of earthworms were not impacted by either ZnO NPs or ZnS NPs at a concentration of 400 mg Zn per kg soil. Further, while ZnO NPs activated earthworm antioxidative system, ZnS NPs resulted in significant alleviation of oxidative damage in earthworm. The presence of earthworms significantly decreased the bioavailability of Zn in ZnO NPs contaminated soil, whereas significantly increased the bioavailability of Zn in ZnS NPs contaminated soil. These findings implied that the earthworm E. fetida could play an important role in altering the mobilization of metals originating from metallic NPs in soils, which may further aid in the development of a method for the treatment of metallic NPs pollution in soils.

Effects of selenium combined with zinc amendment on zinc fractions and bioavailability in calcareous soil.

Selenium (Se) and zinc (Zn) are two important trace elements for human being and animals. The interaction between Se and Zn on the bioavailability of Zn in soil is still unclear. Therefore, pot experiments exposed to different dosages of zinc sulfate (ZnSO4) (0, 20, and 50mg/kg soil) and sodium selenite (Na2SeO3) (0, 0.5, 1.0, and 2.5mg/kg soil) were conducted to investigate the effects of selenite application on Zn bioavailability in calcareous soil and its related mechanisms. The total Zn content of different tissues (roots and shoots) of pak choi (Brassica chinensis L.) and the changes in Zn fraction distribution in soil before planting and after harvest were determined, and the mobility factor (MF) and distribution index (DI) of Zn in soils were calculated. In addition, the Pearson correlation and path analysis were conducted to clarify the relationships between Zn fractions in soil and the Zn uptake of pak choi. Results showed that Se amendment elevated soil Zn bioavailability at appropriate levels of Se and Zn. When 1.0 and 2.5mg/kg of Se and 20mg/kg of Zn were applied in soil, the proportion of exchangeable Zn (Ex-Zn) and Zn weakly bound to organic matter (Wbo-Zn) to the total content of Zn was significantly increased by 28.14%-82.52% compared with that of the corresponding single Zn treatment. Therefore, the Zn concentration in the shoots of pak choi was significantly increased by 27.2%-31.1%. High Zn (50mg/kg) and Se co-amended treatments showed no significantly beneficial effect on the bioavailability of Zn. In addition, the potential available Zn content in soil (weakly bound to organic matter and carbonate bound Zn) and MF and DI values were all positively correlated with the Zn concentrations in pak choi, indicating that these indexes can be used to predict the bioavailability of Zn in soil. This study can provide a good reference for Se and Zn biofortification of plants in calcareous soil.

Chemical Fractions and Availability of Zinc in Winter Wheat Soil in Response to Nitrogen and Zinc Combinations.

Nitrogen (N) is critical for zinc (Zn) accumulation in winter wheat grain via enhancing Zn absorption into plant roots. This paper explored a possible mechanism for enhanced absorption of Zn in winter wheat by N combined with Zn application based on the Zn bio-availability in soil. A pot experiment with three N application rates (0.05, 0.2, and 0.4 g kg-1), two Zn application rates (0 and 10 mg kg-1), without and with plants was conducted. The results showed that high N (N0.2 and N0.4) combined with Zn (Zn10) application significantly increased the yield, yield components and Zn and N concentrations in winter wheat shoots and grain. The available Zn concentration in soil with and without plants was increased by N0.2Zn10 and N0.4Zn10 treatment at each growth stage. N0.2Zn10 and N0.4Zn10 treatment significantly decreased the pH in soil without plants but had different influences on the pH in soil with plants, which depended on the different N application rates and growth stages. Meanwhile, N0.2Zn10 and N0.4Zn10 treatment decreased the exchangeable Zn but increased loose organic-, carbonate- and Fe-Mn oxides-bound Zn concentrations in soil without plants. The exchangeable, loose organic- and carbonate-bound Zn concentrations in soil with plants was increased by N0.2Zn10 and N0.4Zn10 treatment at different growth stages. Different rates of N combined with Zn application influenced the proportion of Zn in different fractions in soil with and without plants at different growth stages. At Zn10, N0.4 treatment showed higher yield, N and Zn concentrations in shoot and grain, and available Zn concentration in soil, but lower pH in soil than N0.2 treatment. In addition, soil without plants had higher available Zn concentrations and lower pH than did the soil with plants. There were significant differences in Zn chemical fractions concentrations and proportions between the soils with and without plants at each growth stage. Therefore, combined influence of roots and the combination of N and Zn (especially N0.4Zn10 treatment) improved the bio-availability of Zn in soil via changing the soil pH and promoting the transformation and distribution of Zn in different fractions.

Effects of soil sterilization and metal spiking in plant growth promoting rhizobacteria selection for phytotechnology purposes

The contamination of the soil with heavy metals (e.g. Zn) is a serious and crosscutting issue worldwide. Phytotechnologies can minimize the negative impact of this problem using plants and microorganisms in soil rehabilitation. However, the efficiency of proper plant-microbe combinations is usually assessed using spiked and/or sterilized soils, which do not mimic the conditions in situ, and therefore can lead to outcomes that will not be observed under field situations.This study aimed to quantify the effect of soil origin and sterilization on the performance of the two plant growth promoting rhizobacteria (PGPR), Ralstonia eutropha 1C2 and Chryseobacterium humi ECP37, for promoting the growth and metal accumulation of maize plants. A two-experiment approach was applied: the PGPR were inoculated in maize plants growing in (i) sterilized soils spiked with Zn (0, 100, 500 and 1000 mg Zn kg−1); and in (ii) a field-contaminated soil, under sterilized and non-sterilized conditions (599 mg Zn kg−1). Biomass and Zn accumulation in the root and shoot, and Zn bioavailability in soils were determined. Additionally, lipid peroxidation, activity of antioxidant enzymes catalase (CAT) and superoxide dismutase (SOD) were assessed in the shoots of plants grown in the field-contaminated soil, as well as the composition of the rhizospheric bacterial community.Zn in the soils negatively affected maize growth, and its effect was strongest in the field-contaminated soil. Overall, PGPR attenuated the negative effects of Zn by improving plant growth, although less pronounced in non-sterilized soils. Sterilization significantly reduced soil Zn availability and affected its' accumulation in plant tissues. Bioinoculants performance was also different in sterilized soil, i.e., bacteria had no effect in the accumulation of Zn but tended to increase the biomass of maize plants. Despite the higher Zn accumulation in shoot tissues, lipid peroxidation was lower whereas antioxidant enzymes were enhanced in non-sterilized soils, suggesting that plant antioxidant system functioned properly. PGPR tended to decrease the diversity of the rhizospheric community.This study highlights that while inoculation with PGPR is effective in increasing Zn bioavailability in soil, accumulation in the plant and maize growth in Zn-contaminated soils, the extent of their effect can be different depending on whether the soil is field-contaminated or metal spiked, and on whether is sterilized prior contaminated. Consequently, the effect of bacterial inoculants assessed exclusively in metal spiked soil and/or sterilized soil may be overestimated, and potentially not transferable to field conditions.

Combining Path Analysis and X‐Ray Absorption Spectroscopy to Unravel the Zn Sorption Mechanism on Soils

Core Ideas pH, clay content, and FeDCB played direct roles in Zn adsorption. Zinc sorption on alkaline soils was higher than on acidic soils as a result of the formation of Zn‐precipitate. Clay content affected Zn sorption on soils via FeDCB and cation exchange capacity. Zinc accumulation in soils due to increasing industrial activities can change this plant nutrient into a pollutant. Soil properties significantly affect the sorption and bioavailability of Zn in soils. Twenty‐one soils with different characteristics were collected from the Jiangsu Province, China, to ascertain the effect of soil properties on Zn sorption. A combination of path analysis and x‐ray absorption spectroscopy (XAS) was conducted to elucidate the sorption mechanism of Zn on soils. The results of XAS indicated that Zn sorption on alkaline soils was higher than on acidic soils because of the formation of a Zn‐precipitate [Zn‐Al layered double hydroxide (LDH), ZnCO3, and Zn(HO)2–2ZnCO3]. Although cation exchange is involved, it was not the primary mechanism of Zn sorption on soils. The correlation and path analysis indicated that pH, clay content, and dithionite‐citrate‐bicarbonate extractable Fe (FeDCB) played direct roles in Zn sorption, in which pH was the dominant factor. The clay content affected Zn sorption on soils measured via FeDCB and cation exchange capacity (CEC) based on the indirect coefficient. The high proportion of Zn‐clay minerals (Zn‐kaolinite, Zn‐illite, Zn‐montmorillonite, and Zn‐vermiculite) further illustrated that soil clay content was the primary factor affecting Zn sorption on soils.

A novel multi-reaction model for kinetics of Zn release from soils: Roles of soil binding sites

The kinetics of Zn release from soil is a key process affecting the fate and bioavailability of Zn in soil, which may be complicated by multiple soil binding sites and reactions during the Zn release process. In this study, we developed a novel quantitative model for the dynamic Zn speciation in soil during the Zn release process, by integrating the mechanistic equilibrium model WHAM 7 and molecular level investigation of Zn speciation with X-ray absorption spectroscopy (XAS). The kinetics model specifically considered Zn reactions with soil organic matter (SOM), iron (hydr)oxides, and clay minerals, and dissolution of Zn precipitates. Batch and replenishment experiments were conducted to study kinetics of Zn release from laboratory-spiked soils and XAS was employed to identify soil Zn speciation. Overall, the model described the experimental data reasonably well with minimal model fitting parameters. We quantitatively assessed the dynamic changes of Zn speciation in different soil components and SOM binding sites at various time scales and reaction conditions. Our kinetics model provides a quantitative framework for predicting the dynamic behavior of Zn in soil environments.

Zinc solubilizing Bacillus spp. potential candidates for biofortification in maize

Bioaugmentation of Zn solubilizing rhizobacteria could be a sustainable intervention to increase bioavailability of Zn in soil which can be helpful in mitigation of yield loss and malnutrition of zinc. In present study, a number of pure rhizobacterial colonies were isolated from maize rhizosphere and screened for their ability to solubilize zinc oxide. These isolates were screened on the basis of zinc and phosphate solubilization, IAA production, protease production, catalase activity and starch hydrolysis. All the selected isolates were also positive for oxidase activity (except ZM22), HCN production (except ZM27) and utilization of citrate. More than 70% of isolates produces ammonia, hydrogen cyanide, siderophores, exopolysaccharides and cellulase. More than half of isolates also showed potential for urease activity and production of lipase. The ZM31 and S10 were the only isolates which showed the chitinase activity. All these isolates were evaluated in a jar trial for their ability to promote growth of maize under axenic conditions. Results revealed that inoculation of selected zinc solubilizing rhizobacterial isolates improved the growth of maize. In comparison, isolates ZM20, ZM31, ZM63 and S10 were best compared to other tested isolates in stimulating the growth attributes of maize like shoot length, root length, plant fresh and dry biomass. These strains were identified as Bacillus sp. (ZM20), Bacillus aryabhattai (ZM31 and S10) and Bacillus subtilis (ZM63) through 16S rRNA sequencing. This study indicated that inoculation of Zn solubilizing strains have potential to promote growth and can be the potential bio-inoculants for biofortification of maize to overcome the problems of malnutrition.

Bioavailability of Zn in ZnO nanoparticle-spiked soil and the implications to maize plants

Little is known about the relationships between Zn bioavailability in ZnO nanoparticle (NP)-spiked soil and the implications to crops. The present pot culture experiment studied Zn bioavailability in soil spiked with different doses of ZnO NPs, using the diethylenetriaminepentaacetic acid (DTPA) extraction method, as well as the toxicity and Zn accumulation in maize plants. Results showed that ZnO NPs exerted dose-dependent effects on maize growth and nutrition, photosynthetic pigments, and root activity (dehydrogenase), ranging from stimulatory (100–200 mg/kg) through to neutral (400 mg/kg) and toxic effect (800–3200 mg/kg). Both Zn concentration in shoots and roots correlated positively (P < 0.01) with ZnO NPs dose and soil DTPA-extractable Zn concentration. The BCF of Zn in shoots and roots ranged from 1.02 to 3.83 when ZnO NPs were added. In most cases, the toxic effects on plants elicited by ZnO NPs were overall similar to those caused by bulk ZnO and soluble Zn (ZnSO4) at the same doses, irrespective of some significant differences suggesting a higher toxicity of ZnO NPs. Oxidative stress in plants via superoxide free radical production was induced by ZnO NPs at 800 mg/kg and above, and was more severe than the same doses of bulk ZnO and ZnSO4. Although significantly lower compared to bulk ZnO and ZnSO4, at least 16 % of the Zn from ZnO NPs was converted into DTPA-extractable (bioavailable) forms. The dissolved Zn2+ from ZnO NPs may make a dominant contribution to their phytotoxicity. Although low amounts of ZnO NPs exhibited some beneficial effects, the accumulation of Zn from ZnO NPs into maize tissues could pose potential health risks for both plants and human.

Application of Biochar for Enhancing Cadmium and Zinc Phytostabilization in Vigna radiata L. Cultivation

Cadmium (Cd) is a toxic and nonessential element. Because of its toxicity, Cd soil contamination is a major environmental risk to living organisms. Several studies have reported on the successful use of biochar to immobilize Cd in soil as it reduces Cd accumulation in plant parts. This research reports on the contrasting effect of biochar on enhancing Cd uptake by plants. A cassava stem biochar produced through low-temperature pyrolysis was applied to natural Cd-contaminated soil that also had a high zinc (Zn) concentration. Vigna radiata L. (a green bean) was grown in treatments receiving three biochar rates, i.e., 5, 10, and 15 %, respectively. The results showed that the 10 % biochar-amended soil had a positive effect on promoting plant growth and seed yield. Unfortunately, 15 % biochar-amended soil caused an adverse effect to plant growth. Cadmium uptake by plants increased with increasing biochar application rate. Zinc uptake by plants tended to decrease with biochar application. Cadmium and Zn bioavailability in soil was significantly reduced with an increasing biochar application rate. The results also showed that the biochar-amended soil could be an alternative and cost-effective method to promote plant growth and decrease Cd mobility in soil. The ratio of Cd concentration in plant root to soil was higher than 1, while the translocation factor from root to shoot was less than 1. These results indicate that the cultivation of V. radiata L. coupled with biochar application is an appropriate method to enhance Cd phytostabilization efficiency of V. radiata L. in Cd-polluted sites.

Comparison of diffusive gradients in thin film technique with traditional methods for evaluation of zinc bioavailability in soils.

The technique of diffusive gradients in thin film (DGT) has been shown to be a promising tool to assess zinc (Zn) bioavailability in soils, but there exists considerable debate on its suitability. In this study, Zn bioavailability was systematically investigated in wheat- and maize-grown soils using this technique and seven traditional methods, including soil solution concentration and six widely used single-step extraction methods (HAc, EDTA, NaAc, NH4Ac, CaCl₂, and MgCl₂). The concentrations of Zn in the shoots and roots of these two plant species increased continuously with increasing additions of Zn to the soils, accompanied by significant decreases in shoot biomass and root biomass at Zn concentrations greater than 400 mg kg(-1) for maize and 800 mg kg(-1) for wheat. Zinc uptake and accumulation was higher in maize roots than in wheat roots. Both the concentrations of bioavailable Zn measured by DGT (CDGT) and soil solutions (Csol) increased linearly with increasing additions of Zn to the soils, while no strong linear relationships were observed for the extraction methods. Higher concentrations of extractable Zn, lower values of Csol, and larger values of R (i.e., the ratio of CDGT to Csol) were observed in maize-grown soils compared with those of wheat-grown soils, while the values of C DGT between the two plants were similar. These findings demonstrate that there likely exists a stronger resupply of Zn from the soil solid phases in maize-grown soils to satisfy a higher Zn uptake and accumulation in this plant. Linear correlation analyses showed that CDGT had the highest correlation coefficients with plant Zn concentrations compared with other traditional methods, implying that the DGT technique is more sensitive and robust in reflecting Zn bioavailability in soils to plants.

Zinc Transformation in a Calcareous Soil as Affected by Applied Zinc Sulfate, Vermicompost, and Incubation Time

An experiment was conducted to evaluate the effect of zinc (Zn) rates and vermicompost levels on distribution of Zn forms of a calcareous soil. After incubation periods, soil samples were air dried, and a sequential extraction scheme was used to fractionate Zn into soluble and exchangeable, bound to carbonate, organically bound, bound to manganese (Mn) oxide, bound to amorphous iron (Fe) oxide, bound to crystalline Fe oxide, and residual forms. In untreated soil, Zn was mainly in the residual fraction. Increasing rates of applied Zn significantly increased all forms of Zn. Carbonate and residual forms showed the greatest increase. Application of vermicompost significantly increased all fractions except Mn-oxide form. This increase was more pronounced for organically bound, soluble, and exchangeable forms, indicating an increase in bioavailability of soil Zn. Incubation time significantly decreased soluble, exchangeable, and organically bound forms but increased other forms of Zn, meaning a significant reduction in Zn phytoavailability in soil with time.

Zinc mobilisation from a contaminated soil by three genotypes of tobacco as affected by soil and rhizosphere pH

The aim of this research was to evaluate the effect of soil and rhizosphere pH on the mobilisation of Zn by various tobacco genotypes. One-month-old tobacco plants were grown for 8 days on top of a thin layer of an arable soil that had been sampled near a Zn smelter. A range of rhizosphere pH values was obtained either by growing nitrate-fed tobacco on top of the soil amended with various amounts of acid or lime, or by growing tobacco on top of the unamended soil with nitrate or ammonium supply. In the latter case, we used three genotypes that were assumed to differ in their ability to accumulate Zn or acidify the rhizosphere and, hence, mobilise soil Zn. In spite of the moderate level of contamination of the soil, tobacco took up substantial amounts of soil Zn. No difference was found between the three genotypes. Exchangeable Zn steeply increased with decreasing soil pH, which could be adequately modelled with a simple model. Whatever the source of nitrogen supplied, a significant acidification occurred in the rhizosphere. This explains why the observed Zn mobilisation was larger than expected on the basis of bulk soil pH values. Taking account of the change of pH induced by tobacco roots is thus of prime importance for better predicting the actual amount of exchangeable Zn in the rhizosphere and, thereafter the bioavailability of soil Zn.

ZINC EXTRACTABILITY AS A FUNCTION OF pH IN ORGANIC WASTE-AMENDED SOILS

The distribution of various forms of heavy metals after addition to soils needs study to improve predictions of metal solubility, especially when the metal is added to soils with large quantities of organic carbon that oxidize over time. The extractability of added zinc (Zn) was determined in a laboratory study using sequential extractions as a function of pH and organic matter (OM as biosolids) additions to A and B horizon samples of three soils from the Coastal Plain of Maryland. The study used Galestown loamy sand (Psammentic Hapludults), Bertie silt loam (Typic Hapludults), and Othello silt loam (Typic Endoaquults), chosen to represent a typical toposequence in this physiographic province. Watersoluble Zn in the six horizons, with and without biosolids added, decreased with increasing pH from 4.0 to 7.0 and increasing cation exchange capacity (CEC) from 2.2 to 106 mmol c /kg soil. Although this decrease with increasing pH was expected, exchangeable levels of Zn were positively correlated with the CEC only up to pH 5.8 but were negatively correlated at pH > 5.8 in all the soils, suggesting that CEC was not a major contributor to Zn exchangeability at pH > 5.8. The biosolids amendment increased the solubility of Zn in all soils at pH 5.8 in all of the soils. The results indicate that pH was a controlling variable for the redistribution of water-soluble, exchangeable, and nonexchangeable Zn affected by biosolids addition. The higher level of nonexchangeable Zn in the Bertie soil compared with other soils, about 0.80 mmol/kg soil for the biosolids treatments and 0.47 mmol/kg soil for the soils without biosolids at pH 7.0, was attributed to the presence of higher native organic C levels and Fe(III) and Mn(III, IV)(hydr) oxide contents that enhance surface-induced hydrolysis and chemisorption of Zn. The effect of biosolids addition on nonexchangeable Zn retention was more pronounced in B horizons of all the soils, especially at pH > 5.8. The results have implications for predicting the mobility and bioavailability of Zn in soils in which pH and organic C levels are modified purposefully for the disposal of heavy metal and C-rich wastes.

Zinc accumulation by Thlaspi caerulescens from soils with different Zn availability: a pot study

The role of Zn bioavailability in soil on Zn hyperaccumulation by Thlaspi caerulescens was investigated. Thlaspi caerulescens from Prayon, Belgium, and Clough Wood, UK, were grown in pots containing unenriched soil (35 μg Zn g−1), or five treatments enriched with Zn compounds of different solubility (ZnS, Zn3(PO4)2, ZnO, ZnCO3, and ZnSO7ċ7H2O). The Zn-enriched treatments had similar total Zn contents (1000 μg Zn g−1), but differed greatly in their concentrations of extractable-Zn. In the treatments with little extractable-Zn (unenriched and ZnS-enriched) T. caerulescens accessed Zn fractions that were not initially soluble; the mass of Zn accumulated in the shoots on Day 90 was greater than the mass of ammonium nitrate extractable-Zn in the soil on Day 0. Moreover, the decrease in ammonium nitrate extractable-Zn in the unenriched treatment after growth accounted for only 50 and 24% of the Zn accumulated by plants of the Clough Wood and Prayon populations, respectively. Despite accumulation of Zn from the previously non-labile fraction in soil, Zn hyperaccumulation from the unenriched and ZnS-enriched treatments was less than from the four treatments with highly extractable-Zn. The mechanisms involved in the solubilization of Zn were therefore not strong. The dissolution of Zn in the soil might have resulted from the very high root density in the pots either enhancing weak mobilization mechanisms, and/or highly efficient uptake in to the roots coupled with replenishment of the Zn taken up through the soil buffering capacity.

Evaluation of surrogate measures of cadmium, lead, and zinc bioavailability to Eisenia fetida

We evaluated weak-electrolyte (0.1 M Ca(NO 3) 2) soil extractions and ion-exchange membranes coated with a metal chelator as measures of Cd, Pb, and Zn bioavailability in spiked artificial soil by comparing their metal availability estimates to acute lethal toxicity in the earthworm Eisenia fetida. Ca(NO 3) 2 extractions were precisely related to toxicity in all toxicity tests, and enabled the development of time-independent LC 50s (incipient lethal-levels, ILLs) calculated using exposure levels based on extraction data. ILLs with 95% CIs for the Cd, Pb, and Zn toxicity tests were 9.8 (9.4–10.3), 1.16 (1.11–1.22), and 6.33 (6.18–6.49) Ca(NO 3) 2-extractable mmol metal/kg soil, respectively. Mixture toxicity of Cd, Pb, and Zn, assessed using the toxic unit (TU) approach, was 1.35 TU, suggesting additivity. Chelating ion-exchange membrane uptake was variable, and not well related to toxicity. Weak-electrolyte extractions show promise as precise, inexpensive surrogate measures of Cd, Pb, and Zn bioavailability in soil.