Trace elements in soils and crops.

To demonstrate the total amounts to be expected in soils, the ranges of contents of some 60 trace elements in ten representative Scottish arable surface soils are compared with ranges in soil-forming rocks and with crustal averages. It is, however, the amounts potentially available to plants rather than the total contents that are biologically significant. In temperate climates, trace element mobilization is greatest when weathering takes place under conditions of impeded pedological drainage, leading to the formation of gleyed soils. Mobilized trace elements occur in arable surface soils largely in adsorbed and chelated forms, which are available to plants to a greater or smaller extent depending on the prevailing soil parameters and on the element in question. Different species take up different amounts of trace elements: the proportions in the various plant parts vary with the element and the stage of growth. Information is required about the mobilization and uptake of many elements about which little is at present known but which may affect the functions of essential elements through inter-element interactions. Systematic soil surveys in which soils are mapped by associations related to parent material, with their series related to genetic soil types, provide a useful countrywide guide to trace element status.     

Trace elements in agroecosystems and impacts on the environment.

Trace elements mean elements present at low concentrations (mg kg-1 or less) in agroecosystems. Some trace elements, including copper (Cu), zinc (Zn), manganese (Mn), iron (Fe), molybdenum (Mo), and boron (B) are essential to plant growth and are called micronutrients. Except for B, these elements are also heavy metals, and are toxic to plants at high concentrations. Some trace elements, such as cobalt (Co) and selenium (Se), are not essential to plant growth but are required by animals and human beings. Other trace elements such as cadmium (Cd), lead (Pb), chromium (Cr), nickel (Ni), mercury (Hg), and arsenic (As) have toxic effects on living organisms and are often considered as contaminants. Trace elements in an agroecosystem are either inherited from soil parent materials or inputs through human activities. Soil contamination with heavy metals and toxic elements due to parent materials or point sources often occurs in a limited area and is easy to identify. Repeated use of metal-enriched chemicals, fertilizers, and organic amendments such as sewage sludge as well as wastewater may cause contamination at a large scale. A good example is the increased concentration of Cu and Zn in soils under long-term production of citrus and other fruit crops. Many chemical processes are involved in the transformation of trace elements in soils, but precipitation-dissolution, adsorption-desorption, and complexation are the most important processes controlling bioavailability and mobility of trace elements in soils. Both deficiency and toxicity of trace elements occur in agroecosystems. Application of trace elements in fertilizers is effective in correcting micronutrient deficiencies for crop production, whereas remediation of soils contaminated with metals is still costly and difficult although phytoremediation appears promising as a cost-effective approach. Soil microorganisms are the first living organisms subjected to the impacts of metal contamination. Being responsive and sensitive, changes in microbial biomass, activity, and community structure as a result of increased metal concentration in soil may be used as indicators of soil contamination or soil environmental quality. Future research needs to focus on the balance of trace elements in an agroecosystem, elaboration of soil chemical and biochemical parameters that can be used to diagnose soil contamination with or deficiency in trace elements, and quantification of trace metal transport from an agroecosystem to the environment.     

Levels, distribution and chemical forms of trace elements in food plants

The content of trace elements in plants can vary widely, depending upon the composition of the soil in which they grow, other environmental factors, and the species or cultivar of the plant. A high growth rate of the plant may cause internal 'dilution' of trace elements. Complex formation with soil organic colloids and compounds, cell wall material and ligands in and inside the cell membranes are of critical importance in uptake, though most evidence shows that it is the free metal ion in the external solution that is absorbed; the detailed mechanisms are still unknown. Other processes such as excretion of organic compounds, reductants and hydrogen ions from the root greatly alter availability of trace metals, and iron has to be reduced to the ferrous form before uptake. The mean composition of plant shoots is affected by age and season; element mobility in the xylem and phloem determines translocation, and hence concentrations in individual parts of the plant. The rate of retranslocation can be strongly affected by the abundance of the element. Symptoms of deficiency or excess are well documented, but are often not dependable. The essentiality of the trace metals depends upon their function as part of enzymes, and these are briefly reviewed, with stress on processes in plants. Only a small fraction of the total amount of an element is bound in the enzyme; of the remainder, some is present as the free metal ion (Mn) or as complexes of small molecular mass (Cu, Zn, Ni, Fe), the rest being bound to cell wall material. Certain species or genotypes have resistance against high levels of some elements in the soil. Several mechanisms may be involved, one being very strong binding to root cell walls. There are also large genetic differences in susceptibility to trace element deficiencies.     

Multivariate statistical analysis of nutrients and trace elements in plants and soil from northwestern Russia

Results are summarized of several field and greenhouse experiments designed to estimate differences in the ability of some plant species to take up from soil essential nutrients and various trace elements and transfer them from roots to upper plant parts. Instrumental neutron activation analysis was used to determine concentrations of 22 elements in plant and soil samples. Correlation and principal component analysis were applied for interpreting a large volume of experimental results. In many cases there was no statistically significant positive correlation between element concentrations in soil and concentrations of these elements in plants. Moreover, relationships between elements were often different in soil and in different plant parts, thereby suggesting quite different element behaviours in soil and in plants. Our experimental results and data published in the literature revealed that macro- and trace element concentrations might serve as a specific indicator of plant taxonomy, thus allowing for differentiation of the plants in accordance with concentrations of certain elements in roots or in leaves. Short-term variations in concentrations of elements typical for different plant species and factors affecting these variations indicated that diurnal dynamics of plant element concentrations were regular and species-specific.     

The Phytoremediation Potential of Native Plants on New Zealand Dairy Farms

Ecological restoration of marginal land and riparian zones in agricultural landscapes in New Zealand enhances the provision of above-ground ecosystem services. We investigated whether native endemic plant assemblages have remediation potential, through modifying soil nutrient and trace element mobility. Analysis of native plant foliage in situ indicated that selective uptake of a range of commonly deficient trace elements including Zn, B, Cu, Mn and Co could provide a browse crop to avoid deficiencies of these elements in livestock, although some native plants may enhance the risk of Mo and Cd toxicity. Native plant rhizospheres were found to modify soil physico-chemistry and are likely to influence lateral and vertical fluxes of chemical elements in drainage waters. Native plants on marginal land in agricultural landscapes could add value to dairy production systems whilst helping to resolve topical environmental issues.     

The biogeochemistry and anatomy of plants in areas of recent volcanism

Studies of the active Mendeleev Volcano on Kunashir Island have shown that the vents of volcanic gases and acidic thermal springs influence the soil geochemistry and the vegetation of the adjacent areas. Some species thrive even though they grow on a hot substratum and accumulate heavy metals, rare and trace elements, which bedrocks are enriched in. It has been shown how the anatomy of woody plants is adapted to habitats near the vents of volcanic gases and thermal springs.     

Chemical Speciation and Mobility of Some Trace Elements in Vermicomposted Fly Ash

Fly ash, a by-product of power plants, is currently being used extensively in India for soil amendment. However, the toxic elements sorbed in the fly ash might pose a serious threat to the environment, causing soil and water contamination. Vermicomposting of fly ash is expected to reduce the contamination of toxic trace metal and could improve the mobility of essential trace element. The current study is focused on characterizing different species of trace metals and their bio-availability in the vermicomposted fly ash (VCFA)-treated lateritic soil. As a fertilizer, different doses (10%, 20%, 30%, 40%, and 50%) of VCFA were applied to the soil and sequential extraction was carried out to analyze trace elements. In the different fractions, Cr < Mn < Pb < Fe were found to be sorbed more to Fe-Mn oxide-bound fractions, whereas Cd, Cu, and Zn were bound more to organic-matter-bound fractions; Cr and Ni were mostly bound to residual fraction. The Fe-Mn oxides and organic-matter-bound fractions may be bio-available with the appropriate environmental condition, whereas chromium and nickel mostly associated with residual fraction are very difficult to release into the environment. The mobility factor index showed the midlevel substitution (i.e., 10% to 30% of VCFA to lateritic soil) to be beneficial as these doses increased the bio-availability of some essential trace elements and restricted the availability toxic trace metals in the soil. At higher doses, the toxic trace metals were found to be released in the bio-available form, which could be hazardous to the environment.     

The uptake of trace elements by spinach and bean varieties of different root parameters

The uptake by plants of trace elements from the soil depends to a large extent on root characteristics and activities. Differences between plant species and varieties in the uptake of trace elements are well known. Less understood, however, are the mechanisms governing these differences and the relative significance of various root parameters.Spinach and bean varieties were, therefore, compared with respect to their root lengths and number of root apices, and to the uptake of Mn, Fe and Zn from soil-sand mixtures. The results showed significant differences among the varieties tested, both in root characteristics and in the uptake of trace elements. However, no relationships were evident between the trace element uptake by spinach varieties and their root characteristics. Contrary to this the Fe-uptake by bean varieies exhibited a clear dependence on the total number of root apices. The uptake of Mn and Zn showed a similar relationship with some exceptions. Whether the apical regions of individual roots are the most active sites of uptake or rather affect the solubility of trace elements will be the subject of further investigations.     

Long-Term Biomonitoring of Soil Contamination Using Poplar Trees: Accumulation of Trace Elements in Leaves and Fruits

Phytostabilization aims to immobilize soil contaminants using higher plants. The accumulation of trace elements in Populus alba leaves was monitored for 12 years after a mine spill. Concentrations of As and Pb significantly decreased, while concentrations of Cd and Zn did not significantly over time. Soil concentrations extracted by CaCl₂ were measured by ICP-OES and results of As and Pb were below the detection limit. Long-term biomonitoring of soil contamination using poplar leaves was proven to be better suited for the study of trace elements. Plants suitable for phytostabilization must also be able to survive and reproduce in contaminated soils. Concentrations of trace elements were also measured in P. alba fruiting catkins to determine the effect on its reproduction potential. Cadmium and Zn were found to accumulate in fruiting catkins, with the transfer coefficient for Cd significantly greater than Zn. It is possible for trace elements to translocate to seed, which presents a concern for seed germination, establishment and colonization. We conclude that white poplar is a suitable tree for long-term monitoring of soil contaminated with Cd and Zn, and for phytostabilization in riparian habitats, although some caution should be taken with the possible effects on the food web.

Supplemental materials are available for this article. Go to the publisher's online edition of International Journal of Phytoremediation to view the supplemental file.     

Trace elements in man.

It is likely that most, if not all, of the elements found to be essential in animals will be shown to be so for man, and the clinical picture produced by deficiency of the elements in the human patient will differ little from that seen in the animal, although this has been established for only five elements (I, Fe, Cu, Co and Zn). However, the link between lack of a given element in the soil and a human patient is far less direct and much more complex than that met with in the animal grazing on deficient pastures, except in isolated primitive communitis. Zn is the most protean of the trace elements and has been chosen to illustrate this in human practice. Excesses of essential elements (both trace and major) give rise to toxic effects and the importance of a proper balance especially of the transitional elements in the human diet is discussed with special reference to Cu, Zn and Fe. Certain non-essential trace elements are individual and community hazards: Cd, Pb and Hg are the principal offenders for humans. Mankind is now largely dependent on grassland products, cereals and livestock with increasing dominance of the former in human nutrition. This has reduced the bioavailability of trace elements so that study of trace metals, especially Zn and Cu, in skeletal and dental remains at human burial and occupation sites should prove useful in assessing the consequences of this striking change in dietary habits.     

The use of transgenic plants in the bioremediation of soils contaminated with trace elements

The use of plants to clean-up soils contaminated with trace elements could provide a cheap and sustainable technology for bioremediation. Field trials suggested that the rate of contaminant removal using conventional plants and growth conditions is insufficient. The introduction of novel traits into high biomass plants in a transgenic approach is a promising strategy for the development of effective phytoremediation technologies. This has been exemplified by generating plants able to convert organic and ionic forms of mercury into the less toxic, volatile, elemental mercury, a trait that occurs naturally only in some bacteria and not at all in plants. The engineering of a phytoremediator plant requires the optimization of a number of processes, including trace element mobilization in the soil, uptake into the root, detoxification and allocation within the plant. A number of transgenic plants have been generated in an attempt to modify the tolerance, uptake or homeostasis of trace elements. The phenotypes of these plants provide important insights for the improvement of engineering strategies. A better understanding, both of micronutrient acquisition and homeostasis, and of the genetic, biochemical and physiological basis of metal hyperaccumulation in plants, will be of key importance for the success of phytoremediation.     

Bioavailability index for quantitative evaluation of plant availability of extractable soil trace elements

A new index, Bioavailability Index (BI) and the corresponding experimental method were developed for quantitative evaluation of bioavailability of the extractable soil trace elements. Soils were first treated with various extractants (DTPA, HCl, NH₂OH·HCl+HCl) separately to remove the extractable elements. The soils after extraction were washed with deionised water to eliminate the extractant and its pH was adjusted with Ca0 and finally restored to its original pH level. Wheat (Triticum aestivum L.) and rape (Brassica chinensis) were planted in the untreated and treated soils for 8 weeks. The concentrations of the trace elements in plants were determined after harvest. Nutrient accumulation by plants is significantly reduced due to removal of extractable trace elements from the soil. BI of the extractable fraction was proportional to the ratio of plant accumulation reduction to trace element extractability. In the present study, BI value of the total content of soil trace elements was designated as 1. Though only a minor fraction of the total soil nutrient, generally less than 5%, was removed by DTPA, the nutrient accumulation by plants, especially for wheat, was reduced greatly, leading to relatively large BI values. For wheat, the average BI values of the eight nutrients Cu, Mn, Zn, Ni, Co, Pb, Cr, and V were found to be 22.7, 17.6 and 17.4 for the three testing soils, and for rape, the corresponding values of 8.9, 10.0 and 11.0 were obtained, indicating that the DTPA-extractable elements represent the highly available fraction of the total content. The BI values for HCl-extractable elements were much lower compared with those for DTPA. For wheat, the average BI values for the three soils are 2.0, 1.9 and 2.4, and for rape, the corresponding values are 4.8, 4.1 and 3.7. The high availability of DTPA-extractable trace elements and relatively low availability of HCl-extractable trace elements highlight the significant role that chelation action might play in plant nutrient acquisition. The different responses of wheat and rape to the soils previously subjected to the same extraction procedure could be explained by their genotypical differences in sensitivity to nutrient deficiencies. The quantitative nature of BI makes it valuable in the study of nutrient bioavailability and plant accumulation mechanisms.     

Soil as the source of trace elements

The nature of the parent rock determines the trace element content of soils. Ultrabasic and basic rocks, which solidified first from the molten magma, incorporated bioessential trace elements such as Co, Ni, Zn and Cr by isomorphous replacement of Fe and Mg in ferromagnesian minerals, while acidic rocks, the last to solidify, tended to be richer in other elements such as Ba and Pb. Cu, Mn and, to a lesser extent, B, Mo and Se are more evenly distributed. The weathering of rocks by pedological and biological processes such as glacial and hydrodynamic comminution, secretion of acids and liganding species by microbes and plants leads to the formation of sands, silts and clays, and finally the incorporation of organic matter causes humification and the formation of soils as we know them. Part of the soil's store of bioessential elements is held in forms that are available to plants. Availability is controlled by the forms of occurrence and the nature of binding of the trace elements in the soil, which in turn is affected by soil acidity, redox balance (drainage) and organic matter content. These and other factors are discussed along with measures for alleviation of deficiency problems. Future progress in this area will depend to a large extent on interdisciplinary research by biologists, chemists, physicists and statisticians.     

The effect of sulfur-containing nitrogen fertilizers on the elemental composition of celery (Apium graveolens) grown on a polluted marsh soil

Summary In order to investigate elemental composition of celery, and to quantify the influence of sulfur-containing N-fertilizers on the trace element uptake, a field trial with celery was carried out on marsh soil polluted with municipal wastes. The research yielded the following results:Compared to leaves at harvest time, bulbs showed significantly lower concentrations of Mo, S and Sb, but higher contents of B, Br and Cr and Cu. Since the acidifying effect of the fertilizers was suppressed by the free calcium carbonate in the soil, no significant changes in concentrations of cationic trace elements were detected in plants fertilized with ammonium sulfate compared to those which received urea or calcium ammonium nitrate. On the other hand, in these plants the conspicuous increase in total sulfur was accompanied by a significant decrease in concentrations of up to 30% for B, Br and Sb, 50% for As, 60% for Se and 80% for Mo.According to these results, in plant production on contaminated soils certain plant parts may be marketable due to their low tendency to accumulate toxic elements, and furthermore it may be feasible to reduce the contents of some of these elements in plants by the use of sulfur-containing fertilizers.     

Role of soil microbes in the rhizospheres of plants growing on trace metal contaminated soils in phytoremediation

This article reviews recent developments in in situ bioremediation of trace metal contaminated soils, with particular reference to the microbial dynamics in the rhizospheres of plants growing on such soils and their significance in phytoremediation. In non-agricultural conditions, the natural role of plant growth promoting rhizobacteria (PGPR), P-solubilizing bacteria, mycorrhizal-helping bacteria (MHB) and arbuscular mycorrhizal fungi (AMF) in maintaining soil fertility is more important than in conventional agriculture, horticulture, and forestry where higher use of agrochemicals minimize their significance. These microbes initiate a concerted action when a particular population density is achieved, i.e. quorum sensing. AMF also recognize their host by signals released by host roots, allowing a functional symbiosis. AM fungi produce an insoluble glycoprotein, glomalin, which sequester trace elements and it should be considered for biostabilization leading to remediation of contaminated soils. Conclusions drawn from studies of metal uptake kinetics in solution cultures may not be valid for more complex field conditions and use of some combination of glasshouse and field experiments with organisms that occur within the same plant community is suggested. Phytoextraction strategies, such as inoculation of plants to be used for phytoremediation with appropriate heavy metal adapted rhizobial microflora, co-cropping system involving a non-mycorrhizal hyperaccumulator plant and a non-accumulator but mycorrhizal with appropriate AMF, or pre-cropping with mycotrophic crop systems to optimize phytoremediation processes, merit further field level investigations. There is also a need to improve our understanding of the mechanisms involved in transfer and mobilization of trace elements by rhizosphere microbiota and to conduct research on selection of microbial isolates from rhizosphere of plants growing on heavy metal contaminated soils for specific restoration programmes. This is necessary if we are to improve the chances of successful phytoremediation.     

Role of soil microbes in the rhizospheres of plants growing on trace metal contaminated soils in phytoremediation

This article reviews recent developments in in situ bioremediation of trace metal contaminated soils, with particular reference to the microbial dynamics in the rhizospheres of plants growing on such soils and their significance in phytoremediation. In non-agricultural conditions, the natural role of plant growth promoting rhizobacteria (PGPR), P-solubilizing bacteria, mycorrhizal-helping bacteria (MHB) and arbuscular mycorrhizal fungi (AMF) in maintaining soil fertility is more important than in conventional agriculture, horticulture, and forestry where higher use of agrochemicals minimize their significance. These microbes initiate a concerted action when a particular population density is achieved, i.e. quorum sensing. AMF also recognize their host by signals released by host roots, allowing a functional symbiosis. AM fungi produce an insoluble glycoprotein, glomalin, which sequester trace elements and it should be considered for biostabilization leading to remediation of contaminated soils. Conclusions drawn from studies of metal uptake kinetics in solution cultures may not be valid for more complex field conditions and use of some combination of glasshouse and field experiments with organisms that occur within the same plant community is suggested. Phytoextraction strategies, such as inoculation of plants to be used for phytoremediation with appropriate heavy metal adapted rhizobial microflora, co-cropping system involving a non-mycorrhizal hyperaccumulator plant and a non-accumulator but mycorrhizal with appropriate AMF, or pre-cropping with mycotrophic crop systems to optimize phytoremediation processes, merit further field level investigations. There is also a need to improve our understanding of the mechanisms involved in transfer and mobilization of trace elements by rhizosphere microbiota and to conduct research on selection of microbial isolates from rhizosphere of plants growing on heavy metal contaminated soils for specific restoration programmes. This is necessary if we are to improve the chances of successful phytoremediation.     

Study of Zn,Cu and Pb content in plants and contaminated soils in Sardinia

Trace elements in soils exist as components of several different fractions. We have analyzed the correlation between total and extractable (EDTA, calcium chloride and deionized water) Zn, Pb and Cu concentrations in soils and the concentration of these elements in plant leaves. Soil and plant samples have been taken from Sulcis-Iglesiente (Sardinia), an area rich in mining tailings. This has made that the concentrations of the trace element under study in soils were varied. Three plants have been studied: Dittrichia viscosa, Cistus salviifolius, and Euphorbia pithyusa subsp. cupanii. Soil samples beneath each of them at depths of 0–30 and 30–60 cm have been considered. The highest concentration of trace elements in the leaves of the studied species has been found for Zn. The calcium carbonate content and the crystalline and amorphous forms of iron in the soil have determined the concentration of metal in plant leaves. The soil concentrations that have been found with the extraction methods are uncorrelated with Pb and Cu concentrations in plants, but Zn is correlated with the fraction extracted with EDTA and calcium chloride. The concentrations of trace metals in plants are most closely related to the soil contents of CaCO₃, electrical conductivity, Fe_ox, and Fe_dc.     

The scientific and practical importance of trace elements

E. J. Underwood's discovery of the essentially of cobalt for ruminant animals is the classic example of the vast benefits to agricultural production of research into the nutritional significance of trace elements. The extension of this discovery, culminating in the identification of vitamin B12, resulted in similar benefits for human health, notably the conquest of pernicious anaemia. Since then, additional essential trace elements have been discovered. Deficiency or imbalance, whether occurring naturally or from human activities, has been shown to present significant problems for the health of man and animals. Essentiality has been proved for a rapidly growing range of 'new' trace elements, whose biochemical mechanisms of action and implications for human health are unknown. In spite of an increasing knowledge of significant changes in the exposure of man and animals to trace elements from diet and environment, the concern of nutrition policy planners for inorganic micro-nutrients remains overshadowed by that for the bulk components of the diet. The application of existing knowledge of trace element nutrition to problems of human and animal health will depend on a clear understanding of events that link molecular, biochemical mechanisms to the clinical manifestation of deficiencies.     

Effects of nitrogen and water addition on trace element stoichiometry in five grassland species

A 9-year manipulative experiment with nitrogen (N) and water addition, simulating increasing N deposition and changing precipitation regime, was conducted to investigate the bioavailability of trace elements, iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn) in soil, and their uptake by plants under the two environmental change factors in a semi-arid grassland of Inner Mongolia. We measured concentrations of trace elements in soil and in foliage of five common herbaceous species including 3 forbs and 2 grasses. In addition, bioaccumulation factors (BAF, the ratio of the chemical concentration in the organism and the chemical concentration in the growth substrate) and foliar Fe:Mn ratio in each plant was calculated. Our results showed that soil available Fe, Mn and Cu concentrations increased under N addition and were negatively correlated with both soil pH and cation exchange capacity. Water addition partly counteracted the positive effects of N addition on available trace element concentrations in the soil. Foliar Mn, Cu and Zn concentrations increased but Fe concentration decreased with N addition, resulting in foliar elemental imbalances among Fe and other selected trace elements. Water addition alleviated the effect of N addition. Forbs are more likely to suffer from Mn toxicity and Fe deficiency than grass species, indicating more sensitivity to changing elemental bioavailability in soil. Our results suggested that soil acidification due to N deposition may accelerate trace element cycling and lead to elemental imbalance in soil–plant systems of semi-arid grasslands and these impacts of N deposition on semi-arid grasslands were affected by water addition. These findings indicate an important role for soil trace elements in maintaining ecosystem functions associated with atmospheric N deposition and changing precipitation regimes in the future.     

Physiological and molecular advances in magnesium nutrition of plants

Plant and Soil - Phytoremediation of soil contaminated by trace elements is a technology using plants and microorganisms to sequester, inactivate, or extract contaminants from the soil. The...