BackgroundZinc (Zn) and cadmium (Cd) compete with each other for adsorption on the cell wall-charged sites, translocation via plasma membrane transporters, and storage in vacuoles. Therefore, the subcellular distribution of Cd is suggested to be under the influence of Cd:Zn ratio in root-growing media.AimSubcellular fractionation of Cd and Zn in lettuce roots and its contribution to shoot Cd and Zn accumulation at low to phytotoxic levels of Cd2+ and Zn2+ activities was investigated.MethodsAn EGTA-buffered nutrient solution was used to provide the desired activity of free metal cations. Higher distributions of Cd in root cell wall in comparison with soluble and organelle fractions demonstrated Cd retention capacity of root apoplasmic spaces.ResultsAt each level of Cd2+ activity, the elevated activity of Zn2+ in the root growth solution caused a significant reduction of Cd concentration in cell wall and soluble fractions, whereas organelles' Cd fraction increased. In contrast, at each constant Zn2+ activity, by increasing Cd2+ activity in nutrient solution, cell wall Zn fraction was decreased, whereas the soluble and organelles' Zn fractions were increased or remained unchanged. The highest concentrations of Cd in the cell wall (34.6 mu g g-1 FW), organelles (5.64 mu g g-1 FW), and soluble (12.3 mu g g-1 FW) fractions were found at the Cd-to-Zn ratios of 0.8, 0.1, and 0.8, respectively.ConclusionsThe subcellular partitioning of Cd in lettuce root is highly influenced by Cd:Zn ratio in the root media, and increasing Zn substantially reduced the overall uptake-translocation of Cd to shoots.

Metals and metalloids (hereafter, metal(loid)s) in plant-based foods are a source of exposure to humans, but not all metal(loid)-food interactions are the same. Differences exist between metal(loid)s in terms of their behavior in soils and in how they are taken up by plants and stored in the edible plant tissue/food. Thus, there cannot be one consistent solution to reducing toxic metal(loid)s exposure to humans from foods. In addition, how metal(loid)s are absorbed, distributed, metabolized, and excreted by the human body differs based on both the metal(loid), other elements and nutrients in the food, and the nutritional status of the human. Initiatives like the United States Food and Drug Administration's Closer to Zero initiative to reduce the exposure of young children to the toxic elements cadmium, lead, arsenic, and mercury from foods warrant careful consideration of each metal(loid) and plant interaction. This review explores such plant-metal(loid) interactions using the example of spinach and the metals cadmium and lead. This review highlights differences in the magnitude of exposure, bioavailability, and the practicality of mitigation strategies while outlining research gaps and future needs. A focus on feasibility and producer needs, informed via stakeholder interviews, emphasizes the need for better analytical testing facilities and grower and consumer education. More research should focus on minimization of chloride inputs for leafy greens to lessen plant-availability of Cd and the role of oxalate in reducing Cd bioavailability from spinach. These findings are applicable to other leafy greens (e.g., kale, lettuce), but not for other plants or metal(loid)s.

Background When studying metallophytes and hyperaccumulator plants, it is often desired to assess the level of tolerance of a specific trace metal/metalloid in a putative tolerant species, to determine root and shoot accumulation of the trace metal/metalloid of interest, or to establish whether a trace metal/metalloid has an essential function. The use of hydroponics has proven to be a powerful tool in answering such questions in relation to the physiological regulation of metal/metalloids in plants. Carefully designing experiments requires considering nutrient solution formulation, dose rate regime, and environmental conditions, but this is often overlooked. Aims This review aims to bring together key information for hydroponics studies in physiological, evolutionary, and genetics/molecular biological research of trace metal/metalloid tolerance and accumulation in plants, focussing on metallophytes and hyperaccumulator plants. Conclusions It is not possible to define a ‘universal’ nutrient solution that is both sufficient and non-toxic for all plants, although it is often possible, dependent on plant species under study and the research question to be addressed, to ‘adapt’ commonly used ‘standard formulations’. Well-designed and executed hydroponics experiments can yield powerful insights in the regulation of essential and toxic metal/metalloid trace elements, and this extends far beyond hyperaccumulator plants.

Background and aims Nickel (Ni) deficiency has been reported to occur in soybean ( Glycine max ) grown on leached tropical soils in Brazil. We aimed to determine whether an internal or external Ni supply can compensate for low Ni within the seed by assessing whether the amount of Ni in the seed whether the foliar-application of aqueous NiSO 4 influenced the uptake of Ni by the leaf, the nutritional status of the plant, urease activity and growth. Methods We used Ni-depleted seeds (<0.35 μg Ni per g) and Ni-sufficient seeds (11.1 μg Ni g −1 ) for hydroponic experiments. Seedlings were grown either with or without an external Ni supply (0 or 0.85 μM Ni in nutrient solution) and either with or without an internal Ni supply (with or cotyledons removed). In addition, we used synchrotron-based micro-X-ray fluorescence analysis to examine the distribution of foliar-applied Ni (50 and 100 mg L -1 ). Key results Leaf Ni concentration and urease activity were both enhanced by increasing either the internal (cotyledon seed store) or external (solution) Ni supply. In addition, plants derived from Ni-depleted seed that received external Ni supply had 9.2% higher biomass relative to plants derived from Ni-sufficient seeds which received Ni. When foliar-applied, Ni accumulated in the pedicles of the trichomes within 15 minutes of application, and then moved to the vascular bundles before dispersing further into tissues within 3 hours. Conclusions Trichomes are an important pathway for foliar Ni absorption in soybean, but there are still major knowledge gaps our understanding of the physiological function of trichomes in the uptake of metal ions from foliar micro-nutrient treatments.

Potential for lifetime risk from food cadmium has been the focus of regulations ever since Cd poisoning of subsistence rice farm families was identified in Japan. But research has shown that absorption of Cd from subsistence rice diets is very different from other diets/foods. Further, much dietary Cd risk assessment has not considered the normal association of Zn with Cd in soils and foods with at least 100-fold higher Zn than Cd in most soils and crops except rice. In the current experiments, lettuce was grown to maturity on soil amended with biosolids which had been spiked with Cd and Zn, equilibrated, and then incorporated into field plots. Lettuce was grown to maturity, dried, ground and mixed with complete diets for rats. Rats were fed the test diets for 90 days. Strong inhibition of lettuce accumulation of Cd resulted from increased soil Zn (and reduced Cd:Zn ratio). Rat liver and kidney Cd levels were markedly affected by Cd:Zn ratio in the lettuce; the lowest ratio caused little Cd change compared to zero added Cd control, while the highest Cd:Zn treatment caused adverse effects on liver cells. These results strongly confirm results from previous studies of biosolids fertilized crops fed to test animals, and confirm that regulations on Cd in biosolids and in foods should be based on the bioavailable Cd in crops, which is strongly reduced by increased levels of Zn and (low level of Cd:Zn ratio) in soils and fertilizer amendments.

Odontarrhena corsica was grown for three months on Chrome loam topsoil and subsoil from near Reisterstown, MD, to examine the effects of varying soil masses (2.8 and 5.6 kg pot-1) and soil layers (topsoil vs. subsoil) on plant growth and Ni accumulation. The subsoil position effect was simulated by placing a pot of topsoil on top of a pot filled with subsoil. Shoot Ni concentrations were similar for all treatments at 7 g Ni kg-1. Shoot yield was significantly higher in the 5.6 kg treatments compared to the 2.8 kg treatments (>18 g pot-1 vs. ∼12 g pot-1) and also greater in the topsoil treatment compared to the subsoil treatment (24.0 g pot-1 vs. 18.6 g pot-1), resulting in significantly higher phytomining. Soil depth had no statistically significant effect on shoot and root yield. Subsoil fertilization increased yield (25.8 g pot-1 vs. 19.7 g pot-1), enough to suggest that further research is warranted to optimize Ni phytomining. This study confirms the importance of soil volume and root access to the subsoil when evaluating the potential for Ni phytomining by Odontarrhena species. The use of small pots may lead to an underestimation of phytomining potential.

Arsenic exposure through rice consumption is a growing concern. Compared to Continuous Flooding (CF), irrigation practices that dry the soil at least once during the growing season [referred to here as Alternate Wetting and Drying (AWD)] can decrease As accumulation in grain; however, this can simultaneously increase grain Cd to potentially unsafe levels. We modelled grain As and Cd from field studies comparing AWD and CF to identify optimal AWD practices to minimize the accumulation of As and Cd in grain. The severity of soil drying during AWD drying event(s), quantified as soil water potential (SWP), was the main factor leading to a reduction in grain total As and inorganic As, compared to CF. However, lower SWP levels were necessary to decrease grain inorganic As, compared to total As. Therefore, if the goal is to decrease grain inorganic As, the soil needs to be dried further than it would for decreasing total As alone. The main factor driving grain Cd accumulation was when AWD was practiced during the season. Higher grain Cd levels were observed when AWD occurred during the early reproductive stage. Further, higher Cd levels were observed when AWD spanned multiple rice growth stages, compared to one stage. If Cd levels are concerning, the minimum trade-off between total As and Cd accumulation in rice grain occurred when AWD was implemented at a SWP of -47 kPa during one stage other than the early reproductive. While these results are not meant to be comprehensive of all the interactions affecting the As and Cd dynamics in rice systems, they can be used as a first guide for implementing AWD practices with the goal of minimizing the accumulation of As and Cd in rice grain.

It has been recognized for many years that plants differ remarkably in relative adaptation to limiting soil mineral problems. Research has shown that cultivars of soybean can differ in both ability to obtain nutrients from a soil and to utilize the absorbed nutrients to produce plant dry matter and yield. Deficiencies of microelements occur in some crops on local or regional problem soils where the soybean is often grown in rotation, and soybean plants suffer Mn, Fe, and possibly Mo and B on particular soils. Plant breeding was considered soon after chlorosis was noted. One of these lines has been used for many research studies and helped scientists recognize the importance of plant breeding to solve soil problems. Several soybean nutrition problems have been identified as clearly requiring plant breeding to correct serious, extensive, yield loss–Fe-deficiency chlorosis on calcareous soils, and Al toxicity, which limits rooting into strongly acid sub-soils.

Excessive Cd accumulation in rice grain has caused chronic Cd diseases in humans. In most crops, 100 times more Zn than Cd strongly inhibits Cd uptake and translocation. However, this response is not found for rice (Oryza sativa L.), which was found to have an unusual Cd uptake pattern compared with other crops, such as spinach (Spinacia oleracea L.). Moreover, studies on shared transporters between Zn and Cd using normal solution experiments with traditional high concentrations of metal ions may result in irrelevant interactions. Therefore, we developed ethyleneglycoltetraacetate-buffered nutrient solutions in this work. Rice and spinach seedlings were grown under calibrated low Cd2+ activity and low to phytotoxic Zn2+ activity levels while buffering other micronutrient cations at sufficient levels. Results showed that as rice grew with pZn(2+) = 8.1-5.4, root Cd and shoot Ni decreased significantly and gradually. However, shoot Cd and Mn in rice decreased slightly with the increase of solution Zn2+ from deficiency to sufficiency and then increased at toxic Zn2+ solution (pZn(2+) = 5.4). The shoot/root ratios of Cd in rice under toxic pZn(2+) (5.6 and 5.4 pZn(2+) activity) were significantly increased (p < .05). It could be concluded that rice absorption of Cd is not inhibited by co-contaminating (toxic) Zn. For spinach, with Zn varying from pZn(2+) = 8.1-5.7, both shoot and root Cd substantially decreased, as did shoot Ni. This work revealed that, to understand food chain Cd risks, one needs to consider the inhibitory role of Zn in limiting Cd absorption in all crops studied except rice.

Health risks caused by food containing Cd is a concern worldwide. Interaction between Mn and Cd has been widely studied in normal hydroponic solution with high ion activities (e.g., the study on sharing of transporter Natural Resistance-Associated Macrophage Protein 5 between Mn and Cd in rice [Oryza sativa L.]). However, interaction of Mn and Cd in crops like rice and spinach (Spinacia oleracea L.) at field ion activity level is still unknown. Thus, an ethyleneglycoltetraacetate-buffered solution experiment was conducted to explore the effect of Mn on the uptake and accumulation of Cd and other mineral elements in rice and spinach. In rice, antagonism of Mn and Cd was only observed in roots at deficient and toxic levels of external Mn2+ . Compared with those at Mn2+ sufficiency (pMn2+ 6.7-5.3), average root Cd levels were elevated significantly by 1.85-3.05 times at Mn2+ deficiency (pMn2+ 8.2) but decreased by 1.57-2.59 times at Mn2+ toxicity (pMn2+ 4.8). The antagonism between Mn and K/Mg in rice shoots might be caused by their common role in physiological processes in plants. Antagonism of Mn/Ni in spinach in this work was consistent with their shared transporters in dicots. Results about the antagonism of root Cd/Mn at Mn2+ deficiency suggest that sufficiently available Mn2+ is significant to reduce Cd uptake in rice under field levels of ion activity, but it was not for spinach because the change of tissue Cd was insignificant with the increase of Mn2+ activity from deficiency to toxicity.

Background and Aims In tropical ultramafic soils, potassium (K) is typically the most growth limiting nutrient. However, tropical nickel (Ni) hyperaccumulator plants, including Phyllanthus rufuschaneyi and Rinorea cf. bengalensis (which are ‘metal crops’ used in agromining) from Malaysia , have unusually high K shoot accumulation compared to other species, despite naturally growing on severely K-impoverished ultramafic soils. This study aimed to establish the response to soil K availability in relation to uptake of K and other elements in the roots and shoots of P. rufuschaneyi and R. cf. bengalensis. Methodology We undertook an experiment in which soluble K was dosed to ultramafic soil in pots with P. rufuschaneyi and R. cf. bengalensis in Sabah (Malaysia). Results The results show that root K concentrations increased markedly as the soil K availability increased by 35-fold, whilst the corresponding effect on K accumulation in the shoots of P. rufuschaneyi and R. cf. bengalensis was not significantly different in relation to soil K dosing. Observed divergent responses between root and shoot K accumulation in these species suggests a separate genetic control of K uptake and xylem loading in P. rufuschaneyi and R. cf. bengalensis . Conclusion The tight control of root-to-shoot K translocation and constrained K accumulation in shoots under a soil K gradient is likely an adaptive mechanism to the evolution of these species to grow in highly nutrient-impoverished ultramafic soils. This study provides information that will be useful for better nutrient management of tropical Ni metal farms that use K-efficient Ni ‘metal crops’.

Background and aims Phosphorus is one of the major nutrients that directly or indirectly affects all aspects of plant growth. Tropical nickel hyperaccumulators, including Phyllanthus rufuschaneyi and Rinorea cf . bengalensis from Borneo Island (in the Malaysian state of Sabah), have evolved to grow in extremely P-impoverished ultramafic soils. This study aimed to establish the response of the root and shoot ionome of these two agromining ‘metal crops’ to soil P availability. Methods We undertook a soil P dosing trial on P. rufuschaneyi and R. cf. bengalensis in Sabah (Malaysia) over a period of 12 months. We measured the elemental concentrations in the soil solution and roots, as well as in the developed and developing stems and leaves. Results The results show that root and shoot P accumulation increased markedly as soil P availability increased by 80-fold in P. rufuschaneyi , whereas R. cf. bengalensis did not increase as strongly to P supply, despite a 135-fold increase in soil solution P. The contrasting observations on the root and shoot P accumulation patterns in these species suggests distinct P acquisition strategies in these species. The non-responsiveness of R. cf. bengalensis to increasing soil P availability may be related to its possible association with mycorrhizal fungi. Conclusion The findings of this study reveal the complexities of phosphate uptake, transport and accumulation in these ‘metal crops’. This information is essential to develop appropriate nutrient management in nickel agromining operations.

Substantial unrealized opportunities exist for economic Ni agromining in the tropics. However, until recently this technology has remained relatively unexploited in this part of the world. In this chapter, we discuss the progress of tropical Ni agromining in two regions, namely Southeast Asia and the neotropical region. Significant advances have been made in Ni agromining operations in Southeast Asia, particularly Sabah (Malaysia), in the past few years: (i) exploring for suitable locations, (ii) screening for hyperaccumulator plants in native flora, (iii) selecting candidate hyperaccumulator species with high biomass production and shoot Ni concentrations ('metal crops'), (iv) testing the agronomy of 'metal crops' to be used in viable agromining operations, and (v) demonstration of real-life agromining operations at field scale. The two most promising 'metal crops' in Sabah are Phyllanthus rufuschaneyi and Rinorea cf. bengalensis. However, Ni agromining developments in neotropical regions are still in their infant stages. Preliminary investigations have led to the discovery of a potential 'metal crop', Blepharidium guatemalense, having highly desirable attributes for possible Ni agromining. It is envisaged that tropical Ni agromining could be a productive alternative land use in these regions, and as such will significantly improve the livelihood of potential local 'metal farmers'.

Background Phytoextraction is an in situ technique that can be applied to minerals and mining wastes using hyperaccumulator plants to purposely bio-concentrate high levels of metals or metalloids into their shoots in order to remove them from the substrate, while achieving monetary gain. Phytoextraction can be applied to a limited number of elements depending on the existence of hyperaccumulator plants with suitable characteristics. Although phytoextraction has been trialled in experimental settings, it requires testing at field scale to assess commercial broad-scale potential. Scope The novelty and purported environmental benefits of phytoextraction have attracted substantial scientific inquiry. The main limitation of phytoextraction with hyperaccumulators is the number of suitable plants with a high accumulation capacity for a target element. We outline the main considerations for applying phytoextraction using selected elemental case studies in which key characteristics of the element, hyperaccumulation and economic considerations are evaluated. Conclusions The metals cobalt, cadmium, thallium and rhenium and the metalloids arsenic and selenium are present in many types of minerals wastes, especially base metal mining tailings, at concentrations amenable for economic phytoextraction. Phytoextraction should focus on the most toxic elements (arsenic, cadmium, and thallium) or especially valuable elements (selenium, cobalt, and rhenium). The value proposition is in the clean-up of contaminated land in the case of toxic elements, whereas it is in the 'bio-ore' generated by the process in the case of valuable elements.

Knowledge on the ecophysiology of cobalt hyperaccumulator species is limited. The nickel hyperaccumulator Rinorea cf. bengalensis from Borneo can accumulate high concentrations of cobalt in nature. This study investigates the cobalt accumulation potential of Rinorea cf. bengalensis in relation to nickel concentrations in soils and the subsequent tissue and cellular-level distributions of cobalt, nickel and major cations. Seedlings of Rinorea cf. bengalensis were grown in mixed treatments on ultramafic soil containing a high concentration of available nickel. Cobalt and nickel salts were then added to the soil to study their interactions. The tissue and cellular-level distributions of cobalt, calcium, nickel, and potassium were investigated using synchrotron-based X-ray fluorescence microscopy. The maximum foliar cobalt concentration reached 1200 μg g−1. Accumulation of cobalt competed with nickel accumulation although nickel seems to stimulate cobalt phloem translocation. Plants suffered toxicity in the treatment with the highest soil cobalt concentration. Cobalt and nickel have contrasting distribution patterns in the leaves of Rinorea cf. bengalensis, with cobalt mainly excreted on the surface of the leaves, whereas nickel is localised in foliar epidermal cells. Rinorea cf. bengalensis can accumulate high concentrations of cobalt, but is intrinsically more tolerant to nickel. It does not rely on a similar sequestration mechanism for both metals, which could explain the lesser tolerance for cobalt. Nickel appears to be essential for the plant to tolerate high cobalt concentrations. Further studies intending to develop agronomic practices are needed to determine the viability of Rinorea cf. bengalensis for nickel-cobalt agromining.

This study investigated the physico-geochemical properties of three types of ultramafic substrates in Sabah (Malaysia) and further characterised their influence on nickel (Ni) accumulation in two selected tropical ‘metal crops’ (Phyllanthus rufuschaneyi and Rinorea cf. bengalensis). Three experiments, consisting of a randomised block growth trial in large pots over 12 months in Sabah, were conducted: i) soil ratio experiment (Cambisol:Ferralsol:Leptosol), ii) soluble Ni dosing experiment with added 0, 60, 240, 600 mg Ni kg−1, and iii) soil pH adjustments (pH 5.2, 5.8 and 6.4) experiment. The results show that the low pH status of Ferralsol did not result in increased extractable Ni concentrations, indicating that Ni-bearing phases and mineralogy play a major role in Ni extractability in these substrates. The increasing order of extractable Ni concentrations was Ferralsol < Leptosol < Cambisol. The selected ‘metal crops’ had remarkable shoot biomass (~40 g pot−1) and foliar Ni concentrations (>15 g kg−1 in P. rufuschaneyi) in the Cambisol, >5-fold higher relative to that recorded in Leptosol and Ferralsol. Mixing the Cambisol with the other substrates significantly improved Ni yield in both species relative to the individual substrates. The effect of Ni addition on the selected ‘metal crops’ was species-dependent, whereas reduction of soil pH in Cambisol significantly reduced Ni yield in both species. Nickel accumulation patterns in ‘metal crops’ in response to the diverse ultramafic soils in the tropics are influenced by the physico-geochemical characteristics of the soil types. Soil Ni extractability in these soils is more dependent on the Ni-bearing phases and mineralogy rather than the pH status. This study provides useful information on plant-soil interactions required for identifying suitable substrates for implementing tropical Ni agromining.

Agromining is an emerging technology that utilizes selected 'metal crops' (= hyperaccumulator plants) to extract valuable target metals from unconventional resources for profit from mineralised soils. Growth characteristics, shoot metal concentrations, and agrominable locations are important considerations in economic agromining. Globally, the greatest potential for nickel (Ni) agromining exists in the tropics. However, the agronomic systems of tropical 'metal crops' have not been previously tested. Furthermore, it is currently unknown whether nutrient dosing of prospective tropical agromining Ni 'metal crops' could possibly cause a shoot Ni-dilution effect which may ultimately limit economically viable Ni yields. We undertook a pioneering study on Ni uptake and growth responses to nutrient dosing in two promising tropical 'metal crops' (Phyllanthus rufuschaneyi and Rinorea cf. bengalensis). The experiment consisted of a large randomised block growth trial in large pots over 12 months in Sabah (Malaysia). At 3-month intervals, the plants were exposed to soluble treatments that altered available concentrations of nitrogen (N), phosphorus (P), and potassium (K). We found strong positive growth responses to N and P additions in P. rufuschaneyi, whereas K additions had negative growth effects. In R. cf. bengalensis, all treatments had positive growth effects. The increases in biomass in response to nutrient dosing did not significantly reduce shoot Ni concentrations in both species, with the exception of N addition in P. rufuschaneyi. This study reveals that Ni uptake and growth responses to nutrient dosing are species-dependent, primarily influenced by the ecophysiology of the species. Inorganic fertilization could possibly be an important component of the management of local 'metal crops' to be used in viable commercial agromining in the tropics, but this needs to be tested in the field with different formulations of N, P, and K.

Previous research has shown that ground rubber from tyres can be used to supply fertiliser zinc (Zn) for prevention of Zn-deficiency in crops, and that inoculation of the ground rubber with several bacterial species hastens the release of Zn to the soil. We evaluated the ability of several microbial combinations to speed the release of Zn from ground rubber and to decrease soil pH to favour phytoavailability of Zn to crops. In a batch experiment, treatment combinations of two rates of ground crumb rubber (nil or 300mg kg–1, equal to 0 or 3.4mg Zn kg–1) and 24 bacterial inoculants were incorporated into a Zn-deficient calcareous soil. In a pot experiment, two wheat cultivars were grown on the soil without or with ground rubber amendment or with equivalent Zn from ZnSO4 (15mg kg–1) in combination with two selected microbial treatments. All microbial treatments significantly decreased soil pH at week 3, most notably the inoculant comprising Rhodococcus erythropolis and Acinetobacter calcoaceticus (RA)+Pseudomonas putida P41 (P1)+mixed Thiobacillus spp. (Mt). In the presence of tyre rubber, soil pH at week 10 was still significantly lower than the initial value, and soil DTPA-extractable Zn concentration increased until week 6 and then remained unchanged or slightly reduced at week 10. The greatest increase in DTPA-Zn concentration occurred with the RA inoculation. Microbial inoculation treatments were classified by cluster analysis into eight groups based on soil pH and concentrations of iron (Fe) and Zn. Group 8 produced the lowest pH and highest concentrations of DTPA-Fe ( average 6.92mg kg–1) and DTPA-Zn (average 2.67mg kg–1). Inoculations with RA and with RA+P1+T. thioparus were the most effective in hastening an increase in DTPA-extractable Zn and significantly enhanced Zn uptake by wheat plants, whereas inoculations with P. putida P168 and with RA+P2+Mt were most effective in decreasing soil pH and increasing plant Fe concentration.

Cadmium is a nephrotoxic metal with no known biological function in man. The diet is the main source of cadmium exposure. Smokers may accumulate more Cd through the inhalation of cadmium-containing tobacco smoke (smoking doubles kidney Cd at age 50) than thru diet. The metal is present in all soils and foods, but is in highest concentrations in shellfish, liver, kidney, oil seeds, cocoa beans, and certain wild mushrooms. However, more than 80% of the diet-cadmium comes, however, from cereals, vegetables, and potatoes. Risk from diet Cd is related to the “bioavailable” Cd, not total Cd in foods. Research has shown that the bioavailability (net absorption) of diet Cd is highly affected by levels of Fe, Zn, Ca, phytate and fiber in diets and foods. Nearly all Cd enrichment in the environment is accompanied by 100-fold higher Zn; soil Zn inhibits both Cd uptake by crops and bioavailability of crop Cd in the intestine for all foods except rice. All foods except rice with relatively high Cd levels also contain high levels of Zn and Fe which inhibit Cd absorption by humans from those foods. Long term consumption of rice home-grown on soils Cd-contaminated by mining or smelting of Zn, Pb or Cu ores has caused the major significant diet Cd risks to humans (other than Cd industry workers). Rice is deficient in Fe, Zn and Ca for adequate human nutrition, greatly increasing Cd absorption from rice-based diets. Essentially all humans who suffered Cd induced kidney disease obtained their excessive dietary Cd from rice grown on mine waste contaminated soils. Cadmium accumulates in the kidney with a very long biological half-life, and the first sign of a toxic effect is renal tubular dysfunction. Prolonged tubular dysfunction lowers Ca absorption causing the “Itai-itai” osteomalacia disease first seen in Japan in 1969. Urinary cadmium serves as a good estimate of Cd accumulation in the kidney. CODEX recommends Cd lifetime exposure from foods should be no higher than 25 μg Cd/kg body weight/month. Studies of a large Japanese urban middle-aged women cohort (with much higher daily Cd intake due to commercial rice grown on contaminated soils) showed that no adverse effects on kidney tubule function occurred while urinary Cd remained ≤ 3 μg Cd/g creatinine, a level which EU toxicologists argued caused early adverse Cd effects. Study of strongly affected Japanese rice farm families showed that clearly adverse renal effects began at 10 μg Cd/g creatinine in urine.

Agromining is the chain of processes of phytoextraction of economically valuable elements by selected hyperaccumulator plants, and subsequent processing of biomass to produce targeted metals or commercial compounds of high value. Although substantial unrealized opportunities exist for developing economic nickel (Ni) agromining in the tropics, this technology has remained relatively unexplored. This study investigated the soil chemistry of a newly established tropical 'metal farm' and elucidated the performance of a prospective 'metal crop' species (Phyllanthus rufuschaneyi) to be used in a large-scale tropical Ni agromining program on ultramafic soils in Sabah (Malaysia). We found that a major portion of the site ( > 90%) had high total Ni concentrations ( > 2000 mu g g(-1)) in the soil (shallow Eutric Cambisol Magnesic). This study also recorded high phytoavailable soil Ni concentrations in the field site, which is a desired property of soils intended for Ni agromining. Moreover, the average soil pH of the field (pH 6.4) is ideal for maximum Ni uptake in the local candidate species. We recorded low concentrations of Ca, K and P, suggesting the need for a fertilizer regime in the farm. The extraordinary shoot Ni concentrations ( > 2 wt%), coupled with the high purity of the 'bio-ore' derived from Phyllanthus rufuschaneyi, confirm its high potential for economic Ni agromining. The success of our first field trial is critical to provide 'real-life' evidence of the value of large-scale tropical 'metal farming'. Research priorities include the need to intensify the search for candidate species, determine their agronomy, develop mass propagation methods, and to test technologies to process the biomass to recover valuable products.