Cd Phytoavailability Research Articles

Heterogeneity of humic/fulvic acids derived from composts explains the differences in accelerating soil Cd-hyperaccumulation by Sedum alfredii

The hyperaccumulating mechanism concerning heavy metal activation or passivation and plant response triggered by fulvic acid (FA) and humic acid (HA) recruitments are investigated herein. We carefully examine the Cd activation effect by various FA and HA, tracing from pig, goat, and duck manure composts to straw compost and commercial materials (i.e., PC, GC, DC, SC, and CM), as well as their roles in plant growth promotion and Cd uptake. Our results indicate that due to the decrease of soil pH and their multiple functional groups, the contents of available Cd (AE–Cd) increased by 4.3–4.8% and 3.6–6.3% when all FA and HA sources were applied for 30 days. A 13.1–19.9% increase in AE–Cd was observed when CFA, DFA, and PFA were applied for five days, and a 9.5% increment was found when PHA was applied for 10 days. In the pot experiment, the Cd accumulation in plants increased by 2.78 and 2.17 folds with PFA and PHA applications, respectively, compared to the blank control group. This result can be attributed to the stimulative effects of the simultaneous Sedum alfredii growth and Cd phytoavailability. Notably, the Cd accumulation increased by 2.26 times with the SFA amendment due to the predominant stimulation effect to the phytoavailable Cd rather than plant growth. However, slight inhibitory effects were observed upon plant growth or Cd uptake, which led to the reduction of the Cd accumulation with DHA, SHA, and CHA employments. Consistently, the corresponding soil Cd removal efficiencies were 43.5% and 34.6% with PFA and PHA, respectively, which hold abundant O- and N-containing groups. Our research aims to gain insights into the ternary interaction in the presence of heavy metal, humic substances, and S. alfredii to simultaneously accelerate Cd activation and hyperaccumulation.

The role of different organic amendments to improve maize growth in wastewater irrigated soil

Wastewater irrigation in croplands is recognized as one of the major threat, seriously affecting soil health and sustainable agriculture around the globe. Heavy metals presence especially cadmium (Cd) in wastewater irrigated soils is posing serious physiological and morphological disorder in plants due to its highest mobility. Therefore, metals mobility in wastewater irrigated soils can be curtailed through eco-friendly and cost effective organic soil amendments compost (CP), press mud (PM) and moringa leaf extract (ME) at 3% rate that eventually reduces its translocation from polluted soil to plant. This study explored the possible effects of various types of organic soil amendments on Cd phytoavailability in wastewater degraded soil and its subsequent accumulation in maize tissues. Maize plant was grown in Ghazi University as a test plant and Cd accumulation was recorded in its tissues, translocation from root to shoot, chlorophyll contents, plant biomass, yield and soil properties (pH, NPK, OM and Soluble Cd) were also examined. Results revealed that the addition of amendments significantly minimized Cd mobility in soil by 45.8%, 23% and 19.4% when CP, PM and ME were added at 3% over control. Comparing the control soil, Cd uptake effectively reduced via plants shoots by 33.3%, 27.7% and 19.4% when CP, PM and ME. In addition, NPK were significantly increased among all the added treatments in the soil-plant system as well as improved chlorophyll contents relative to non-treated soil. The Current study suggested that among all the amendments, compost at 3% rate performed well and can be considered a suitable approach for maize growth in polluted soil.

Modelling evaluation of key cadmium transformation processes in acid paddy soil under alternating redox conditions

The intermittent flooding conditions in acid paddy field can substantially affect the phytoavailability of cadmium (Cd), but the underlying mechanisms controlling Cd transformation are still poorly understood, and furthermore the corresponding models for predict the Cd phytoavailability during pre-harvest drainage phase are still lacked. In this study, Cd-spiked paddy soil was incubated under anaerobic condition for 40 days and then aerobic condition for 15 days. The transformation of Cd speciation and the redox cycling of Fe/N/S was investigated. During anaerobic incubation, the dissolved and exchangeable Cd rapidly decreased, which were mainly attributed to enhanced adsorption on the surface of organic matter with the increasing soil pH. Moreover, the reductive dissolution of Fe oxides induced the release of soil colloids, which result in increasing the surface sites to immobilize Cd. In addition, the formation of Cd sulfide contributed to slight decrease the availability of Cd. However, when oxygen was re-introduced into the system, the rapid decrease of soil pH and surface sites resulted in the release of adsorption on soil surfaces, and oxidative dissolution of Cd sulfide also contributed to the increase of Cd availability. The kinetic models were established based on the key elementary reactions for soil biogeochemical processes on the transformation of Cd fractions. The kinetic models were applied to calculate the potential available Cd in acid paddy soils samples (pH ≤ 6.0) from the field, and moreover, the dissolved Cd was estimated from the surface complexation model (SCM) by incorporating the changes of soil pH. The concentration of model-predicted dissolved Cd during drainage phase had a stronger correlation with the Cd content in rice grain than that of chemical extraction methods. Our results demonstrated that it was feasible to use the kinetic models and SCM to predict Cd phytoavailability during drainage phase of acid paddy field.

Effects of sheep bone biochar on soil quality, maize growth, and fractionation and phytoavailability of Cd and Zn in a mining-contaminated soil

Biochar prepared from various feedstock materials has been utilized in recent years as a potential stabilizing agent for heavy metals in smelter-contaminated soils. However, the effectiveness of animal bone-derived biochar and its potential for the stabilization of contaminants remains unclear. In the present study, sheep bone-derived biochar (SB) was prepared at low (500 °C; SBL) and high temperatures (800 °C; SBH) and amended a smelter-contaminated soil at 2, 5, and 10% (w/w). The effects of SB on soil properties, bioavailable Zn and Cd and their geochemical fractions, bacterial community composition and activity, and the response of plant attributes (pigments and antioxidant activity) were assessed. Results showed that the SBH added at 10% (SBH10) increased soil organic carbon, total nitrogen, and phosphorus, and also increased the oxidizable and residual Zn and Cd fractions at the expense of the bioavailable fractions. The SBH10 lowered the Zn and Cd contents in maize roots (by 57 and 60%) and shoot (by 42 and 61%), respectively, compared to unamended control. Additionally, SBH10 enhanced urease (98%) and phosphates (107%) activities, but reduced dehydrogenase (58%) and β-glucosidase (30%) activities. Regarding the effect of the pyrolysis temperature, SBH enhanced the activity of Acidobacteria, Bacteroidetes, Firmicutes, Nitrospirae, Verrucomicrobia, Chlorobi, and Microgenomates, but reduced Actinobacteria and Parcubacteria in comparison to SBL. However, only the SBL10 reduced the Proteobacteria community (by 9%). In conclusion, SB immobilized Zn and Cd in smelter-affected soils, enhanced the bacterial abundance and microbial function (urease, phosphates), and improved plant growth. However, validation of the results, obtained from the pot experiment, under field conditions is suggested.

Long-term effects of biochar on trace metals accumulation in rice grain: A 7-year field experiment

Biochar (BC) is an emerging amendment for minimizing the phytoavailability of trace metals in soil-crop system, but its relevant durability and mechanism are unclear. A 7-year field study was conducted in double rice paddy land of mid-subtropical China. Wheat straw BC was once applied at rates of 24 and 48 Mg ha−1 (LBC and HBC, respectively). Results showed that the increase in soil total organic C (TOC, 41.1–54.6%), pH (0.41–0.86 units) and cation exchange capacity (13.7–23.3%) for the HBC was kept for 14, 11 and 4 seasons (p < 0.05), respectively. Meanwhile, the reduction in soil CaCl2-extractable Cd, Pb, Zn and Mn (29.5–61.6%, 38.2–81.0%, 36.5–70.7%, 19.1–56.7%, respectively, p < 0.05) was maintained for 9–11 seasons. And, the reduction in grain Cd, Pb, Zn and Mn (23.0–59.5%, 15.2–49.8%, 17.5–22.9%, 13.4–34.2%, respectively, p < 0.05) was maintained for 4–6 seasons. Similar but weaker effects were observed for the LBC. And, their durable periods (except for TOC) for the LBC were about 1/3–1/2 of those for the HBC. Phytoavailability of Cd, Pb, Zn and Mn in soil-rice system was negatively correlated with soil pH and TOC (p < 0.05). Therefore, the immobilization durability of the BC on trace metals in soil-rice system are mainly attributed to the primary fixation of stable microstructure and functional groups, and mediated by the associated changes in soil pH. Alkaline materials should be taken as supplemental agents to strengthen the immobilization durability of the BC on trace metals in soil-rice system.

Impact of Biochar and Lime on Phytoavailability of Pb and Cd in a Contaminated Soil

Current study was conducted to evaluate the efficiency of water hyacinth (<em>Echhornia crassipes</em>) biochar and lime to remediate cadmium (Cd) and lead (Pb) contaminated soils. A pot experiment was carried out treating soil with Cd and Pb (both separately and in combination) at the rates of 3, 6, 9 mg/kg and 250, 500, 750 mg/kg, respectively. Biochar and lime were applied separately to the metal spiked soils at the rate of 5.54 g/kg and 2.77 g/kg, respectively. After growing metal accumulator plant <em>Eleusine indica </em>for five months, metal contents were examined in plant biomass (composite sample prepared mixing equal amount of both root and shoot). Water hyacinth biochar had higher efficiency (up to 72%) to reduce phytoavailability of Pb compared to Cd for <em>E. indica </em>when applied in metal spiked soil. However, lime application restricted the availability of Cd to <em>E. indica </em>up to 38.8% compared to control. Documented greater microbial biomass carbon (MBC) and chlorophyll content under application of water hyacinth biochar than lime indicates its higher efficacy for growth of plants and microbes in Pb and Cd contaminated soil due to supply of nutrient in the soil. Therefore, the water hyacinth biochar and lime have potential to reduce bioavailability of Pb and Cd in contaminated soil.

Phytoavailability of Cd, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Tl and Zn in Arable Crop Systems Amended for 13 to 15 Years with Organic Waste Products

Repeated applications of organic waste products (OWP) are a source of trace elements (TE) inputs to agricultural topsoils. The present study aimed at (i) assessing the effects of repeated OWP inputs on the chemical properties of topsoils in two long-term field experiments (13 and 15 years; calcareous and non-calcareous soils), (ii) evaluating TE phytoavailability and their transfer to grain (winter wheat and maize) and (iii) identifying the underlying factors causing alterations of TE phytoavailability. In both field experiments, receiving compliant or slightly high doses of OWP in compliance with regulations, OWP and soil physicochemical properties and TE concentrations in soils and grains were determined. In situ phytoavailability of TE was assessed at two juvenile crop growth stages by analyzing TE concentrations in shoot plantlets. Depending on the OWP input amount, results showed that compared to the soil receiving no organic amendment, repeated OWP inputs significantly increased soil organic carbon content, pH, cation exchange capacity, total soil Cu, Mo and Zn concentration and the phytoavailability of Mo, while the phytoavailability of Cd, Mn, Ni and Tl was significantly reduced. No notable effect was observed for Cr, Cu, Hg, Pb and Zn phytoavailability. Statistical approaches suggested that due to the repeated OWP applications, increased soil organic carbon content and pH, were likely responsible for decreased TE phytoavailability (e.g., Cd).

Inoculation of Bacillus megaterium strain A14 alleviates cadmium accumulation in peanut: effects and underlying mechanisms.

A cadmium (Cd)-tolerant Bacillus megaterium strain A14 was used to investigate the effects and mechanisms of bacterial inoculation on peanut growth, Cd accumulation in grains and Cd fixation in Cd-contaminated soil. Spectroscopic analysis showed that A14 has many functional groups (-OH, -NH2 and -COO et al.) distributed on its surface. The pot experiment indicated that compared to the Cd-contaminated soil alone treatment, inoculation with strain A14 increased shoot and root biomass by 59·93 and 58·31% respectively. The accumulation of Cd in grains decreased by 48·14%, while the proportion of exchangeable Cd in soil decreased from 40 to 26% in A14 inoculated soil. Inoculation with B. megaterium A14 improved peanut plant growth via (i) adsorbing Cd2+ through functional groups on cell surface, (ii) immobilization of Cd in soil through extracellular secretions, (iii) scavenging the reactive oxygen species through production of antioxidant enzymes, and (iv) by reducing the phytoavailable Cd through regulation of Cd transport gene expression. This study provided a new sight on microbial approach for the chemical composition transformation of soil Cd and associated food safety production, which pointed out an efficient way to improve peanut cultivation.

CATTLE MANURE APPLICATION AFFECTS BIOAVAILABILITY OF CADMIUM IN SOIL

Cadmium (Cd) in soil could enter human body via food chain contamination. Farmers regularly apply manure to the vegetable growing fields in the up-country in Sri Lanka. Regular manure application could increase concentration of labile C in soil, which could maintain relatively high soluble Cd fraction in soil, resulting soil Cd more bioavailable. A pot experiment was conducted in a greenhouse with soils (Ultisols) collected from an urban vegetable garden in Seethaeliya, Sri Lanka to assess whether the manure application affects the phytoavailability of soil Cd to lettuce. Lettuce was grown for three consecutive crop cycles with and without cattle manure. For one manure-added treatment, dried manure was added as a bulk (i.e. one time at the beginning of the first crop at the rate of 75g of manure per 5kg of soil in fresh weight basis) and as a split dose for the other treatment at the rate of 25g of manure per 5 kg of soil at the beginning of every crop. The concentration of total Cd was 0.72 mg/kg in soil and 0.60 mg/kg in cattle manure. Only 2% of total Cd was easily extractable in soil, whereas 21% of total Cd in the manure was easily extractable. Concentration of Cd in lettuce was significantly higher in manure-added treatments compare to the unamended soil. Manure application (both bulk and split) increased Cd concentration in lettuce. Incorporation of easily extractable Cd and increase of labile C via manure application may have contributed to elevate Cd concentrations in lettuce.

PGPR inoculation of a contaminated soil affects plant growth and phytoavailability of Cd and Pb

ABSTRACT To evaluate the effects of cabbage varieties and PGPR inoculation on the mobility and phytoavailability of Pb and Cd in a contaminated soil, a factorial (3 × 6) pot experiment with completely randomized design and three replications was performed. The cabbage varieties were ornamental cabbage, broccoli and cabbage and the PGPR strains included Pseudomonas putida PTCC1694, Bacillus megaterium PTCC1656, Azotobacter chroococcum, Proteus vulgaris PTCC1079 and Bacillus subtilis PTCC1715. The results indicate that the PGPR inoculation increased the amounts of plant biomass, the Pb and Cd concentrations in the root and shoot, the Pb and Cd uptake performances, the mobility, phytoavailability and translocation factor. The Pb bound to Fe-Mn oxides (42.7%) and the Cd bound to carbonate (38.1%) were the predominant heavy metal fractions. The percentages of the Pb and Cd in the soluble, exchangeable and organic matter-bound fractions increased while those of the Pb and Cd in the carbonate and Fe-Mn oxide-bound fractions decreased by lowering the soil pH and increasing the concentration of dissolved organic carbon (DOC). It is concluded that inoculation of heavy metal contaminated soils by the PGPR strains, especially Bacillus megaterium PTCC1656, enhances the phytoremediation process by increasing the metal phytoavailability and biomass production of the plant. Abbreviations HMe: Heavy metals; PGPR: Plant growth-promoting rhizobacteria; DOC: Dissolved organic carbon; HCN: Hydrogen cyanide; IAA: Indole-3-acetic acid; MIC: Minimum Inhibitory Concentrations; LB: Liquid broth; EC: Electrical conductivity; OC: Organic carbon; FC: Field capacity; WS: Water soluble; EX: Exchangeable; CAR: Bound to carbonates; OX: Bound to Fe-Mn oxides; OM: Bound to organic matter; RES: Residual; BOA: Ornamental cabbage; BOC: Cabbag; BOI: Broccoli; Pp: Pseudomonas putida PTCC1694; Bm: Bacillus megaterium 107 PTCC1656; Ac: Azotobacter chroococcum; Pv: Proteus vulgaris PTCC1079; Bs: Bacillus subtilis 108 PTCC1715; C-: Uninoculated non-rhizosphere soil; C+: Uninoculated rhizosphere soil; TF: Translocation factor; MF: Mobility factor; ANOVA: Analysis of variance

Cadmium Immobilization in the Rhizosphere and Plant Cellular Detoxification: Role of Plant-Growth-Promoting Rhizobacteria as a Sustainable Solution.

Food is the major cadmium (Cd)-exposure pathway from agricultural soils to humans and other living entities and must be reduced in an effective way. A plant can select beneficial microbes, like plant-growth-promoting rhizobacteria (PGPR), depending upon the nature of root exudates in the rhizosphere, for its own benefits, such as plant growth promotion as well as protection from metal toxicity. This review intends to seek out information on the rhizo-immobilization of Cd in polluted soils using the PGPR along with plant nutrient fertilizers. This review suggests that the rhizo-immobilization of Cd by a combination of PGPR and nanohybrid-based plant nutrient fertilizers would be a potential and sustainable technology for phytoavailable Cd immobilization in the rhizosphere and plant cellular detoxification, by keeping the plant nutrition flow and green dynamics of plant nutrition and boosting the plant growth and development under Cd stress.

Effect of aging on stabilization of Cd and Ni by biochars and enzyme activities in a historically contaminated alkaline agricultural soil simulated with wet-dry and freeze-thaw cycling.

Natural aging alters the surface physicochemical properties of biochars, which can affect the retention of heavy metals. This work investigated the effect of biochar aging on stabilization of heavy metals (Cd and Ni) and soil enzyme activities simulated with laboratory wet-dry (WD) and freeze-thaw (FT) cycling. A wheat straw (WS) biochar and a corn straw (CS) biochar were subjected to 30 WD or FT cycles, and Cd- and Ni-contaminated alkaline soils amended with the two fresh biochars (at 5% w/w) were subjected to 30-day constant moisture incubation and 30 WD or FT cycles. WD and FT aging caused slight reduction in the pH of the biochars, significant increases in their O contents and surface areas, and formation of new carbonate minerals. WS biochar was more effective than CS biochar at reducing the phytoavailable Cd in the soil, with reduction of 12.1%, 14.6%, and 12.9% under constant moisture incubation, WD aging, and FT aging, respectively. Reduction in phytoavailability of Ni by the addition of biochars was observed only under WD aging, by 17.0% and 18.5% in the presence of WS and CS biochars, respectively. Biochar amendment also reduced the distribution of Cd in the acid soluble and reducible fractions in all aging regimes. The addition of biochars decreased catalase activity in almost all aging regimes and invertase activity under FT aging, but increased urease activity under FT aging. Comparison of the enzyme activities in the soils amended with biochars under constant moisture and accelerated aging conditions indicates WD aging significantly decreased the activities of catalase, invertase, and urease in all treatments, while FT aging significantly increased urease activity in all treatments. These findings suggest that biochars can stabilize Cd in alkaline soils under changing environmental conditions, although the activities of some soil enzymes could be negatively impacted.

Evaluation of geogenic cadmium bioavailability in soil-rice system with high geochemical background caused by black shales

Recently, the cadmium (Cd) accumulation in the black shale area has attracted increasing attentions, due to its extensive distribution and extremely high background values. The aim of our study is to explore the impacts of soil properties on Cd uptake by rice, and identify the key factors of Cd transfer from soil to rice in the black shale area. A total of 40 pairs of topsoil and rice samples were collected from the paddy fields in the black shale area and the control area. The relevant parameters in the soils, as well as the Cd concentrations in different rice tissues, were analyzed. The results show that 87.5% of the soil samples in the study area exceed the risk screening values of Cd, and 42% of the rice grain samples exceed the allowable limit of Cd. The key factor influencing Cd uptake by rice in this area is the soil phytoavailable Cd concentration. Compared to those in the control area, the soils in the black shale area have a much higher Cd level but a lower phytoavailability due to the higher pH, which is caused by carbonate. Even so, the level of Cd in the rice from the black shale area indicates a much higher ecological risk. Soil pH and iron (Fe) are the most important variables exhibiting direct effects on Cd fractions in soils. In light of the neutral or weakly alkaline soil in the black shale area, we concluded that increasing the soil pH was not a feasible way to reduce Cd levels in the rice in this area.

A phytoremediation coupled with agro-production mode suppresses Fusarium wilt disease and alleviates cadmium phytotoxicity of cucumber (Cucumis sativus L.) in continuous cropping greenhouse soil

Cadmium (Cd) contamination and continuous cropping obstacle often coexist in greenhouse soil and seriously restrict cucumber production. In this study, hyperaccumulator Sedum alfredii Hance was intercropped with spring cucumber (Cucumis sativus L.), then rotated with low accumulator water spinach and autumn cucumber under rational water regime, composited amendment was applied to soil before transplanting autumn cucumber. The results showed that, compared with conventional crop rotation system (Chinese cabbage and cucumber rotation), superposition management practice suppressed Fusarium wilt disease by 28.4 and 57.4% and increased yield by 35.2 and 383% for spring and autumn cucumbers, respectively. Meanwhile, photosynthetic characteristics, antioxidant system and fruit quality were significantly improved. Furthermore, this mode modified soil microbial community structure, enhanced soil enzyme activities, and simultaneously reduced soil total and phytoavailable Cd by 30.3 and 47.7%, respectively. These results demonstrated a feasible technical system to achieve phytoremediation coupled with argo-production in Cd contaminated greenhouse soil with continuous cropping obstacle and provided useful information for further revelation of interaction mechanisms between multicropping and comprehensive biofortification measurements.

Ecological-health risk assessment and bioavailability of potentially toxic elements (PTEs) in soil and plant around a copper smelter.

Soil and the dominant plant species in the vicinity of Khatoon Abad copper smelter in Kerman province of Iran are examined to determine contamination, bioavailability, and ecological-health risk of potentially toxic elements (PTEs) based on 23 collected soil samples and 13 Artemisia siebri plant species. Cu, Mo, As, and Sb display a significant level of enrichment in soil. Ecological risk assessment shows that Cu, As, and Cd pose the highest ecological risk. The results of PTEs fractionation reveal that, on average, Cu, As, Cd, Pb, Zn, and Mo are mostly distributed between non-residual fractions reflecting higher mobility and potential ecological risk, while Cr, Ni, and Co are significantly distributed within the residual fraction, and do not pose a serious ecological risk. Mobility factor suggests high bioavailability of Cu for plants followed by As, Cd, Pb, Mo, Co, Ni, and Cr. Biological accumulation coefficient displays higher phytoavailability of Mo and Cd. PTEs transfer within plant follows the order of Mo > As > Pb > Zn > Cu > Ni > Co > Cr > Cd. The results of phytoavailability indicate the high tendency of Cd to bioaccumulate in Artemisia's root, while Mo, As, and Pb tend to translocate towards Artemisia's shoot. Calculated hazard index and incremental lifetime cancer risk revealed that As poses the highest non-carcinogenic health risk, and As and Pb pose the greatest carcinogenic health risk in both adults and children.

Dynamic interactions between soil cadmium and zinc affect cadmium phytoavailability to rice and wheat: Regional investigation and risk modeling

Characterizing the interactions between Cd and Zn with respect to the soil soluble Cd and crop Cd uptake allows the development of risk-based approaches to the performance of grain crops. By means of a three-year survey of 358 rice fields and 206 wheat fields across China, this study investigated the effect of Cd–Zn interactions on the phytoavailability of Cd to rice and wheat. The interactive nature between the Cd:Zn ratio and pH of soil affected crop Cd uptake, and the resulting grain Cd intake risk, were examined by the Free-Ion Activity-based model and probability analysis. In highly acidic rice soils (pH < 5.9), soil Zn had no effect on rice Cd uptake, whereas, under near-neutral conditions (pH > 5.9), a site-specific influence of soil Zn on grain Cd concentration was found. Soil Zn could inhibit Cd uptake and translocation by the plant in soil-wheat system when the soil Cd:Zn ratio decreased to 0.0083 and lower. Rice grain poses a significant health risk to local consumers due to its high Cd accumulation and its low Zn accumulation. In order to reduce the health risks from dietary Cd to local consumers, approximately 63.9% of the rice fields and 30.5% of the wheat fields require strategies ameliorating soil acidity in rice soils and increasing Zn concentrations in wheat soils.

Surface soil liming reduces cadmium uptake in cacao seedlings but subsurface uptake is enhanced.

Cadmium concentrations in cacao (Theobroma cacao L.) beans from South America often exceed trade limits. Liming soil is advocated as a remediation option, but amendments cannot be incorporated into the entire root zone without harming the trees. An experiment was set up to identify how Cd uptake varies within the root zone when surface and subsurface soil layers are either limed or not. The experiment used 22-cm-height pots with top and bottom layers using surface and subsurface soil samples from a cacao field. The potted soils were either surface limed or not or fully limed and layers spiked with stable 108 Cd isotope in various combinations to trace the plant Cd provenance. The root distribution was neither affected by liming nor by soil source; 70% of the root biomass was present in the top layer. Plants grown on the fully limed surface soil had 1.7 times lower Cd concentrations in leaves than the unlimed treatments, whereas this concentration was 1.2 times lower when only the top layer was limed (surface soil used in both layers). The isotope dilution data showed that surface soil liming enhanced Cd uptake from the unlimed bottom layer compared with the unlimed soil, suggesting compensating mechanisms. The pots containing surface soil over subsurface soil also showed that compensating effect but, due to lower phytoavailable Cd in the subsurface soil, surface liming still effectively reduced foliar Cd. We conclude that liming might be a feasible mitigation strategy, but its effectiveness is limited when Cd phytoavailability remains untreated in the subsurface layer.

Efficient models for predicting durum wheat grain Cd conformity using soil variables and cultivars

Contamination of durum wheat grain by cadmium (Cd) threatens food safety and is of increasing concern because regulations concerning Cd are becoming stricter due to its toxicity. This work aimed at using soil variables and cultivar types to build models to predict whether durum wheat grain Cd will conform with current and possibly lower regulatory thresholds. We combined multiple Gaussian and logistic regressions and the random forest algorithm to take advantage of their strength. Models tested using cross-validation produced excellent performances including for the lowest regulatory threshold of 0.1 mg Cd/kg, half of the current one: 79–85% of the non-conformity cases were detected and the reliability of predictions was 69–82%. The models enabled identification of a x1.4 variability in grain Cd content between cultivars that do not have the low Cd accumulation allele of the Cdu1 gene. The models confirmed that for the grain Cd content, the between-cultivar variability had much less influence than the phytoavailability of Cd in soil, the critical contexts of which were characterized by the models. For farmers, these models are valuable tools to predict whether durum wheat production will conform with existing and future Cd regulation in foodstuffs.

Phytoavailability-based threshold values for cadmium in soil for safer crop production

The phytoavailability of heavy metals in soils is important for both food safety and environmental management. Hence soil metal phytoavailability threshold values need to be established based on a firm scientific basis. In this study, optimal Cd phytoavailability threshold values, were determined for bean, rice and sesame cultivated in 100 soils varying widely in soil chemical characteristics by comparing the soil Cd phytoavailability obtained using three commonly used extraction procedures. Subsequently, the transfer functions derived in this study, were used to establish soil Cd phytoavailability threshold value standard limits for each specific crop. In addition, independent experimental data were used to supplement the obtained soil phytoavailable Cd threshold value for rice. Soil phytoavailable Cd concentrations extracted by 1 M NH4NO3, 0.05 M EDTA and Mehlich3 solutions were each more significantly correlated with plant Cd concentrations than total soil Cd concentrations. Thus, the soil Cd phytoavailability threshold values proposed in this study provide a more effective means of ensuring safer agricultural food production. Therefore, it is recommended that current agricultural soil heavy metal management policy; which is based on total concentrations; should be changed to embrace soil metal phytoavailability for safer agricultural food production.

Si-Ca-K-Mg amendment reduces the phytoavailability and transfer of Cd from acidic soil to rice grain.

Cadmium (Cd) contamination in the soil-rice chain is the major threat to human health in China. It is very necessary to lower Cd phytoavailability in contaminated soils and reduce Cd transfer from soil to rice for food safety. This study applied the Si-Ca-K-Mg amendment (SCKM) to immobilize Cd in acidic soils and then reduce its accumulation in rice grain (Oryza sativa L.). Two agricultural soils (Alfisol and Ultisol) collected from Eastern China were treated with three levels of Cd concentration (0, 0.4, and 2.0 mg/kg), respectively, for pot experiment. The phytoavailability and chemical forms of Cd in two soils were determined using ethylenediaminetetraacetic acid (EDTA) and the European Community Bureau of Reference (BCR) extraction procedures. At 2.0 mg Cd/kg-treated soils, application of SCKM amendment increased the yield of rice grain by 10-17% for Alfisol and 14-39% for Ultisol, and reduced the concentrations of EDTA-extractable Cd by 6-27% for Alfisol and 5-25% for Ultisol, compared with treatment without amendment. SCKM amendment significantly (p < 0.05) reduced the bioconcentration factor (BCF) of Cd in root, straw, and grain of rice. Compared with treatment without amendment, the application of amendments decreased the Cd concentrations of rice grains by 35-76% for Alfisol and 31-72% for Ultisol, respectively. The BCR sequential extraction revealed that amendment reduced acid soluble Cd fraction by 6.2-13.6% for Alfisol and 6.1-13.5% for Ultisol, respectively, indicating that amendment could effectively transform the highly phytoavailable Cd into a more stable form. SCKM amendment addition significantly (p < 0.05) increased soil pH and exchangeable K+, and decreased exchangeable Al3+ contents in both soils. Our results demonstrated that SCKM amendment was effective in reducing the phytoavailability and transfer of Cd in soil-rice system, and ameliorating soil acidity. The SCKM amendment had greater potential as a low-cost and friendly environmentally amendment for safe production of rice in Cd-contaminated soils.