Effects of intercropping of oat (Avena sativa L.) with white lupin (Lupinus albus L.) on the mobility of target elements for phytoremediation and phytomining in soil solution

The Uptake of Rare Trace Elements by Perennial Ryegrass (Lolium perenne L.)

Technological development has increased the use of chemical elements that have hitherto received scant scientific attention as environmental contaminants. Successful management of these rare trace elements (RTEs) requires elucidation of their mobility in the soil-plant system. We aimed to determine the capacity of Lolium perenne (a common pasture species) to tolerate and accumulate the RTEs Be, Ga, In, La, Ce, Nd, and Gd in a fluvial recent soil. Cadmium was used as a reference as a well-studied contaminant that is relatively mobile in the soil-plant system. Soil was spiked with 2.5-283 mg kg-1 of RTE or Cd salts, representing five, 10, 20, and 40 times their background concentrations in soil. For Be, Ce, In, and La, there was no growth reduction, even at the highest soil concentrations (76, 1132, 10.2, and 874 mg kg-1, respectively), which resulted in foliar concentrations of 7.1, 12, 0.11, and 50 mg kg-1, respectively. The maximum no-biomass reduction foliar concentrations for Cd, Gd, Nd, and Ga were 0.061, 0.1, 7.1, and 11 mg kg-1, respectively. Bioaccumulation coefficients ranged from 0.0030-0.95, and increased Ce < In < Nd ≅ Gd < La ≅ Be ≅ Ga < Cd. Beryllium and La were the RTEs most at risk of entering the food chain via L. perenne, as their toxicity thresholds were not reached in the ranges tested, and the bioaccumulation coefficient (plant/soil concentration quotient) trends indicated that uptake would continue to increase at higher soil concentrations. In contrast, In and Ce were the elements least likely to enter the food chain. Further research should repeat the experiments in different soil types or with different plant species to test the robustness of the findings.

Mechanisms underlying cereal/legume intercropping as nature-based biofortification: A review

The deficiencies of micronutrients known as hidden hunger are severely affecting more than one-half of the world’s population, which is highly related to low bioavailability of micronutrients, poor quality diets, and consumption of cereal-based foods in developing countries. Although numerous experiments proved biofortification as a paramount approach for improving hidden hunger around the world, its effectiveness is highly related to various soil factors, climate conditions, and the adoption rates of biofortified crops. Furthermore, agronomic biofortification may result in the sedimentation of heavy metals in the soil that pose another detrimental effect on plants and human health. In response to these challenges, several studies suggested intercropping as one of the feasible, eco-friendly, low-cost, and short-term approaches for improving the nutritional quality and yield of crops sustainable way. Besides, it is the cornerstone of climate-smart agriculture and the holistic solution for the most vulnerable area to solve malnutrition that disturbs human healthy catastrophically. Nevertheless, there is meager information on mechanisms and processes related to soil-plant interspecific interactions that lead to an increment of nutrients bioavailability to tackle the crisis of micronutrient deficiency in a nature-based solution. In this regard, this review tempted to (1) explore mechanisms and processes that can favor the bioavailability of Zn, Fe, P, etc. in soil and edible parts of crops, (2) synthesize available information on the benefits and synergic role of the intercropping system in food and nutritional security, and (3) outline the bottlenecks influencing the effectiveness of biofortification for promoting sustainable agriculture in sub-Saharan Africa (SSA). Based on this review SSA countries are malnourished due to limited access to diverse diets, supplementation, and commercially fortified food; hence, I suggest integrated research by agronomists, plant nutritionists, and agroecologist to intensify and utilize intercropping systems as biofortification sustainably alleviating micronutrient deficiencies.

Graphical Abstract

Effects of intercropping on Se accumulation and growth of pakchoi, lettuce and radish

Selenium is one of the most basic trace elements in the human body. It is necessary to improve the selenium content in agricultural products through agricultural planting technology to ensure that human nutrition and health need selenium. Therefore, our research passed the effect of intercropping on the growth and selenium accumulation of pakchoi, lettuce and radish were studied through pot experiments to determine whether intercropping of the three crop species can improve their selenium accumulation ability. The results showed that intercropping increased the root and shoot biomass of pakchoi and radish compared with the monocultures, while the biomass of roots and shoots decreased in other intercropping combinations. Intercropping also affected the photosynthetic pigment content of the three crop species. Specifically, the photosynthetic pigments increased in pakchoi and decreased in radish after intercropping. Notably, intercropping the three crop species together increased the SOD (superoxide dismutase) activities of the three crops compared with the monocultures. Meanwhile, intercropping radish with lettuce significantly increased the activities of SOD and CAT (catalase) in radish. Intercropping also increased the soluble sugar content in pakchoi and soluble protein content of radish relative to the monocultures. Furthermore, intercropping decreased the selenium content and the bioconcentration factor of the roots of the three vegetable crops, but improved the shoot selenium content, the bioconcentration factor and the transport factor of Se in pakchoi and radish. In conclusion, intercropping combination of pakchoi and radish can improve selenium accumulation in the edible parts of the crops, which is significant for efficient production of selenium-enriched vegetables.

Accumulation and Transfer of Cadmium Isotope (112Cd) by Sonchus asper Intercropped with Vicia faba and Implications for Phytoremediation

Accumulation and transfer of Cd by Sonchus asper intercropped with a legume crop, Vicia faba were determined via ¹¹²Cd as a tracer to assess the potential of Sonchus asper as a hyperaccumulator. In this study, Sonchus asper and Vicia faba were planted side by side in soils amended with arbuscular mycorrhizae, and ¹¹²Cd was applied to either Sonchus asper or Vicia faba planted soils while the neighboring plant received ¹¹²Cd across a nylon net impermeable to the roots. Mean concentration of ¹¹²Cd in the shoot of Sonchus asper was 7.0 times higher than Vicia faba. The translocation factor of ¹¹²Cd in Sonchus asper were 39 and 400 times higher than in Vicia faba under the ¹¹²Cd direct and indirect treatments, respectively. The results suggested that the intercropping of Sonchus asper as a hyperaccumulator with Vicia faba, might be a feasible approach for phytoremediation of Cd contaminated soil.

A review of in situ phytoextraction of rare earth elements from contaminated soils

Rare earth elements (REE), with their distinct physical and chemical properties, are critical components of green economic development. Intensive exploitation and application of REE are wreaking havoc on the environment. But research on REE is still limited to a small number and in a few countries. With the growing interest of REE in modern technologies and their potential ecological risks, phytoextraction seems promising for both REE pollution reduction and resource circulation. This paper summarizes the recent findings in the literature concerning REE hyperaccumulating plants and relevant accumulation mechanisms. Additional interests should be focused on a broader range of plant species and a global scale to achieve a sustainable REE supply.Novelty statementThis paper summarized the referenced potential rare earth elements (REE) hyperaccumulator plants that accumulated higher than REE 100 µg/g and discussed their accumulation and translocation mechanisms.We addressed the synonyms of Dicranopteris pedata, Dicranopteris dichotoma Bernh., and Dicranopteris linearis.Although Dicranopteris pedata has been extensively studied in the sense of REE hyperaccumulation, active phytoextraction outside of its native range, as well as in accumulation of the precious heavy rare earth elements, may be difficult. Thus, further interests should take these disadvantages into account.

Enhancing cadmium extraction potential of Brassica napus: Effect of rhizosphere interactions

Brassica napus L. (oilseed rape) was grown with daikon and white lupin in a polyvinyl chloride split pot experiment (with no barrier between the compartments or by a nylon mesh barrier (37 μm) to license partial root interaction, or a solid barrier to stop any root interactions) to examine the effect of rhizosphere interaction on the cadmium uptake. The results showed that shoot and root biomasses of oilseed rape were 40.66% and 26.94% less than that of the monocropped treatment (solid barrier) when intercropping with daikon under the rhizosphere complete interaction. However, the intermingling of roots between oilseed rape and white lupin notably enhanced the dry biomass of oilseed rape by 40.23% and decreased with the reduction of root contact. Oilseed rape intercropping with daikon enhanced the shoot Cd concentration of oilseed rape. The shoot Cd concentration (44.8 mg/kg) of oilseed rape when intercropped white lupin under complete rhizosphere interaction were greater than those of other treatments. Additionally, the intermingling of roots played a positive role in the content of citric and malic acids when intercropping with white lupin. In all systems, the BCF values of oilseed rape >5. Therefore, intercropping with white lupin may contribute to higher biomass and increased uptake Cd by oilseed rape. We can toward sustainable positive effects on phytoremediation that based on a better understanding of rhizosphere processes.

Effect of Wheat-Solanum nigrum L. intercropping on Cd accumulation by plants and soil bacterial community under Cd contaminated soil

Using accumulators for intercropping in agricultural production can change the heavy metal concentration in the target plants. This study aims to investigate how intercropping wheat (Triticum aestivum L.) and Solanum nigrum L. affects soil bacterial community and cadmium (Cd) absorption in response to Cd-contaminated soil. We compared the concentrations and accumulations of Cd by plants, the activities of soil enzymes and the bacterial community structures of rhizosphere soil in monoculture and intercropping system. Principal component analysis (PCA) ordinations showed that soil bacterial communities were significantly separated by MW and IW, which illustrated intercropping with Solanum nigrum L. impacted the bacterial community structure of wheat. Firstly, the results showed that the biomass of shoots and roots in intercropped wheat (IW) were significantly decreased by 16.19% and 29.38% compared with monoculture wheat (MW) after 60 days after transplanting (DAT). Secondly, the Cd concentration and accumulation of shoots in IW was higher than MW. The Cd accumulation of IW shoots and roots were increased 12.87% and 0.98%, respectively after 60 days DAT. Besides, the enzymes activity [catalase (CAT), urease (UA) and alkaline phosphatase (ALP)] of IW were decreased 35%, 6% and 21%, respectively after 60 days DAT. Finally, the diversity indexes [Abundance-based Coverage Estimator (ACE), Chao and InvSimpson] of IW were lower than MW. These results indicated that intercropping with Solanum nigrum L. inhibited the wheat growth and decreased the bacterial community diversity in wheat rhizosphere, increased the Cd concentration and accumulation in plant tissues of wheat. Therefore, intercropping Solanum nigrum L. and wheat with Cd-contaminated soil might increase the risk of excessive Cd in wheat.

Cadmium accumulation in oilseed rape is promoted by intercropping with faba bean and ryegrass

The mechanisms of intercropping increasing plant biomass, cadmium accumulation, and organic acids secreted in rhizosphere soil are still unclear. Oilseed rape and intercrops were grown in boxes separated either with no barrier between the compartments or by a nylon mesh barrier (37 μm) to license partial root interaction, or a solid barrier to stop any root interactions. Two intercropping systems (oilseed rape-faba bean and oilseed rape-ryegrass) were carried out in soil with Cd content of 5 mg/kg. The intermingling of roots between oilseed rape and faba bean enhanced the biomass of oilseed rape. However, the biomass was negatively affected implying the higher nutrient apportionment to the ryegrass than oilseed rape. Oilseed rape intercropping with both faba bean and ryegrass played a positive role in the shoot Cd concentration of oilseed rape. The intermingling of roots played a positive role in the citric and malic acids when intercropping with faba bean. A remarkable increase in water-soluble Cd and DTPA-Cd content was observed during oilseed rape-faba bean complete root interaction treatment, up to 175.00% and 46.65%, respectively, which compare with the monoculture treatment. In both systems, the translocation factor values were higher for oilseed rape (O-F system) than for the other test plants and were always >1. Thus the Cd removal potential of oilseed rape can be further improved in the future by optimizing agronomic practices and intercropping with faba bean.

The phytoremediation efficiency of Eucalyptus globulus treated by static magnetic fields before sowing

Eucalyptus globulus pre-treated by static magnetic fields of 30, 60, 120, 150 and 400 mT (mT) before sowing were used in a 45-day experiment to remediate soil containing Cd, Hg, Pb, Zn, Cr and Cu. The influence of magnetic fields on its remediation efficiency was evaluated. Magnetic fields with strength of 30, 60, 120 and 150 mT increased the biomass yield of the species by 3.1, 19.4, 48.1 and 60.9%, respectively, while 400 mT decreased the yield by 16.7%. Comparing with the control exposed only to the earth's geomagnetic field, all plants pre-treated by static magnetic field had significantly higher metal concentrations with the highest values achieved in the field of 400 mT. Higher transpiration rate of the plants along with exposure to static magnetic fields induced lower soil moisture content and was beneficial to environmental control because it could reduce the leachate during the phytoremediation process. Among all static magnetic field treatments, 150 mT was the best to improve the phytoremediation and alleviate the environmental risk, which shortened the time to purify Cd, Pb and Cu by 27.8-73.2%, 27.3-74.7% and 2.5-50.6%, respectively and intercepted 31.6-86.1% of the leachate. Therefore, static magnetic field with appropriate intensity is a suitable candidate to improve phytoremediation efficiency through enhancing the biomass production, toxin uptake and leachate interception.

Phytoremediation of barium-affected flooded soils using single and intercropping cultivation of aquatic macrophytes

Aquatic macrophytes are potentially useful for phytoremediation on flooded areas. A field study in Brazil was conducted to evaluate Eleocharis acutangula (E), Cyperus papyrus (C) and Typha domingensis (T) in monocropping and intercropping, aiming to phytoremediate barium-polluted flooded soils. The treatments were: monocroppings (E, C and T); double intercroppings (EC, ET and CT); and triple intercropping (ECT). The 180-d field trial was performed in a flooded area with high barium content, with a randomized complete block design and three replicates. Plant stand size, biomass yield, and Ba concentration aboveground/Ba concentration in roots (translocation factor - TF) as well as Ba mass aboveground/Ba mass in roots (mass translocation factor - mTF) were determined. Most of the treatments did not differ on dry biomass, except for EC, which showed the lowest yield. Consistently with its biology, E. acutangula in monocropping showed the largest plant stand. Otherwise, intercroppings with T. domingensis achieved the highest amounts of barium absorbed from the soil and transferred most of the barium content from belowground to aboveground (mTF > 1.0), especially ET, which showed the highest mTF among the intercroppings (2.03). Remarkably, TF values did not reflect such phytoextraction ability for CT and ECT. Thus, mTF was more appropriate than TF to assess phytoextraction capacity. Furthermore, it was demonstrated that intercropping can increase barium uptake from flooded soils. Particularly, the intercropping ET constituted the most cost-effective treatment, with the cyperaceous species providing high plant coverage while T. domingensis facilitated barium removal by translocating it to the aboveground biomass.

Germanium in the soil-plant system-a review

Germanium (Ge) is widespread in the Earth's crust. As a cognate element to silicon (Si), Ge shows very similar chemical characteristics. Recent use of Ge/Si to trace Si cycles and changes in weathering over time, growing demand for Ge as raw material, and consequently an increasing interest in Ge phytomining have contributed to a growing interest in this previously rather scarcely considered element in geochemical studies. This review deals with the distribution of Ge in primary minerals and surface soils as well as the factors influencing the mobility of Ge in soils including the sequestration of Ge in secondary mineral phases and soil organic matter. Furthermore, the uptake and accumulation of Ge in plants and effects of plant-soil relationships on the availability of Ge in soils and the biogeochemical cycling of Ge are discussed. The formation of secondary soil minerals and soil organic matter are of particular importance for the concentration of Ge in plant-available forms. The transfer from soil to plant is usually low and shows clear differences between species belonging to the functional groups of grasses and forbs. Possible uptake mechanisms in the rhizosphere are discussed. However, the processes that are involved in the formation of plant-available Ge pools in soils and consequently its biogeochemical cycling are not yet well understood. There is, therefore, a need for future studies on the uptake mechanisms and stoichiometry of Ge uptake under field conditions and plant-soil-microbe interactions in the rhizosphere as well as the chemical speciation in different plant parts.

Alteration of trace element concentrations in plants by adhering particles - Methods of correction

Trace element concentrations in plants may be influenced by airborne dust or adhering soil particles. Neglecting adhering particles in plant tissue leads to misinterpretation of trace element concentrations in research fields such as phytomining, phytoremediation, bio-monitoring, uptake of micronutrients and provenance studies. In case washing or brushing the samples prior to analysis is insufficient or impossible due to fragile or pre-processed samples mathematical correction should be applied. In this study three methods are presented allowing to subtract the influence of adhering particles in order to obtain the element concentrations in plants resulting only from uptake. All mathematical models are based on trace elements with negligible soil to plant transfer. A prerequisite for the correction methods is trace element analytics with good accuracy and high precision, e.g. through complete acid digestion. In a data set of 1040 plant samples grown in open field and pot trials most plants show a small but detectable amount of adhering particles. While concentrations of nutrients are nearly unaffected trace element concentrations such as Al, Cd, Co, Cr, Fe, Mn, Ni, Pb, REEs, Ti and U may be significantly altered. Different sampling techniques like cutting height can also significantly alter the concentrations measured in the samples.