This review paper assessassesses the progress of the Diffusive Gradients in thin films (DGT) application and the latest advances with a focus on the mobility and bioavailability of heavy metals in soil. Soil chemical extractions are extensively used to predict nutrient elements in the soil. However, these measurements have their weakness and shortcoming. Comparing DGT with conventional extraction methods, DGT is a sampling technique with significant advantages; including speciation capabilities, sensitivity, time-integrated signal, low risk of contamination, and, time-averaged concentrations. These findings strengthened the use of DGT as a potential monitoring tool for soil with heavy metal contamination.
Studies that have used the DGT technique to evaluate processes important to bioavailability have been booming in the last 13 years, especially its application in soils. As element accumulated by DGT and in-plant depends on the plant species and the soil study, DGT has not yet been accepted universally as a dependable analytical tool. Some recent studies have shown a good relationship between the measurements of metals concentration in soil and plant by DGT, and cohesive results have been obtained from these measurements when they are based on the DGT technique.
DGT is a newly established procedure to assess the bioavailability of trace elements in sediments and soils, and its applications are still in the early stage of testing. Therefore, future applications of DGT are likely to include the studies of heavy metals contamination in soil for risk assessment and transfer rates to the food chain, as some studies have indicated the potential of DGT in this area.
Keywords: DGT, Gels, Mobility, Bioavailability, Soil, metals
Heavy metals (HMs) such as cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn) generally refer to metals and metalloids having densities greater than (>5g/cm3). Although HMs may occur naturally in the environment, humans may promote HMs pollution through agriculture, urbanization, industrialization, and mining (Alloway, 2013; Wuana & Okieimen, 2011; Zimmerman & Weindorf, 2010). The convertible, persistent and irreversible pollution not only degrades the quality of the atmosphere, water bodies, and soil but also threatens the health of animals and human beings by transport through the food chain. As the major sink for HMs in the terrestrial ecosystem, soils polluted with HMs have been attracting more and more interest. However, the assessment of eco-environmental and human risks remains limited.
The mobility of HMs such as Cd, Cu, Pb, and Zn, and their bioavailability to sensitive receptors in terrestrial environments, is strongly influenced by separating the metal between the solid and dissolved fractions of the soil (mostly spoken of as the Kd) (Dočekalová, Kovaříková, & Dočekal, 2015; Rieuwerts, 2015; Warnken, Zhang, & Davison, 2006). The measurement of the total metal concentration is not always, or even usually, appropriate for considerations of metal mobility and bioavailability (Docekalova, Skarpa, & Docekal, 2015; S. Li, Zhang, Zhou, Zhang, & Chen, 2009; Speir, Van Schaik, Hunter, Ryburn, & Percival, 2007). These metal fractions may occur differently in the soil, and their mobility and bioavailability are governed by physical-chemical soil properties (Glaser, Lehmann, & Zech, 2002) and their metal chemical compositions (Camobreco, Richards, Steenhuis, Peverly, & McBride, 1996).The Critical concentrations of these heavy metals in the soils environment, whether measured or predicted from models, are useful in risk assessment, for example, to assess plant exposure to soil metals via root uptake or transport into surface or ground waters (Hattab, Motelica-Heino, Bourrat, & Mench, 2014; Rieuwerts, 2015). Determining the bioavailability of heavy metal in a contaminated environment is a crucial step in risk assessment for metal-polluted soils (Sungur, Soylak, & Ozcan, 2015). However, several soil removal measures have been developed to improve the measurement of the bioavailability of these elements. Metals toxicity is usually predicted from the relationships with the soil solution concentration, the free metal ion in soil solution, or some functionally defined extractable fraction (Athar & Ahmad, 2002; Soriano-Disla et al., 2010). It is necessary, therefore, to understand the impact of the presence of heavy metals in soils and be able to develop techniques to determine the potential risks from soil contamination. In this context, the Diffusive Gradients in thin-Films (DGT) technique based on the diffusion of metals from the soil solution can be considered an effective alternative compared to other traditional sampling procedures (Hooda, Zhang, Davison, & Edwards, 1999; H. Zhang, F.-J. Zhao, B. Sun, W. Davison, & S. P. Mcgrath, 2001; H. Zhang, F. J. Zhao, B. Sun, W. Davison, & S. P. McGrath, 2001).
The use of the DGT technique in soils and sediments provides unique information relating to the system dynamic. The DGT technique was developed for in-situ measurements of trace metals elements such as Cd, Cu, Co, Ni, Pb, and Zn (Denney, Sherwood, & Leyden, 1999), which have been used to characterize soils (Cattani et al., 2008; Dočekalová, Kovaříková, & Dočekal, 2012; Nolan, Zhang, & McLaughlin, 2005; Nowack, Koehler, & Schulin, 2004), and have been extended to the measurement of metal fluxes in sediments and soils (Noh, Hong, & Han, 2016; Hao Zhang et al., 2001). Since the extension of DGT to soils and sediments, the technique has been evaluated extensively in several studies in geochemical and health disciplines (Nolan et al., 2005; Hao Zhang et al., 2001). A good Predicting element availability to plants with DGT correlation between concentrations of metals in plants and their measurement by DGT has been observed in several studies (Degryse, Smolders, Zhang, & Davison, 2009; Pérez & Anderson, 2009; Tandy et al., 2011). From these studies, DGT has been recognized to be suitable for the evaluation of metal bioavailability for plants. Based on itson usefulness, the DGT technique has shown to be superior to other sampling procedures in that it accumulates chemicals continuously from sediments and soils and can provide time-weighted average concentrations of pollutants over the exposure period base on Fick’s first law of diffusion (H. M. Conesa, R. Schulin, & B. Nowack, 2010). The DGT technique is an effective technique for determining labile metal species in soils and sediments (Martin, 2008), and has its application to a broad suite of metals common to mine-influenced environments (e.g., Cu, Ni, Zn, Cd, Pb, Hg). The Metal values obtained using DGT also have been shown to conform reasonably well to predict the labile metal fraction using speciation models (Arevalo-Gardini, Arevalo-Hernandez, Baligar, & He, 2017; Unsworth et al., 2006). However, the work efficiency of DGT flux can reach higher levels when the soil’s moisture is at the maximum water holding capacity (MWHC) levels. The DGT technique is a relatively simple research tool that allows the metalstesting of bioavailable metals and helps us understand how the biota cooperate with their environment. In this review, we discuss the applications and advances of the DGT technique in soil. We begin with a brief introduction of the theory of DGT and then discuss the gels used in DGT as a key part, and the recent advances in bioavailability and toxicity of heavy metals assessed by DGT. Finally, we make an outlook on possible aspects of DGT application in the soil in the future.
The DGT technique is based on the involvement of a polypropylene device comprising of two pieces, the piston, and the cap. The piston work to support the gel layer that is placed inside the devices; a membrane, diffusive layer, and a functional binding layer, which may vary based on the procedures and the targeted analyst being sample (Amauri Antonio Menegário, Yabuki, Luko, Williams, & Blackburn, 2017; Tafurt-Cardona et al., 2015). The base of the piston and membrane or diffusive layer is enclosed by the cap, which ensures the pathway of ions from the bulk solution to the inner layer through a well-defined area; while the diffusive layer forces ion transport to occur completely by diffusion, thus allowing analyst concentration to be determined (Amauri Antonio Menegário et al., 2017; Zhang & Davison 2010). DGT samplers are very easy to install and easy to use at a reasonable cost. However, detail on installation, deployment time, storage, calibration, and other procedures can vary according to the needs, and environments (Ernstberger, Davison, Zhang, Tye, & Young, 2002; Mengistu, 2015). For the deployment of the devices in soil, the unit is placed in close contact with wet soils using a twist and turns methods or inserted into sediments (Hanousek et al., 2016). The labile forms of chemical elements diffuse through the filter and diffusive gel and are absorbed in the binding gel (Hao Zhang & Davison, 2001, 2015). As the unit is being deployed, there is a diffusive boundary layer (DBL) that is formed between the diffusive layers, the diffusive gel, and filter membrane, and the resolution (Y. Zhang, S. Mason, A. McNeill, & M. J. McLaughlin, 2014). After all the deployment time, a linear concentrations gradient is well established between the solution and the binding gel (Martin, 2008; S. Mason, R. Hamon, A. Nolan, H. Zhang, & W. Davison, 2005). The DGT technique work with Fick’s first law of diffusion, which monitored the diffusion of dissolved species such as Cd, As, Mn, Cu, Pb, Zn, etc. through adding a membrane-diffusive layer, which could also control and restrict the flux accumulated in an ion-exchange resin (J. Luo, Zhang, Santner, & Davison, 2010; Y. Zhang et al., 2014). Assuming the concentration gradient of the ions remains constant during deployment time (t), the flux F (mol cm−2 s−1) of an ion through the diffusive gel layer is given by (eq 1) and the concentration of ions measured by the DGT (CDGT) can be calculated using eq 2 (Huynh, Zhang, & Noller, 2012).
F=DC/ ∆g (1)
CDGT=M∆ g DtA (2)
where D is the diffusion coefficient (cm2 s−1) for a given metal ion, C is the bulk concentration of an ion, A, the area of hydrogel membrane (cm2) exposed to the bulk solution, and M, the mass of metals (ng) accumulated in the diffusive layer over time, t (s). Until now, various types of materials have been evaluated as diffusive layers within the DGT samplers to assess the labile species of heavy metals in soil (Docekalova & Divis, 2005; Menegario, Yabuki, Luko, Williams, & Blackburn, 2017).
As discussed above, the first demonstration of DGT potential as a predictor of bioavailability was shown in 1999 (Hooda et al., 1999), and its application of it for bioavailability assessment in the soil is still in its early stage of development. Several different materials have been defined and proposed by many researchers in many kinds of literature for application as DGT binding layers. These binding layer, however, comprises solid resins that are combined into a gel matrix, for example, polyacrylamide to form a binding layer (Heidari, Reyhanitabar, Oustan, & Olad, 2016). In expanding the range of analysts determined in a single DGT deployment, mixed binding layer (Sean Mason, Rebecca Hamon, Annette Nolan, Hao Zhang, & William Davison, 2005), and multiple binding layer (Naylor, Davison, Modelica-Heino, Van Den Berg, & Van Der Heijden, 2004) samplers have been developed. To use solid material as a binding agent within a DGT binding layer, the material must be meaningfully small in particle size to allow simple and evenly combined into a gel matrix. As shown with different binding agents, many options are employed to consider which binding layer can be used for a particular geometry. The DGT suitability study has revealed thatmethod binding layer suitability depends on the environment in which it will be deployed, with many of the binding layers only appropriate to freshwaters owing to the high concentrations of potentially competing ions present in more complex environments like seawater (Héctor Miguel Conesa, Rainer Schulin, & Bernd Nowack, 2010; J. G. Panther et al., 2013).
The Solute measure by DGT accurately relies on the strength of interaction with the binding layer. However, these binding layers have been compared with different materials to assess trace metals in sediments and soils. It is not possible to show all the working performances of every binding layer described; only the binding layer which has gained a widespread application and has stood the test of time are considered. This includeishasChalex-100, ferrihydrite, Metsorb, zirconium dioxide (ZrO2), 3-mercaptopropyl functionalized silica, AG50W-X8, and XAD18. These DGT binding layers have been extensively evaluated with different materials in several types of research for trace elements. This is done to determine whether these binding materials are capable of accumulating analysts of concern (Uptake). For example, (J. G. Panther, Teasdale, Bennett, Welsh, & Zhao, 2010) evaluated Metsorb and Ferrihydrite DGT for measuring dissolved reactive phosphorus in the freshwater environment for deployment time up to four days. Their work revealed that, while both techniques (binding layer, ferrihydrite, and Metsorb) accurately measured the analysts of concern over the four-day deployment period, only Metsorb DGT was capable of measuring the Dissolved Reactive Phosphorus (DRP) in seawater after four days. However, the ferrihydrite DGT techniques underestimated the DRP in freshwater by 31%. (Bennett, Teasdale, Panther, Welsh, & Jolley’s, 2010) work revealed that the measurements of Mn by chalet-100 experienced similar limitations in synthetic seawater, underestimating Mn concentration by 49% after four days of deployment. They concluded that accurate measurement of Mn was possible with a longer deployment time, up to 48h. Suggesting that the short-term validations reported in many kinds of literature may not reveal these limitations of the binding layer. Of all the above-mentioned DGT binding layers, the chalet-100 DGT has stood the test of time to measure heavy metals in soils. Chale-100 as a DGT binding layer has proven to be effective in measuring heavy metals in contaminated soils.
Originally, the DGT technique was developed for in situ measurements (Davison & Zhang, 2012a; Scally, Davison, & Zhang, 2003). Since then, the DGT technology has developed rapidly. Significant progress has been made within two aspects of the technology, which include the development of newa newbinding gel and the 2D high-resolution measurements (Davison & Zhang, 2012a; Sochaczewski, Tych, Davison, & Zhang, 2007). It is well established that the Chelex resin can take up trace metals, as it contains paired with iminodiacetate ions which act as chelating groups in the binding polyvalenmetaltelexsulfidels ions. Accordingly, the binding agent for the first DGT was the chelex resin (Davison & Zhang, 2012a; Sean Mason et al., 2005). After that, the ferrihydrite gel with a strong affinity for phosphorus was used to measure labile phosphorus (Chaosheng Zhang, Shiming Ding, Di Xu, Ya Tang, & Ming H Wong, 2014; Hao Zhang & Davison, 2015) and silver iodide was included in the gel to take up sulphide (Teasdale et al., 1999). However, the radioactive caesium was adsorbed by a gel containing ammonium molybdophosphate (W. Li, Wang, Zhang, & Evans, 2009; Murdock, Kelly, Chang, Davison, & Zhang, 2001; Puy et al., 2014).
In recentcesiumaa study, (Q. Sun et al., 2014; Chaosheng Zhang et al., 2014) Zr oxide gel was developed to measure phosphorus and inorganic arsenic with high capacities. The hydrous zirconium oxide (Zr oxide) has been combined with silver iodide to measure both phosphorus and sulfide, (Q. Sun et al., 2014; Q. Sun, Chen, Xu, Wang, & Ding, 2013); and combined with Chelex to measure phosphorus and iron (Xu et al., 2013). The mixed Amberlite and ferrihydrite gel has been recently developed to measure potassium and phosphorus (Yulin Zhang, Mason, McNeill, & McLaughlin, 2013; Yulin Zhang, Sean Mason, Ann McNeill, & Michael J McLaughlin, 2014). Recently, it has been found that the capacities of the Zr oxide DGT for As in freshwater and seawater were 5∼19 times and 3∼13 times more than those reported for the commonly used ferrihydrite and Metsorb DGTs, respectively (Q. Sun et al., 2014; Hao Zhang & Davison, 2015). In addition, a titanium dioxide gel-assembled DGT has been used to simultaneously measure arsenic, phsulfideosphorus and, sulfide metals in soil (Bennett et al., 2010; Fauvelle et al., 2015; Jared G Panther, Bennett, Welsh, & Teasdale, 2013; Chaosheng Zhang et al., 2014). The first material used in DGT to fabricate the binding layer was the polyvalent metal chelating resin Chelex-100 (Docekalova & Divis, 2005). It needs to be mentioned that the DGT method has recently been developed to measure organic and inorganic compounds (Q. Sun et al., 2014; Q. Sun et al., 2013). However, the DGT technique measures directly the mean flux of labile species in soils to the device during deployment. Thus, providing a novel and promising approach for the measurement of bioavailable metal concentrations in soils (Lehto, Davison, Zhang, & Tych, 2006; Hao Zhang & Davison, 2015).
Besides measuring individual chemical elements, DGT techniques for simultaneous measurements of multiple elements have been developed, that is, Zn, Mn, Fe, and As (Naylor et al., 2004; Chaosheng Zhang et al., 2014). Two separate gels of silver iodide and Chelex-100 were used together to measure sulphide and metals in sediments (Motelica-Heino, Naylor, Zhang, & Davison, 2003; Chaosheng Zhang et al., 2014). A mixed binding layer (MBL) containing a mixture of ferrihydrite and Chelex-100 was developed to measure phosphorus and cations (Sean Mason et al., 2005; Chaosheng Zhang et al., 2014). Furthermore, various types of materials (e.g. polyacrylamide gel, agarose gel, dialysis membrane, Nafion membrane, chromatography paper,ion-imprinted, and filter paper) have been evaluated as diffusive layers within the DGT samplers (Menegario et al., 2017). Recently, the development of DGT methods has been to use ion imprinted binding layers, wherein are the shows he analysts are sorbed to the ligand to improve species retention, this approach has been successfully carried out for Cd (II) Pb(II) (Dong, Fan, Sui, Li, & Sun, 2014; Stanley et al., 2016; Sui et al., 2016). The development of new binding gels enables DGT technique to analyze diverse analysts in an environment, particularly those of environmental importance (Davison & Zhang, 2012a; Huang, Bennett, Teasdale, Gardiner, & Welsh, 2016). In addition, the primary limitations of the technique include the limited functional pH range (pH 5–9) and the limited application to certain metals/metalloids (Divis et al., 2009; Martin, 2008). However, (Galceran & Pu015) show that it is possible to interpret quantitatively the proportions of metals penetrating into the rear of the binding layer in terms of the dissociation kinetics of metal complexes with humic substances using DGT. Even though the application of DGT for metals analyses is well established, it still needs to be extended to other elements. Despite all the above-described DGT advantages in the tin study of bioavailability, the development of a DGT technique and its validation by an established model for interpreting data obtained from a particular measurement remains a challenge in the study of DGT. Therefore, a comprehensive and standardized DGT method is required to ensure that the DGT results are measured using different binding gels in different environments other.
The applications of DGT to assess the bioavailability of heavy metals in soils have received attention worldwide. A trend that can be observed linked to the development of new methods based on the DGT technique, is the prediction of available metals based on the comparison of the DGT results with others methods for specific analysts (Menegario et al., 2017). Presently, there is an increasing body of research focusing on the use of the DGT technique for predicting the bioavailability of metals and toxicity (Menegario et al., 2017). The Labile and small complexes of metals are the forms that allowed metal to be able to pass through cell membranes and therefore, they are commonly the most bioavailable and harmful to biota (Turner et al., 2012). However, as these metals are the ones sampled by the DGT technique, some studies have been carried out to assess the possibility to use DGT to predict the bioavailable metals and their toxicity to human-human (Tandy et al., 2011; Tella et al., 2016). Some recent studies on bioavailable metals in soil using DGT and some plant species. The DGT technique has been developed mostly for toxic cationic divalent trace metals but, it has also been expanded to test the bioavailability of plant nutrients such as: phosphate, potassium, uranium, methyl mercury, arsenate, molybdate, and most recently been used to understand the effect of nanoparticles in the soil environment (Davison & Zhang, 2012b; Docekalova et al., 2015; Huang, Bennett, Teasdale, Kankanamge, & Welsh, 2017). The concentration of these heavy metals measured by DGT is associated with labile species, including free ions and kinetic resupply of ions from the solid phase, which are considered to be bioavailable (Hao Zhang, Davison, Knight, & McGrath, 1998). Recently, Zhang and Davison (2015) reviewed the use of DGT for studies of speciation and bioavailability. During this study, however, key studies over decades were examined and discussed, by giving an environmental perspective to the theory of DGT (relating to measurements of metal complexes, Too), and the ability of DGT to obtain in situ information was discussed. In addition, the relationships between DGT measurements in soil and plant uptake were deeply discussed example to determine trace element species, the methods based on the DGT technique are mostly used (Amauri A Menegário, Tonello, & Durrant, 2010; Amauri Antonio Menegário et al., 2017). While it is true that, DGT might not be expected to be a universal tool to predict bioavailable metals in soil and sediment, for examples, the measurement of labile species has aidhasofed predicttheyns to where they have emerged (Degryse et al., 2009; Smolders et al., 2009). In a recent study, (Johnson et al., 2012; Menzies, Kusumo, & Moody, 2005) reported that the redox-driven layering in sediments makes them a particularly challenging environment for predicting and assessing bioavailability and toxicity, but, again, there are some encouraging recent results (Hao Zhang & Davison). (Salzberg et al., 2012; Simpson & Batley, 2003) also reported that there is no single technique that can be expected to imitate the range of processes that may be operating during biological uptake. Many studies involving bioavailability of trace elements to DGT measurement have been investigated thoroughly with focused on the uptake of trace metals such as Cd, Cu, Zn, Ni and Pb by plants, using a standard DGT device with a chelex resin layer, but very few studies have compared bioavailability with DGT for other elements (Shahid, Dumat, Khalid, Niazi, & Antunes, 2016; Tandy et al., 2011; P. Wang et al., 2016). For example, (Wang et al., 2014) measured As concentrations in forty-three different soils collected in China with different extractions procedures, including DGT and soil solution measurement. It was concluded that, the DGT-measured and soil solution concentrations of As give a good prediction of the plant As concentrations and show much better than the acid digestion extraction procedures. It has been recognized by many researchers that, in the conventional methods of testing soil solution, metal speciation may change during sampling and extraction and the kinetics of metal resupply from the solid phase to the solution is are not considered (Koster, Reijnders, van Oost, & Peijnenburg, 2005; Chaosheng Zhang et al., 2014). Furthermore, the bioavailability of metals in a given soil is reliant on both, their concentrations in the soil solution and their rate of transport through the soil (H. Zhang & Davison, 2000; Hao Zhang & Davison, 2001). With little exclusions, most of the studies concerning with DGT and bioavailability in soils have focused on measuring the concentrations rather than uptake fluxes (Ahmed et al., 2013; Jun Luo et al., 2013). Up till now, it has been used by (Sean Mason, McNeill, McLaughlin, & Zhang, 2010; Menzies et al., 2005) for predicting yield responses of tomato tomatoes, cultivated at high P levels and for predicting wheat responses to different P fertilizers formulations (Menzies et al., 2005). Recently, (Sean Mason et al., 2010; McBeath et al., 2007) showed that the DGT technique was able to predict the growth response of wheat when cultivated under a range of P conditions.
(Degryse, Shahbazi, Verheyen, & Smolders, 2012a, 2012b) measured Cd uptake by spinach in solution in the absence or presence of synthetic ligands. They suggest, however, concluded that, at constant free ion activity and constant total Cd concentration, the uptake of metal increased with increasing dissociation rate of the complex, and correlated well with DGT measured concentrations, which strongly suggest that Cd uptake by spinach was limited by diffusion. They further reported that, direct uptake of the complex was predicted to be a major contribution only at millmillimolari molar concentrations of the complex or at very large ratios of complex to free ion concentration. The ‘true Km’ for uptake of Cd2+ Zn2+ was estimated at 99%), BPB (TGI, >98%) or BPF (TGI, >99%) compared compare with DGT to assessed bisphenols desorption from soils (Davison & Zhang, 2012b; Fernandez-Gomez, Bayona, & Diez, 2014). The Schematic representation of the concentration gradient of analysts the analyst through the deployed DGT device and the immediately adjacent soil induced the y deployment of activated charcoal based on the DGT device.
The DGT techniques have been considered houses, a potential monitoring tool for soil with heavy metal contamination. The technique involves using a specialized-designed passive sampler that house a binding gel, diffusive gel and filter membranes. During deployment, the elements or compounds pass through the membrane filter and diffusive gel and are assimilated by the binding gel in a rate-controlled manner. However, the post-deployment analysis of the binding gel can be used to determine the bulk solution concentrations of elements or compounds using a simple equation. There is no reverse diffusion of the analyst back into the solution is assumed to occur during deployment of the DGT device. Bearing in mind the difficulty of environmental chemistry, it is not surprising that the ability of DGT to predict bioavailability and toxicity are variables. High solute concentrations are usually associated with conditions of toxicity. There is substantial literature that shows that under these conditions the biotic ligand model applies, which denotes that uptake is affected by the free ion concentration and competition with other ions for the biotic ligand.
In conclusion, there is a great promise concerning the use of DGT in soils, particularly concerning helping understand the uptake processes and predicting the uptake, or requirements for, nutrients when their concentrations are low, which indicates deficient conditions. As a surrogate of plants, DGT is capable of measuring heavy metal concentrations in the soil. However, DGT is not considered to be a universal application tool for the prediction of the bioavailability of metals in soil and sediments. Moreover, there is no single technique that is expected to imitate the wide range of processes that may be functioning during biological uptake. DGT is no exclusion, but it does imitate some significant processes that may be dominant in some situations. Future application of DGT are likely to include the studies of heavy metals contamination in soil for risk assessment and transfer rates to the food chain, as some studies have indicated the potential of DGT in metals in this area. Further investigation in these areas will help the development of DGT as a tool for assessing metals bioavailability in soils and sediments and the risk associated with its contamination. The DGT technique has also been used, in combination with some materials to assess the bioavailability of heavy metals in soils. There is a great promise for the use of DGT in soil, particularly under metal-organicerstandingorganiser standing the uptake processes and for predicting the uptake, or requirements for nutrients when the concentrations are low, which indicates deficient conditions.
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