Plants metal bioavailability

Metal bioavailability is generally increased with decreasing pH. This is due to the presence of phosphoric, sulfuric and carbonic acids, which increasingly solubilize organic- and particulate-bound metals. Particulate-bound metals are considered those bound to secondary minerals, for example, clays, iron and aluminum oxides, carbonates and sulfidic and phosphoric minerals. Due to the heterogeneous nature of soils and sediments, wide fluctuations in pH can exist in a given environment. For instance, metals may be more soluble in surface layers where plant exudates, microbial activity, moisture and leaching lower pH. 

The Heavy Metal Binding Capacity (HMBC) test is a bioassay that helps to quickly determine metal bioavailability in aquatic environments. HMBC can also be applied to soils and to root exudates from aquatic and terrestrial plants. The HMBC test is based on MetPLATE, a bacterial toxicity test that selectively detects metal toxicity. 

Metal bioavailability is the fraction of the total metal occurring in the soil matrix, which can be taken up by an organism and can react with its metabolic system (Campbell, 1995). Metals can be plant-bioavailable, if they come in contact with plants (physical accessibility) and have a form which can be uptaken by plant roots (chemical accessibility). Soil metals become accessible for humans by ingestion, inhalation and dermal contact. Available forms of PTMs are not necessarily associated with one particular chemical species or a specific soil component. Main soil PTMs pools of different mobility, target organisms and routes of transfer are sketched in Fig. 9.2. The most labile fraction, corresponding to the soluble metal pool, occurs as either free ions or soluble complexed ions and is considered the... 

Allen HE, editor. 2002. Bioavailability of metals in terrestrial ecosystems importance of partitioning for bioavailability to invertebrates, microbes, and plants. Metals and the Environment Series. New York SETAC. 

Diethylenetriaminepentaacetic acid (DTPA) possesses amine and carboxylic acid functional groups and could form octadentate complexes with heavy metals. It is used as a chelator to determine metal bioavailability in soil to plants and mobility to the aquatic environment. 

Metal bioavailability is an important parameter in determining the effectiveness of metal-contaminated soils in remediation. Assessment of metal bioavailability varies from humans to plants to microbes, and as such, various methods have been developed for a specific purpose. In this section we provide an overview of various methods used to determine the effectiveness of P-induced Pb immobilization. Reduction of Pb bioavailability by PR was demonstrated by feeding-trial and plant-uptake studies (Laperche et al., 1996). Hydroxyapatite and PR effectively... 

Plant Uptake In addition to metal bioavailability to humans, metal hioavailability to plants is an important parameter for evaluation of a remediation technology. Numerous studies have shown that P is effective in reducing Pb uptake by various plants. For example, P effectively reduced Pb uptake by tobacco from 36 mg kg to 16 mg kg (Mench, 1994). However, P was not effective in reducing Pb uptake by ryegrass in the same soil, although it was effective for other two soils where Pb uptake was reduced from 16.2 mg kg to 12.3 mg kg and from 4.9 mg kg to 1.4 mg kg . Laperche et al. (1996) showed that... 

Many extractants were recommended for use as bioavailable indices based on significant correlation between quantities of metal extracted from the soils by the extractants and metal uptake by plants. The most commonly used extractants were ammonium bicarbonate-diethyl triamine penta acetic acid (AB-DTPA) (Soltanpour and Schwab, 1977 Norwell, 1984) and ammonium acetate acetic acid-ethylene diamine tetra acetic acid (AAAc-EDTA) (Lakanen and Ervio, 1971 Sillanpaa and Jansson, 1992). AB-DTPA was used successfully as an extractant for characterizing the bioavailability of both native soil metals as well as metals added to soils in sewage sludges. Some authors also reported insignificant relationships between the AB-DTPA-extracted metals and test plant metal concentrations (e.g. Haq and Miller, 1972 Rappaport et al, 1988). O Connor... 

The uptake of metals from agricultural soil by crops and vegetables is an important pathway through which metals in contaminated soils impose health threats to organisms. On the other hand, the capacity of plant roots to remove heavy metals from contaminated soils is an emerging environmental cleanup and remedial biotechnology. In order to evaluate the risks of metal contamination in the area, it is essential to understand metal bioavailability, which depends on a metal s chemical form in the soil, rather than on the total amount accumulated (Allen, 1997 Zemberyova etal, 1998). 

Concentrations of bioavailable Cu and Zn computed from plant uptake measurements (Fig. 5) partially agreed with concentrations of Ethylene Diamine Tetra Acetate (EDTA) -extractable Cu and Zn (data not shown). Both soil EDTA-extractable Cu and Zn determinations steeply increased roughly 10- to 20-fold from 1991 to 1996 (Cu increasing from 3 to 35 mg kg and Zn from 3 to 65 mg kg ), and tended to decrease between 1996 and 1999. This agrees to some extent with bioavailable Cu but contrasts with bioavailable Zn deduced from plant analyses. Therefore, EDTA appeared to be a poor soil extractant for predicting the bioavailability of Cu and Zn in grasses grown on pig slurry-amended soil. Other chemical extractants were thus compared with the estimates of metal bioavailability (see below). 

S. Ruyters, J. Mertens, E. Vassilieva, B. Dehandsehutter, A. Poffijn, E. Smolders (2011) The red mud aeeident in Ajka (Hungary) plant toxieity and tiaee metal bioavailability in red mud contaminated soi. Environ. Sci. Technol, 45, 1616-1622. 

Upon formation of a metal chelate or complex, the next rate-limiting step in delivering iron to the cell is the diffusion of iron complexes through the. soil in response to diffusion gradients. In the vicinity of plant roots, metal chelates and complexes may also move by bulk flow in the transpiration stream as water moves from the soil into the plant. However, depending on their charge characteristics and hydrophobicity, metal chelators and complexes can become adsorbed to clay and organic matter, which may then decrease their mobility and bioavail-... 

Most trace metals may be precipitated with phosphate into insoluble metal phosphates (Table 7.5). Most metal phosphates have low solubility. High localization of phosphates reduces the bioavailability of Zn in arid soils. The banded application of P near the seeds depresses Zn uptake by com (Adriano and Murphy, 1970 Grant and Bailey, 1993), causing Zn deficiency. However, both N and P fertilizers increase Cd concentration in plants. Cadmium and Zn are antagonistic in root uptake and distribution within plants. 

Phytate (myo-inositol hexaphosphate Fig. 15.3, structure 33) is found in many food species and can be considered as a phytochemical. Its role in the plant is primarily as a phosphate store in seeds, but it is found in other tissues as well, for example, tubers (Harland et al., 2004). Phytate and its hydrolysis products are anti-nutrients that chelate metal ions and thus reduce their bioavailability (Persson et al., 1998 House, 1999). This is particularly a problem with cereal grains, but pre-processing can improve mineral absorption from these foods (Agte and Joshi, 1997). There is some concern that high phytate foods could also contain higher levels of toxic heavy metals caused by natural accumulation. Plants also contain phytate-degrading enzymes that can also influence metal ion bioavailability (Viveros et al., 2000). 

---