Metal at the soil-root interface

Research on the metal speciation of the soil solution has been encouraged by the free metal ion hypothesis in environmental toxicology (Lund, 1990). This hypothesis states that the toxicity or bioavailability of a metal is related to the activity of the free aquo ion. This hypothesis is gaining popularity in studies of soil-plant relations (Parker et al., 1995). However, some evidence is now emerging that free metal ion hypothesis may not be valid in all situations (Tessier and Turner, 1995). Plant uptake of metals varies with the types of chelators present in solution at the same free metal activity. Furthermore, given the same chelate, total metal concentration in solution affects metal uptake by plants. Either kinetic limitations to dissociation of the complex or uptake of the intact complex could explain these observations (Laurie et al., 1991). The possible reactions of complexed metals at the soil-root interface and the potential uptake by plants of metal-organic complexes are depicted in Figure 1.8. 

The literature shows that many fewer data are available on dissolved than on solid-phase heavy metals at the soil-root interface. To date, several approaches have been used to acquire soil solution samples from the rhizospheric environment, includmg water displacement, water extraction, centrifugation, and microsuction cups. However, because of technical constraints, no single method appears to sample unaltered rhizosphere solution. 

The results from these complementary approaches showed that the rhizospheric environment accelerated the weathering of primary minerals. In situ weathering products contributed to the retention of trace metals at the soil-root interface. The reader is referred to Chapter 2 written by S6guin et al. (2005) for a detailed presentation of the methods used to measure weathering and for a comprehensive discussion of the results gathered as part of this project. 

The maps obtained by TOF-SIMS for selected metals are presented in Fig. 3. Lighter colours depict higher relative metal concentrations. These maps show a net accumulation of metal at the soil-root interface. 

Furthermore, abiotic and biotic reactions are not independent but rather, interactive processes in soil environments. Interactions of abiotic and biotic processes are thus very important in governing the dynamics and fate of metals and metalloids in soils, especially at the soil-root interface. Abiotic and biotic interactions in the rhizosphere in influencing the stabilization of contaminants and the efficacy of ameliorants need to be investigated. The impact of physical, chemical, and biological interfacial interactions on risk assessment and management of metal and metalloid contamination and restoration of ecosystem health merits close attention. 

BIOGEOCHEMISTRY OF METALS AND METALLOIDS AT THE SOIL-ROOT INTERFACE... 

Metal and metalloid concentration gradients at the soil-root interface 277... 

Beyond this effect, however, the concentration gradients measured in various solid-phase metal fractions at the soil-root interface can also be attributed to the effects of a variety of processes, such as rhizosphere acidification or alka-linization, adsorption or desorption reactions, and precipitation or dissolution phenomena, which are themselves associated with plant uptake and a range of... 

Sdguin, V., Gagnon, C., and Courchesne, F. (2004). Changes in water extractable metals, pH and organic carbon concentrations at the soil-root interface of forested soils. Plant Soil 260, 1-17. 

The map of total Mn concentrations obtained by Synchrotron XRF is presented in Fig. 4(a). Relative concentrations are presented with lighter colours indicating higher metal concentrations. As for TOF-SIMS, there is clearly a higher relative content of Mn at the soil-root interface. Similar results were also obtained for other metals (e.g. Cu and Fe) (Naftel et al, 2002). 

Metals preferentially accumulate at the soil-root interface (Fig. 3). Identifying the precise location of the interface between the root and the soil is, however, not easy, and the transition from the soil to the root is gradual (Naftel et al, 2002). Moreover, mineral particles can be incorporated in the external tissues of roots (Adamo et al., 1998). 

Similar to Al, Fe also accumulates at the soil-root interface, probably in the form of oxides. Inside the root, there is a close association between the Cu and the Fe distributions. These metals could be associated with organic matter to form complexes. It is known that Cu and Fe are relatively immobile elements in plants and that they are not readily transferred between tissues during growth. (Alloway, 1990). 

It can be speculated that copper, and by analogy other trace metals, is retained in soluble forms and transported through the soil complexed by FA type organic molecules. However, at the soil root interface the metals may be transferred to small biochemical molecules of either plant or microbial origin. 

Manceau, A., Nagy, K.L, Marcus, M.A., Lanson, M., Geoffrey, N., Jacquet, T. and Kirpiditchikova, T. (2008) Formation of metallic copper nanoparticles at the soil-root interface. Environmental Science and Technology, 42,1766-72. 

There may be a cycling of S compounds of different oxidation state between anaerobic and aerobic zones in the soil, such as at the soil—floodwater interface. In reduced lake and marine sediments this leads to accumulation of insoluble sulfides as S04 carried into the sediment from the water above is immobilized. Such deposits function as sinks for heavy metals. Plants absorb S through their roots as S04 H2S is toxic to them. Therefore HS must be oxidized to S04 in the rhizosphere before it is absorbed. 

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