Saturday 12 October 2013

Comments on metal soil-stabilization- in Kirkham (2006), review article in Geoderma


Successful implementation of a stabilization technique for any metal within the soil environment lies heavily on a thorough understanding of the chemistry of the root zone, root exudates, contaminants, and fertilizer or soil amendments, to prevent unintended effect that might increase contaminant solubility and leaching (Pivetz, 2001).  The Knowledge of rhizospheric processes mediated by root exudates has not developed to a definitive level; likewise, the chemo-diversity of root exudates is largely unexplored and it is an obvious place to search for novel biologically active compounds that could enhance phytostabilization. Mucilage, a viscous, high molecular weight, insoluble, polysaccharide-rich material, secreted by root caps and epidermal cells, in addition to other functions, provides protection to plants from desiccation (Walker et al., 2003). It has been speculated that as the soil dries up, exudates will also begin to lose water to soil, and as a result, their viscosity increases. Likewise, the resistance to movement of soil particles in contact with exudates will also increase, and a degree of stabilization within the rhizosphere will be achieved (Walker et al., 2003). This speculation implies that exudates (mucilage) play a major role in the maintenance of root-soil contact, and suggest the possibility of this process in limiting plant nutrient uptake or rhizostabilization of contaminants. According to the Stokes-Einstein relation, diffusion is inversely proportional to fluid viscosity (Sposito, 2008) hence, as the viscosity of the mucilage increases due to loss of water to surrounding soil particles; permeability of the mucilage layer by ions in solution should also be limited, and consequently the amount of contaminant taken up by a plant. Also, the increase in resistance to movement of soil particles in contact with the mucilage could as well imply reduced exposure of roots to the soil matrix from where more metals could be solubilized; at the same time, field water regime of crops could be optimized to exploit this potential.  Therefore, possible research question that could be raised include:

1.       To what extent does the mucilage (galacturonic acid polymers with high viscosity) covering the surface of apical roots limits the diffusion of metal-organic complexes into the root apoplasm?

2.       Is there any difference in plant uptake of a metal with change in the viscosity of the root cap mucilage layer?

3.       How can field water regime be managed to maintain an optimum soil resistivity so as to limit root-soil contact, and thus metal uptake by plant roots?

 

The role of organic molecules that provide exchange sites for the metals in the root zone is ill defined. An assessment of these molecules especially in relation to their interaction with microbial exudates in influencing the chemistry of the rhizosphere could provide vital information on the mechanism of metal exclusion by plants. The negatively charged sites provided by these molecules upon proton dissociation by the acid groups provide complexation site for metals, including Cd. The possibility of this functional group binding Cd in octahedral coordination with the oxygen ligands could result in complexes of high stability which could precipitate out of solution due to soil solution saturation. Equally important is the effect of protons released by the organic acids on solubilization of metals bound to the residual fractions of the soil. This is of concern in soils derived from parent materials with high enrichment factors for heavy metals; our ability to quantify the effect of rhizospheric exudates on this metal fraction could be very important in predicting contaminant transfer to the food chain and the extent of ground water pollution over time. Exogenous materials such as kerogen and black carbon have received less attention in the study of organic compounds in the rhizosphere, a thorough understanding of the reactivity and roles of such materials could reveal vital information on how metal stabilization could be enhanced.

Due to the influence of root morphology on uptake of solutes, the mechanism controlling the variable uptake observed in different plant is worth investigating. The role of root hairs present in sub-apical root zones as it pertains to the increased release of protons and organic compounds in this zone needs a more complete evaluation. Furthermore, because the selectivity in uptake of an ion over another reflect more of physico-chemical similarity rather than essentiality (Marschner, 1995), it is important to understand the extent of this selectivity (for Cd) under varied supply of another physico-chemically similar metal (e.g. Zn). The extent of this inhibition could hint on possible ways to limit plant uptake of Cd.

Other possible questions with regards metal stabilization in the rhizosphere include:

i.                     To what extent do changes in rhizospheric processes affect metal mobility?

ii.                   What are those stress factors that influence plant root modification as an adaptation mechanism to exploit a larger soil volume for improved acquisition of nutrients, and how can such information be used in reducing plant uptake of heavy metals?

A profound understanding of these points is fundamental to successful metal stabilization within the soil environment.

 

 References

Kirkham M. B. 2006. Cadmium in plants on polluted soils: Effects of soil factors, hyperaccumulation, and amendments. Geoderma 137, 1932

Marschener, H. Mineral Nutrition of Higher Plants. Academic Press. London, UK.
Pivetz, B. E. 2001. Phytoremediation of Contaminated Soil and Ground Water at Hazardous Waste Sites. Ground Water Issue. EPA/540/S-01/500

 Sposito G. 2008. The chemistry of soils (2nd ed.). NY: oxford university press.

Walker S. T., Bais H. P., Grotewold, E. and Vivanco, J. M. 2003. Root Exudation and Rhizosphere Biology. Plant Physiology. 132, 1 44-51 .doi: http:/​/​dx.​doi.​org/​10.​1104/​pp.​102.​019661