Studies on soil rhizosphere: speciation and availability of Cd

Abstract

The rhizosphere soils of two durum wheat (Triticum turgidum var. durum L.) cultivars Kyle and Arcola grown in two selected soils of southern Saskatchewan were collected both at 2-week and 7-week plant growth stages. The cadmium availability index (CAl), determined as M NH 4 CI-extractable Cd, pH and the distribution of the particulatebound Cd species of the soils were carried out and the data were discussed in comparison with those of the corresponding bulk soil. At the 2-week growth stage, the pH of the rhizosphere soil was less than that of the corresponding bulk soil and the CAl values were higher in the rhizosphere soil, indicating that more Cd was complexed with the low-molecular-weight organic acids (LMWOAs) at the soil-root interface and was extractable by M NH 4 CI. Compared with the bulk soils, the CAl values were 2-9 times higher in the soil rhizosphere of the plots fertilized with Idaho monoammonium phosphate fertilizer at 2-week growth stage, which is attributed to the combined effects of the Cd introduced into the soil rhizosphere from the fertilizer (Cd content of the fertilizer was 144 mg kg-1) and complexation reactions of phosphate and LMWOAs with soil Cd. At 7-week plant growth stage, such differences were not observed. The increased amounts of carbonate-bound and metal-organic complex-bound Cd species of the rhizosphere soils are due to the increased amounts of carbonate, a product of plant respiration, and the LMWOAs at the soil-root interface, respectively. Simple correlation analysis of the data showed that the CAl of the rhizosphere soils of the control plots correlated at least two orders of magnitude better with the metal-organic complex-bound Cd whereas the CAl of the rhizosphere soils treated with Idaho phosphate correlated better with carbonate-bound Cd species in comparison to other species.

Differential accumulation of Cd in durum wheat cultivars: uptake and retranslocation as sources of variation

“…Rhizosphere soil of 2-week-old Arcola had a higher CAI than Kyle when grown on a soil with high Cd availability and phosphate fertilizer, although the rhizosphere soil pH was the same for both cultivars. The Cd 2+ /Cd total data from this study are consistent with Krishnamurti et al (1996), as the increase in CAI by Arcola was attributed to complexation of particle-bound Cd by LMWOAs. There was no particle-bound Cd in the nutrient solution in this study; however, the high affinity of the LMWOAs for Cd would probably lead to the complexation of Cd 2+ in the nutrient solution.…”

“…The change in speciation noted in the present study could also be a result of the addition of complexing ligands to the solution by the plants, differentially between cultivars. Krishnamurti et al (1996) determined the 'cadmium availability index' (CAI) in rhizosphere soil of Arcola and Kyle at 2 and 7 weeks of age. The young plants (similar in age to those for the Accumulation of Cd in wheat 2577 Cd 2+ /Cd total measurements used here) had appreciable amounts of low molecular weight organic acids (LMWOAs) in the root exudates, whereas the 7-week-old plants did not.…”

Mobility and Bioavailability of Heavy Metals and Metalloids in Soil Environments

“…Total concentrations of metals in soils are a poor indicator of metal toxicity since metals exist in different solid-phase forms that can vary greatly in terms of their bioavailability (Krishnamurti et al, 1995a and1995b;Krishnamurti et al, 1996;Naidu, 2002, 2008;Huang and Gobran, 2005). Most regulations or guidelines used for protecting soil from metal pollution (to set maximum permissible metal concentrations used for definition of contaminated sites, for setting remediation clean-up goals, and for limiting loading of metals to soils in fertilizers and wastes) are still based on assessing the total concentration of metal present in the soil.…”

Coupling the use of computer chemical speciation models and culture techniques in laboratory investigations of trace metal toxicity

“…These include such compounds as sugars, organic acids, amino acids and phenolics (Marschner and Romheld, 1996). Low molecular weight organic acids have been studied in greater detail than the other exudates and have been shown to influence the speciation of Cd in solution (Mench et al, 1988) and in soil (Krishnamurti et al, 1996). However, due to difficulties in collecting rhizosphere soils and the time-consuming procedures for measuring low molecular weight organic acids (Szmigielska et al, 1996), little work has been done to determine free-metal ion concentrations in rhizosphere soil.…”