G. S. R. Krishnamurti

Low-molecular-weight organic acids in rhizosphere soils of durum wheat and their effect on cadmium bioaccumulation

Cadmium (Cd) accumulation has been found to vary between cultivars of durum wheat (Triticum turgidum var. durum), and it is hypothesized that low-molecular-weight organic acids (LMWOAs) produced at the soil-root interface (rhizosphere) may play an important role in the availability and uptake of Cd by these plants. The objective of this study, therefore, was to (1) investigate the nature and quantity of LMWOAs present in the rhizosphere of durum wheat cultivars Arcola (low Cd accumulator) and Kyle (high Cd accumulator) grown in three different soils: Yorkton, Sutherland and Waitville, and (2) determine the relationship between Cd accumulation in these plants and LMWOAs present in the rhizosphere. Plants were grown for two weeks in pot-cultures under growth chamber conditions. Oxalic, fumaric, succinic, L-malic, tartaric, citric, acetic, propionic and butyric acids were found and quantified in the water extracts of rhizosphere soil, with acetic and succinic acids being predominant. No water extractable LMWOAs were identified in the bulk soil. Total amount of LMWOAs in the rhizosphere soil of the high Cd accumulator (Kyle) was significantly higher than that for the low Cd accumulator (Arcola) in all three soils. Furthermore, large differences in amounts of LMWOAs were found in the rhizosphere soil for the same cultivars grown in different soils and followed the pattern: Sutherland > Waitville > Yorkton. Extractable soil Cd (M NH4Cl) and Cd accumulation in the plants also followed the same soil sequence as LMWOA production. Cadmium accumulation by the high and low Cd accumulating cultivars was proportional to the levels of LMWOAs found in the rhizosphere soil of each cultivar. These results suggest that the differing levels of LMWOAs present in the rhizosphere soil played an important role in the solubilization of particulate-bound Cd into soil solution and its subsequent phytoaccumulation by the high and low Cd accumulating cultivars.

Speciation of particulate-bound Cadmium of soils and its bioavailability

A modified sequential chemical extraction procedure was developed for partitioning particulate Cd into eight fractions: exchangeable, carbonate-bound, metal–organic complex-bound, easily reducible metal oxide-bound, organic-bound, amorphous mineral colloid-bound, crystalline Fe oxide-bound, and residual. Results of experimental data on 16 surface soils of Saskatchewan, widely varying in physico-chemical properties, indicate the presence of little exchangeable Cd. Cadmium in these soils was predominantly in the form metal–organic complex-bound, accounting for 31–55%, with an average of 40%, of the total Cd present in the soils. The average relative abundance of the different forms of Cd present in these soils is in the order: metal–organic complex-bound (0.107 mg kg–1) > carbonate-bound (0.052 mg kg–1) > residual (0.042 mg kg–1) > organic-bound (0.035 mg kg–1) > crystalline Fe oxide-bound (0.016 mg kg–1) > easily reducible metal oxide-bound (0.010 mg kg–1) > amorphous mineral colloid-bound (0.002 mg kg–1). Statistical treatment of the Cd availability index, measured as ammonium hydrogencarbonate–diethylenetriaminepentaacetic acid (ABDTPA)-extractable Cd, with different particulate-bound Cd species showed high correlation (r= 0.916, p= 6 × 10–7) of the Cd availability index with the metal–organic complex-bound Cd. The beta coefficients obtained from the multiple regression analyses have given an insight into the importance of Al–organic complex-bound Cd species in estimating the bioavailability of Cd in these soils. The relationship of the metal–organic complex-bound Cd and the mobility and bioavailability of soil Cd merits in-depth research in explaining the toxicity and food chain contamination of Cd in the environment.

A new soil test method for the determination of plant-available cadmium in soils

Abstract A new soil test procedure using 1M NH4Cl was developed for the extraction of plant‐available cadmium (Cd) from soils. Five grams of soil is weighed into a 50‐mL polyethylene vial to which 30 mL of 1M NH4Cl solution is added. The soil suspension is then shaken on a horizontal shaker for 16 h at 25°C at 180 cycles per min. The suspension is then centrifuged at 2,500g for 5 min and the supernatant filtered through a 0.45 μm nitrocellulose filter under vacuum. Cadmium in the extract is then determined at 228.8 nm on a graphite furnace equipped atomic absorption spectrophotometer. A highly significant correlation was observed between the natural logarithm (In) of 1M NH4Cl‐extractable Cd in soils and the Cd content in the grain of durum wheat (Triticum turgidum var. durutn L.) grown on the same soils (r = 0.974, p = 3.8 x 10‐7). In comparison with several commonly used extradants, such as ABDTPA, CaCl2, NH4OAc, and NH4NO3, the 1M NH4Cl‐extracted Cd from soils was found to be a better index of Cd availa...

Sorption and desorption kinetics of cadmium from soils: Influence of phosphate

The mobility and availability of heavy metals is controlled by sorption-desorption characteristics of the soils. There is much literature available about the sorption characteristics of heavy metals by soils. However, the influence of ionic environments on the desorption kinetics of heavy metals, pa

Microwave digestion technique for the determination of total cadmium in soils

Abstract A rapid method is described for the determination of total cadmium (Cd) in soils. The soil samples and standard sediment samples were digested in a microwave oven using concentrated HNO3 and also by the conventional HF‐HC1O4 digestion in platinum crucibles. The Cd content in the extracts was determined using graphite furnace atomic absorption spectro‐photometry (GFAAS). Both digestion methods produced comparable quantitative values for total Cd of standard reference materials obtained from the National Institute of Standards and Technology and a series of surface and subsurface soil samples. Thus, the microwave digestion technique was shown to be an accurate and rapid method for digesting soil samples prior to total Cd analysis.

Studies on soil rhizosphere: speciation and availability of Cd

ABSTRACTThe rhizosphere soils of two durum wheat (Triticum turgidum var. durum L.) cultivars Kyle and Areola grown in two selected soils of southern Saskatchewan were collected both at 2-week and 7-week plant growth stages. The cadmium availability index (CAI), determined as M NH4CI-extractable Cd, pH and the distribution of the particulate- bound 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 CAI 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 NH4CI. Compared with the bulk soils, the CAI 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 o...

Role of tartaric acid in the inhibition of the formation of Al 13 tridecamer using sulfate precipitation

Polynuclear Al13 tridecamer species are the major hydrolyzed species of aluminum, but their occurrence in terrestrial environments has not been established. X-ray diffraction (XRD), 27Al nuclear magnetic resonance (NMR), and scanning electron microscope (SEM) analyses show that the presence of tartaric acid (concentration range of 10−5–10−3 M), one of the commonly occurring low-molecular-weight organic acids, inhibits the formation of the Al13 tridecamer species.In the absence of tartaric acid, the basic aluminum sulfate crystals were of tetrahedral morphology and conformed to isometric symmetry with a = 17.748 Å and space group of P4232. Increasing amounts of tartaric acid [tartaric acid/Al molar ratio (R) ranging from 0.01 to 0.05] modified the crystal morphology from the tetrahedral particles of isometric symmetry (R = 0) to rod-shaped particles of monoclinic symmetry (R = 0.01) to irregularly shaped X-ray noncrystalline microparticles (R = 0.05). Failure to detect the presence of Al13 tridecamer, the dominant hydrolyzed species of aluminum, in terrestrial environments may be partially attributed to the presence of low-molecular-weight organic acids, which inhibit the formation of Al13 tridecamer species.

Desorption kinetics of cadmium from soils using M ammonium nitrate and M ammonium chloride

Abstract The bioavailability and, hence, potential toxicity of a trace metal ion in the soil depends on its concentration in the soil solution and on the soils ability to release the trace metal ions from the solid phase to replenish those removed from the soil solution by plants. Voluminous literature is available on the sorption characteristics of heavy metals by soils. However, study of desorption of heavy metals, particularly cadmium (Cd) from soils has not received much attention. The present study reports the kinetic data of desorption of Cd by M ammonium nitrate (NH4NO3) and M ammonium chloride (NH4Cl) from the Luseland and Jedburgh soils of Saskatchewan, Canada, with contrasting Cd‐availability characteristics, preadsorbed with different amounts of Cd. The desorption kinetics were described best using the parabolic diffusion mathematical model. The overall diffusion coefficient of Cd release by M NH4Cl, obtained using the parabolic diffusion model for the desorption kinetics, from the Luseland so...

Biogeochemistry of soil cadmium and the impact on terrestrial food chain contamination

ABSTRACT Cadmium has been recognized to be a highly toxic element, but it was not until recently that concern has been expressed about the possible effects on human health of long-term exposure to low concentrations of this element. The discovery that Cd pollution from a basic metal mining operation could cause serious illness and possibly death has led to public anxiety as well as medical interest. There are many reviews on the chemistry, biochemistry, and biology of soil cadmium. However, to assess the impact of Cd pollution on food chain contamination and ecosystem health, major biogeochemical pathways, particularly those in the rhizosphere, have to be elucidated, and gaps in our knowledge must be identified for future research planning. This chapter addresses the biogeochemistry of soil Cd and the impact of Cd pollution on terrestrial food chain contamination through the rhizosphere. Knowledge of the total content of Cd does not imply comprehensive knowledge of its chemical behavior; rather, it is the chemical speciation of Cd that influences Cds chemical reactivity, mobility, bioavailability, and toxicity in the ecosystem. Therefore, it is essential to investigate the chemical speciation of Cd in soils and sediments, and especially in the rhizosphere, which is the bottleneck of terrestrial food chain contamination. The fractionation of metal–organic complex-bound Cd species is a recent innovation in sequential extraction schemes. The metal–organic complex-bound Cd is the most common among the particulate-bound Cd species of surface soils in temperate and tropical regions. Cadmium present as metal–organic complex-bound species is especially enriched in the rhizosphere soils after application of phosphate fertilizer. The importance of metal–organic complex-bound Cd fractions in assessing phytoavailability of soil Cd has been demonstrated, and thus merits attention.

Distribution of cadmium in selected soil profiles of Saskatchewan, Canada: Speciation and availability

The distribution of Cd with depth, its particulate-bound speciation and availability index were studied in nine typical soil profiles of Saskatchewan, Canada. In the Ap horizons, Cd was predominantly in the metal-organic complex-bound form, accounting for about 38.8%, on average, of the total Cd present in the soils. The carbonate-bound Cd and the metal-organic complex-bound Cd accounted for 33.5 and 12.9%, respectively, on average, of the total Cd present in the B horizons. In the Ck horizons, Cd was predominantly in the carbonate-bound form, accounting for about 70.9%, on average, of the total Cd present in the soils. The M NH4Cl-extractable Cd of the soils, which is a measure of availability index of soil Cd, generally decreased with depth. Statistical treatment of the M NH4Cl-extractable Cd with different particulate-bound Cd species of various horizons in the soil profiles showed its highest correlation (P = 2.0 × 10−9, n = 42) with the amount of the metal-organic complex-bound Cd species, indicating...