K. C. J. Van Rees

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.

Cadmium uptake and bioaccumulation in selected cultivars of durum wheat and flax as affected by soil type

Accumulation of cadmium (Cd) in crop plants is of great concern due to the potential for food chain contamination through the soil-root interface. Although Cd uptake varies considerably with plant species, the processes which determine the accumulation of Cd in plant tissues are affected by soil factors. The influence of soil type on Cd uptake by durum wheat (Triticum turgidum var. durum L.) and flax (Linum usitatissimum L.) was studied in a pot experiment under environmentally controlled growth chamber conditions. Four cultivars/lines of durum wheat (Kyle, Sceptre, DT 627, and DT 637) and three cultivars/lines of flax (Flanders, AC Emerson, and YSED 2) were grown in two Saskatchewan soils: an Orthic Gray Luvisol (low background Cd concentration; total/ABDTPA extractable Cd: 0.12/0.03 mg kg-1, respectively) and a Dark Brown Chernozem (relatively high background Cd concentration; total/ABDTPA Cd: 0.34/0.17 mg kg-1 respectively). Plant roots, stems, newly developed heads, and grain/seeds were analyzed for Cd concentration at three stages of plant growth: two and seven weeks after germination, and at plant maturity. The results showed that Cd bioaccumulation and distribution within the plants were strongly affected by both soil type and plant cultivar/line. The Cd concentration in roots leaves and stems varied at different stages of plant growth. However, all cultivars of both plant species grown in the Chernozemic soil accumulated more Cd in grain/seeds than plants grown in the Orthic Gray Luvisol soil. The different Cd accumulation pattern also corresponded to the levels of ABDTPA extractable and metal-organic complex bound soil Cd found in both soils. Large differences were found in grain Cd among the durum wheat cultivars grown in the same soil type, suggesting the importance of rhizosphere processes in Cd bioaccumulation and/or Cd transport processes within the plant. Distribution of Cd in parts of mature plants showed that durum grain contained up to 21 and 36% of the total amount of Cd taken up by the plants for the Orthic Gray Luvisol and Chernozemic soils, respectively. These results indicate the importance of studying Cd speciation, bioaccumulation and cycling in the environment for the management of agricultural soils and crops.

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...

Low molecular weight organic acids released from roots of durum wheat and flax into sterile nutrient solutions

Abstract Knowledge of the composition and quantity of organic substances released from roots of different plant species is necessary for understanding the chemical and biological processes in the rhizosphere. The present study was undertaken to quantify low molecular weight organic acids (LMWOAs) released from roots of five cultivars/lines of durum wheat (Triticum turgidum var. durum L.): Kyle, Sceptre, DT618, DT627, and DT637 and four cultivars/ lines of flax (Linum usitatissiumum L.): Somme, Flanders, AC Emerson, and YSED 2. Plants were grown in sterile nutrient solution cultures and amounts of organic acids exuded by roots were analyzed by gas chromatography. The LMWOAs varied significantly among both durum wheat and flax cultivars and oxalic, malonic, fumaric, succinic, acetic, malic, citric and tartaric acids were detected in root exudates of both species. Generally, oxalic and acetic acids were predominant in durum wheat exudates and oxalic, acetic and malic acids were predominant in flax root exuda...

Comparison of liquid and gas chromatography for analysis of low molecular weight organic acids in rhizosphere soil

Abstract Low molecular weight organic acids from rhizosphere soil of durum wheat (Triticum turgidum var. durum L.) were studied by ion exclusion high pressure liquid chromatography (HPLC) and gas chromatography (GC). Ion exclusion HPLC was found unsatisfactory for acid determination because poor peak resolution hindered acid identification, strongly ionized solutes and oxalic acid co‐eluted in the dead volume of the column, and UV detection was not sensitive for aliphatic acid determination. Excellent resolution was obtained when the same low molecular weight organic acids were analyzed by GC. Non‐volatile low molecular weight organic acids were separated as trimethylsilyl (TMS) derivatives while volatile acids were analyzed without sample derivatization. A combination of these two GC techniques allowed for separation of 16 acids. Ten low molecular weight organic acids were identified and quantified in rhizosphere soil of durum wheat grown in three different soils. Succinic and acetic acids were most abun...

Influence of benthic organisms on solute transport in lake sediments

Increased inputs of nutrients into the waters of Lake Okeechobee has raised concern that the lake is becoming hypereutrophic. One aspect in understanding the overall cycling and dynamics of the nutrients in the system is the effect of benthic organisms on solute transport. Various diffusional models have been used to approximate the effect of benthic organisms on solute transport within sediments using diffusion coefficient values which represent the pooled contributions of molecular diffusion (Ds) and enhanced solute mixing due to macrobenthos activity (Di). The objective of this study was to investigate the impact of benthic activity on solute transport by measuringDs (i.e., no benthic activity) and an apparent-dispersion or mixing coefficientDm (i.e., with benthic activity) for the four major sediment types of Lake Okeechobee, Florida using a reservoir technique. This method involved monitoring the depletion of a conservative tracer (tritiated water) from the overlying water (reservoir) resulting from transport into sediments using disturbed sediments repacked in cores (3.2 cm diam.) and undisturbed cores (3.2 to 12 cm diam.) obtained from the lake. Additional estimates ofDm andDs were also obtained by measuring tracer concentration profiles in the sediment cores at the end of a specified diffusion period. Molecular diffusion coefficients (Ds) measured forrepacked cores of sand, littoral, mud and peat sediments ranged from 0.90 to 1.29 cm2 d−1, and estimates ofDs were slightly higher in undisturbed cores without benthic organisms.Dmvalues for undisturbed cores of mud, sand and littoral sediments having macrobenthic populations ranged from 2.09 to 24.78 cm2 d−1; values that were 1.6 to 15 times higher than those in sediments without benthic activity. Undisturbed cores of varying diameter from mud sediments had similar estimates ofDm for tritium; however, the undisturbed cores with larger diameters from littoral sediments yielded larger estimates ofDm, reflecting the heterogeneity of benthic population densities and activity. Therefore,Ds estimates may not adequately represent transport processes for mud, sand and littoral sediments of Lake Okeechobee; hence careful consideration should be given to the role of benthic organisms in the overall transport of solutes across the sediment-water interface.

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...

Wood/sludge ash effects on white spruce seedling growth

The disposal of wood ash and/or pulp and paper mill sludges is becoming increasingly more challenging as landfills are more difficult to site due to diminishing land availability and public opposition, as well as more costly to construct and operate because of increasingly stringent regulations. The most notable alternative to disposal that is receiving the attention of the forest industry is land application. The objective of this study, therefore, was to determine the influence of applying a pulp mill wood/sludge ash mixture on 1) various soil chemical properties of an Orthic Gray Luvisol and 2) the growth and nutrient uptake of white spruce [Picea glauca (Moench) Voss] seedlings. The pulp mill wood/sludge ash mixture from an olivine burner was surface broadcast at rates of 1 and 5 Mg ha-1 around white spruce seedlings planted on disc-trenched microsites. Soil solutions from lysimeters and soil samples were analyzed for exchangeable elements, soil pH and electrical conductivity while white spruce tissue...