Mohammed Kader

Pore-water chemistry explains zinc phytotoxicity in soil.

Zinc (Zn) is a widespread soil contaminant arising from a numerous anthropogenic sources. However, adequately predicting toxicity of Zn to ecological receptors remains difficult due to the complexity of soil characteristics. In this study, we examined solid-solution partitioning using pore-water data and toxicity of Zn to cucumber (Cucumis sativus L.) in spiked soils. Pore-water effective concentration (ECx, x=10%, 20% and 50% reduction) values were negatively related to pH, indicating lower Zn pore water concentration were needed to cause phytotoxicity at high pH soils. Total dissolved zinc (Znpw) and free zinc (Zn(2+)) in soil-pore water successfully described 78% and 80.3% of the variation in relative growth (%) in the full dataset. When the complete data set was used (10 soils), the estimated EC50pw was 450 and 79.2 µM for Znpw and Zn(2+), respectively. Total added Zn, soil pore water pH (pHpw) and dissolve organic carbon (DOC) were the best predictors of Znpw and Zn(2+) in pore-water. The EC10 (total loading) values ranged from 179 to 5214 mg/kg, depending on soil type. Only pH measurements in soil were related to ECx total Zn data. The strongest relationship to ECx overall was pHca, although pHw and pHpw were in general related to Zn ECx. Similarly, when a solution-only model was used to predict Zn in shoot, DOC was negatively related to Zn in shoot, indicating a reduction in uptake/ translocation of Zn from solution with increasing DOC.

Predicting plant uptake of cadmium: validated with long-term contaminated soils

Cadmium accumulates in plant tissues at low soil loadings and is a concern for human health. Yet at higher levels it is also of concern for ecological receptors. We determined Cd partitioning constants for 41 soils to examine the role of soil properties controlling Cd partitioning and plant uptake. From a series of sorption and dose response studies, transfer functions were developed for predicting Cd uptake in Cucumis sativa L. (cucumber). The parameter log Kf was predicted with soil pHca, logCEC and log OC. Transfer of soil pore-water Cd2+ to shoots was described with a power function (R2 = 0.73). The dataset was validated with 13 long-term contaminated soils (plus 2 control soils) ranging in Cd concentration from 0.2 to 300 mg kg−1. The series of equations predicting Cdshoot from pore-water Cd2+ were able to predict the measured data in the independent dataset (root mean square error = 2.2). The good relationship indicated that Cd uptake to cucumber shoots could be predicted with Cdpore and Cd2+ without other pore-water parameters such as pH or Ca2+. The approach may be adapted to a range of plant species.

Copper interactions on arsenic bioavailability and phytotoxicity in soil

Arsenic (As) and copper (Cu) are co-contaminants in the environment but little is known about their ecological impact as mixtures in soil. In this study, we investigated the combined As-Cu interactions on toxicity and uptake as binary mixtures in 5 contrasting soils. The study included solubility, contaminant uptake and toxicity in cucumber (Cucumis sativus L.) as a model plant species. Soils were spiked individually and as a mixtures at 10 different As levels (2, 4, 8 up to 1024mgkg-1). Copper was added with As at two effective concentration levels (EC10Cu and EC50Cu). Arsenic uptake was significantly reduced in the presence of Cu and a higher effect was demonstrated with increasing pore-water pH. Copper accumulation was not significantly influenced by As. An additive response on plant growth was predominant overall when expressed from pore-water parameters with root mean square errors of 12.6 and 13.2 for EC10Cu and EC50Cu treatments, respectively.