Nafiu Abdu

Phytoavailability, human risk assessment and transfer characteristics of cadmium and zinc contamination from urban gardens in Kano, Nigeria

BACKGROUND Quantitative data about phytoavailability and transfer into consumed plant parts for heavy metals in intensively managed urban vegetable production areas of sub-Saharan Africa are scarce. We therefore studied the transfer of zinc (Zn) and cadmium (Cd) from soil to the root and subsequent translocation to edible portions of four vegetables in six urban gardens. RESULTS While respective diethylenetriaminepentaacetic acid (DTPA)-available Zn and Cd concentrations ranged from 18 to 66 mg kg(-1) and from 0.19 to 0.35 mg kg(-1) , respectively, in soils, total Zn and Cd were 8.4-256 mg kg(-1) and 0.04-1.7 mg kg(-1) in shoot parts. Metal transfer factor (MTF) ratios were higher in Zn (0.2-0.9) than in Cd (0.1-0.6). Our data suggest that total Zn concentration in soil is a reliable indicator to assess its transfer from soil to crop in lettuce, carrot and parsley, while for Cd DTPA-extractable concentration may be used to estimate soil-crop transfer of Cd in amaranthus and carrot. Overall, Cd was more easily translocated to the aerial plant parts than Zn. CONCLUSION Zinc and Cd accumulation by vegetables in our soils is mainly a metabolically controlled process. Such accumulation can contaminate the ecosystem but under our conditions intake and ingestion of these metals will likely have to occur over a prolonged period to experience health hazard.

Formulation of a first-order kinetic model and release of added phosphorus in a savanna soil

Adsorption–desorption of added phosphorus (P) was studied in a batch experiment using anion-exchange resin. Total P sorbed by adding 400 mg P kg−1 by Nigerian soil ranged from 10.8 mg kg−1 in the Idofian Basement complex to 35.5 mg kg−1 in Alkaleri Sandstone, representing 3 and 9% of applied P. Phosphorus release kinetics was apparently described by the first-order, second-order, Elovich, parabolic diffusion and fractional power equations, but not in soils derived from sandstone. The mechanism underlying the release pattern was concluded to be dissolution followed by diffusion of sorbed P from the good fit to the Elovich and parabolic diffusion equations. The inability to clearly relate the P sorbed by the soil to OH- and SO4 2- released into the soil solution during the adsorption process further corroborated the above conclusion, thereby overruling the possibility of ligand exchange as a dominant mechanism in the sorption/desorption of P in these soils.

Adsorption-solubility equilibria and speciation of Pb, Cd, and Zn in a savanna soil

Emission of contaminants into the environment is a common feature of industrialization. The danger of these substances has generated interest in their solubility and speciation. In this study, the nature of adsorption, solubility and speciation of Pb, Cd and Zn under high levels of accumulation were evaluated through a model sorption-solubility experiment. Equilibrium concentrations of metals and other soil characteristics were used as input parameters in a metal speciation model. The solubility controls of metal activities were evaluated through the use of activity/stability diagrams. The solubility of Pb, Cd and Zn was enhanced between pH 4.8 and 6.6. Increased solubility at this pH range indicates higher bioavailability and toxicity. Speciation results showed a predominance of free metal species in the soil solution. The activity diagrams used to test the stability of possible mineral bearing compounds in the soils did not show any particular solid phase as possible control of the activities of Pb, Cd and Zn in solution. This suggests that solubility controls might be due to the formation of metastable minerals which are not well crystallized.

Fifteen-year fallow altered the dynamics of soil phosphorus and cationic balance of a savannah Alfisol

This study was designed to evaluate changes in the dynamics of soil phosphorus and cationic balances of a savannah soil subjected to 45 years of continuous cultivation under different fertilizer management and later left fallow for 15 years. It was conducted on the experimental plots at the Institute for Agricultural Research, Ahmadu Bello University, Nigeria. Treatments consisted of nitrogen (N), phosphorus (P), potassium (K), cow dung manure (D) and their combination (DNPK). Results of P fractionation and cationic distribution were compared with previous studies on the same plot 15 years ago. Organic carbon increased from a range of 3–5 g kg−1 in 1997 to 10.9 g kg−1 in 2012. Similarly, the cation exchange capacity (CEC) of the soil increased from 6.40 cmolc kg−1 in 1997 to 16.4 cmolc kg1 in the present study. The degree of saturation of the CEC by Ca2+ was 68–79% and 10–20% for Mg2+, while that of K+ was 1.5–2%. Although there was an uneven trend in depletion and enrichment of the various P pools, however, the fallow period substantially improved the CEC and the plant available P pools of the soil by more than 200% and 6–259%, respectively.