John O. Agbenin

Competitive adsorption of copper and zinc by a Bt horizon of a savanna Alfisol as affected by pH and selective removal of hydrous oxides and organic matter

The preferential retention of heavy metals by soils is critical to their availability and mobility through the soils to contaminate groundwater. We examined the competitive adsorption of Cu and Zn by Bt horizon of a Savanna Alfisol from a dilute salt solution under a variety of conditions. Copper and Zinc adsorption from the Cu–Zn binary solution produced a characteristic H-type isotherm for Cu, and a linear isotherm for Zn. The distribution coefficient Kd, which defines the affinity of the metals for the soil adsorbents, was five times greater for Cu than Zn. The adsorption envelope of the metals showed that maximum Cu adsorption occurred at pH 5.5 as against pH 6.8 for Zn adsorption probably reflecting difference in the first hydrolysis constant of the metals. The removal of organic matter from the soil reduced Kd,Cu 40 times as compared to the natural soil, whereas Kd,Zn was reduced by half as compared to the natural soil. Similarly, removing amorphous hydrous oxide by extraction with oxalate solution prior to the introduction of Cu and Zn reduced Kd,Cu 100 times, and Kd,Zn by 20 times as compared to the natural soil, but when both amorphous and crystalline hydrous oxides were removed prior to the introduction of Cu and Zn, the Kd,Cu was on par with the natural soil, whereas Kd,Zn increased by more than two times as compared to the natural soil. It seems that reactions with organic matter and amorphous oxides are the major controls on Cu, whereas Zn might be largely sorbed by cation exchange reaction when in competition with Cu for binding sites in soils. The metal binding sites in the amorphous hydrous oxides and organic matter were more selective for Cu than Zn.

Carbon, nitrogen and phosphorus dynamics under continuous cultivation as influenced by farmyard manure and inorganic fertilizers in the savanna of northern Nigeria

Changes in the carbon (C), nitrogen (N) and phosphorus (P) status of a savanna Alfisol under continuous cultivation complemented with farmyard manure and inorganic fertilization for 45 years were determined. The objective of the study was to determine the effect of farmyard manure (D), N + P and N + P + K fertilizers and their combinations on soil quality under continuous cultivation. Six field plots from the long-term DNPK experiment at Samaru were sampled for the study: D, N + P, N + P + K, D + N + P, D + N + P + K fertilized plots, a control plot with no fertilization and a native site soil. Continuous cultivation caused significant losses of C, N and P. The N + P or N + P + K fertilization could not mitigate these losses. Losses of N and P were mainly in organic forms. However, combination of farmyard manure with N + P and N + P + K fertilization enabled C, N and P to be maintained equal to, or greater than, the native site soil. It was concluded that farmyard manure applied alone, or in combination with N + P or N + P + K fertilization, was effective in maintaining soil quality under continuous cultivation in the savanna. In contrast, continuous inorganic fertilization was deleterious to soil quality because of depletion of organic matter, the reservoir of plant available N and P in weathered, tropical soils.

Phosphorus transformations in a toposequence of lithosols and cambisols from semi-arid northeastern brazil

Abstract Concepts of phosphorus transformations in soils and landscapes have largely been developed in temperate regions on moderate slopes. Little is known about the P dynamics and availability in semi-arid tropical soils where rainfall extremes cause limited but periodically intensive leaching and erosion. We therefore studied the different forms of inorganic P (Pi) and organic P (Po) as related to mineralogy and landscape position in semi-arid northeastern Brazil. Samples were collected from a catena of Lithosols at upper, and Cambisols at mid and lower slopes. All soils were derived from syenite which had a high total P content near 5500 mg kg−1. Phosphaterich particles were identified by X-ray microprobe as fluor-apatite. The apatite contents of the A horizons were reduced in the weathering sequence from Lithosols to the older Cambisols, while the total P contents diminished to between 1200 and 2300 mg kg−. The lower-slope soils had also lost two thirds of the Ca, Mg and Fe contents from sand and silt fractions. Chemicals fractionation of P showed the lowest Ca-P contents in the C horizons of the Cambisols, indicating a maximum transformation of primary Ca-P in these lowermost horizons. Deposition of partly weathered materials from upper slopes explained some of the differences in the Ca-P contents between R and C horizons and the overlying solum at the mid and lower slope. Similarly, some Lithosols showed an exceptionally high Ca-P contents suggesting that these soils have been replenished with unweathered material from rock outcrops above. Microprobe analysis revealed P-rich silt particles containing Fe, Al and Ca, which may be explained by the impregnation of primary Ca-P with Fe-oxyhydroxides. This absorption of Fe by mineral particles would represent a short-cut in the transformations of primary P to secondary, resistant P forms. The observation of Fe-oxyhydroxide coatings in feldspars from saprolite indicates that such transformations did occur and may have general importance where primary and highly weathered minerals are intimately mixed through erosion, limited leaching and high weathering intensities typical of semi-arid tropical environments. Only 5% of the total P was found in organic forms, mostly in stable forms of low availability with little contribution to P fertility.

Phosphorus sorption by three cultivated savanna alfisols as influenced by pH

An earlier study of phosphate sorption by some savanna soils from Nigeria suggested that increased P sorption when pH was raised might be due to precipitation of exchangeable Al as amorphous polymeric Al species with increased sorption sites. But these savanna soils have Ca as the dominant cation in their exchange sites, and low exchangeable Al. The objective of this study was to determine the role played by Ca in pH-induced P sorption of three savanna soils under continuous cultivation. Phosphorus sorption increased when pH was raised from 4.5 to 7.0. Similarly, Ca retention increased with increasing pH. Regression of P sorption on Ca retention indicated a significant linear relationship in the three soils. Three possible mechanisms were proposed to explain the increasing P sorption with increasing pH: precipitation of Ca-phosphates, Ca-induced P sorption or co-adsorption of Ca and H2PO4− or HPO42− as ion pairs or complexes. Available evidence suggests that all three mechanisms can operate together to enhance P retention as pH increases. The paper proposes that increased P sorption by savanna soils when pH is raised is likely to be related to the chemistry and retention of Ca rather than to hydrolytic reactions of Al.

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.

Lead in a Nigerian savanna soil under long-term cultivation

Concern about heavy metal accumulation in agricultural soils under long-term application of phosphate fertilizers and organic wastes makes investigation of heavy metals in agricultural soils imperative. This study examines the total, available and chemical forms of lead (Pb) in a savanna soil after 50 years of continuous cultivation and application of NPK fertilizers and farmyard manure (FYM). Total Pb concentration ranged from 28 to 42 mg kg(-1) over 2-3 times more than the average Pb concentration of non-polluted soils worldwide. Available Pb was, however, less than 2 mg kg(-1) indicating that Pb in the soils was largely insoluble. There were no detectable concentrations of water soluble and organically-bound Pb fractions. On average, residual Pb accounted for over 80% of total Pb. Compared to the natural site, cultivation and fertilization with NPK and FYM increased total Pb concentration by 19 and 17%, respectively, or, on mass basis, by 10 and 35 kg ha(-1), respectively, after 50 years. Soil Pb showed strong linear relations with sand fraction and inorganic phosphorus in the soils. Thermodynamic equilibrium relations provided some indirect evidence that the control on soluble Pb appeared to be chloropyromorphite [Pb5(PO4)3Cl], an insoluble lead phosphate mineral.

The status and dynamics of some trace elements in a savanna soil under long-term cultivation

A recent study in the savanna suggested that the surface soils might be contaminated with Ti, V, U and Th because of increasing phosphate fertilizer applications. In this study, we determined the dynamics of Ga, V, U, Nb, Y, La, Ce and Nd in a savanna soil that has received NPK fertilizers, farmyard manure (FYM) and FYM + NPK for 50 years using an adjacent uncultivated or native site as a reference. The concentrations of the elements in the soils were determined by X-ray fluorescence. Mass balance calculations indicated that Nb, U, Y, La, Ce and Nd increased by between 8 and 61% in the cultivated soil compared with the uncultivated natural site. Long-term application of FYM increased the masses of Nb (16%), Th (40%), La (26%), Ce (44%) and Nd (54%). There were no discernible changes in the masses of the trace elements that could be unambiguously associated with phosphate fertilizer applications. It was, thus, concluded that physical accumulation as leaching losses and crop removal depleted soluble ions, stabilization by organo-mineral complexes and aeolian deposition are the probable soil enrichment mechanisms of these trace elements during cultivation.

The cation exchange properties and microbial carbon, nitrogen and phosphorus in savanna Alfisol under continuous cultivation

The study reports on the impact of inorganic fertilisers with or without farmyard manure on the cation exchange capacity (CEC), cationic balance and microbial biomass carbon (C), nitrogen (N) and phosphorus (P) in a savanna Alfisol after 45 years of cultivation. Soils for the study were fertilised with N or N + P + K, cow dung (D), D + N and D + N + P + K for 45 years. Inorganic fertiliser significantly reduced CEC, exchangeable cations and upset the cationic balance. The reduction in CEC and exchangeable cations seemed to operate via organic matter depletion and pH reduction. Similarly inorganic fertiliser stressed microbial activity evidenced by the low amounts of microbial biomass C, N and P compared to the soil under native vegetation and the soils fertilised with D + N and D + N + P + K. From the perspective of the flows of C, N and P through soil microbial biomass, and cation exchange properties and prevention of nutritional imbalances involving basic cations, a rational fertilisation program for savanna Alfisols is one that combines inorganic fertilisers with farmyard manures.

Extraction of phytoavailable trace metals from tropical soils by mixed ion exchange resin modified with inorganic and organic ligands

Abstract Chelating agents and organic acid ligands readily solubilize trace and heavy metals in soils. We compared Fe, Mn, Cu and Zn extracted from 16 soils by a mixed ion exchange resin (Amberlite IRA-120+ Amberlite IRA-400) modified with six simple inorganic and organic acid ligands with a conventional chelating agent (DTPA) and bioassay. The mixed cation–anion exchange resins were saturated with Na-salts of chloride (R–CHL), bicarbonate (R–BIC), fluoride (R–FLU), acetate (R–ACT), citrate (R–CIT) and tartrate (R–TAR). Solubilization and adsorption of Fe and Cu from the soils by resin–ligand systems decreased in the order R–CIT≥R–FLU≥R–TAR=R–ACT>R–CHL=R–BIC reflecting differential stability of Fe- and Cu–organic/inorganic complexes. The R–CHL solubilized and adsorbed more Mn and Zn than other resin–ligand systems because of the relatively low pH maintained by the R–CHL-soil suspension. Extraction with DTPA gave similar amounts of Fe and Mn as resin–ligand systems, but DTPA extracted two to three times more Cu and Zn than resin. Resin-extractable Fe, Mn and Cu correlated with the corresponding DTPA-extractable metal contents, while both resin- and DTPA-extractable Cu correlated with the bioassay results. The R–ACT and R–TAR extractable Zn correlated more strongly with the bioassay results than DTPA-extractable Zn. Mixed ion exchange resin is apparently feasible for characterizing plant available Fe, Mn, Cu and Zn in soils.

Bioavailability of copper, cadmium, zinc, and lead in tropical savanna soils assessed by diffusive gradient in thin films (DGT) and ion exchange resin membranes.

The technique of diffusion gradient in thin films (DGT) for assessing bioavailable metals has not been tested under field conditions. We assessed the relationships of DGT- and cation exchange resin-membrane-measured concentrations of Cd, Cu, Pb, and Zn with plant uptake of the metals under greenhouse and field conditions. In the greenhouse, the effective concentrations of Cu, Pb, and Zn by DGT correlated significantly with uptake by sorghum (Sorghum bicolor), but cation exchange resin-membrane-measured concentrations of Cd, Pb, and Zn did not correlate with sorghum uptake. In the field, the DGT-measured concentrations of Cd, Pb, and Zn were not linearly related to uptake Cd, Pb, and Zn by lettuce (Lactuca sativa) except for Cu uptake (r = 0.87, p < 0.05). Similarly, it was only the resin-membrane-extractable Pb that correlated with Pb uptake by lettuce (r = 0.77; p < 0.05). However, fitting non-linear regression models improved the plant metal uptake predictions by DGT-measured bioavailable Cd, Cu, Pb, and Zn under field conditions. In conclusion, the DGT technique was fairly predictive of bioavailability in the greenhouse, but not in the field.