Interactive Effect of Fe and Pb on Decreasing Corn Pb Availability in a Pb-Polluted Soil

Lead (Pb) is of great concern in environment because of its toxicity to animals and humans. Lead is a cumulative toxin and known carcinogen. Although, plants do not require Pb for growth, the bioaccumulation index of Pb in plants exceeds that of most other trace elements. It is therefore important to control Pb concentration in plants, especially in the edible parts of crops to ensure food safety. There are many factors that control Pb accumulation and availability to plants in agricultural soils such as Pb source, Pb loading rate, soil pH, soil cation change capacity (CEC), chloride concentration in soil solution and soil organic matter content. These are important factors that should be considered for evaluating Pb phyto-availability. In addition, element interactions can also affect the elements uptake. Thus, this study was performed to investigate the effect of converter sludge-enriched cow manure on the changes in corn Pb uptake in a Pb-polluted soil under greenhouse condition.
This pot experiment was conducted under greenhouse condition around the city of Arak, using a Fine loamy, mixed and thermic, Typic Haplargids soil. A factorial experiment with a randomized complete block design with 24 treatments in three replications was carried out. The treatments consisted of applying enriched cow manure (0, 15 and 30 t ha-1) with 0% and 5% pure Fe from converter. To investigate the effect of converter sludge-enriched cow manure on the changes in corn Pb uptake, a non-saline soil with low carbon percentage was selected. The soil was polluted with Pb from Pb(NO3)2 source at the concentrations of 0, 200, 300 and 400 mg Pb kg-1 soil and incubated for one month. Cow manure was produced in a local farm and aged for two years before the experiment. The cow manure was enriched with converter sludge and incubated for three months in room temperature. Then, the enriched cow manure was added to the Pb polluted soil and corn (Zea mays L. single grass 704) seeds were sown. After 60 days from the experiment, soil physio-chemical properties and soil and plant Pb concentration were measured.
The greatest and least DTPA-extractable-Pb were determined in the polluted soil (400 mg Pb) without applying cow manure and the polluted soil (200 mg Pb) treated with 30 t ha-1 enriched cow manure, respectively. The DTPA-extractable-Pb in uncontaminated soils was not detectable by atomic absorption spectroscopy (AAS). Increasing the amount of cow manure caused a significant reduction in DTPA-extractable-Pb as applying 15 and 30 t ha-1 cow manure in a polluted soil (300 mg Pb) resulted in a significant decrease in DTPA-extractable-Pb by 11.9 and 23.4 units, respectively. This can be accounted for by the role of organic and inorganic fractions of cow manure in decreasing soil Pb availability. Interactions between Fe and Pb appear to influence the soil Pb availability as application of 15 and 30 t ha-1 converter sludge-enriched cow manure in 300 mg Pb-polluted soil caused a significant decline in soil Pb availability by 10.4 and 9.3 units, respectively. The highest and least root Pb concentration were observed in the polluted soil (400 mg Pb) without applying cow manure and the polluted soil (200 mg Pb) treated with 30 t ha-1 enriched cow manure, respectively. The corn root Pb concentration in unpolluted soils was not detectable by AAS. Applying 5% (W/W) pure Fe from converter sludge in the polluted soil (300 and 400 mg Pb) which were not manured significantly decreased the root Pb concentration by 19 and 9 units, respectively which is explainable by the interaction existing between Pb and Fe in soil. Furthermore, root Pb concentration was affected by converter sludge enriched-cow manure as applying 15 and 30 t ha-1 converter sludge cow manure in a polluted soil (400 mg Pb) significantly decreased the root Pb concentration by 20.8 and 10.9 units, respectively. However, the role of cow manure in increasing pH and decreasing root Pb concentration cannot be ignored. The greatest and least shoot Pb concentration was obtained for the polluted soil (400 mg Pb) without applying cow manure and the polluted soil (200 mg Pb) treated with 30 t ha-1 enriched cow manure, respectively. The corn shoot Pb concentration in unpolluted soils was not detectable by atomic absorption spectroscopy (AAS). Interaction effects were also observed for shoot Pb concentration as using 5% (W/W) pure Fe from converter sludge in the polluted soil (300 and 400 mg Pb) which were not treated by cow manure significantly decreased the shoot Pb concentration by 4.1 and 4.7 units, respectively.
The results of this study showed that interactions between Pb and Fe seem to play an important role in reducing root and shoot Pb concentration. On the other hand, applying cow manure can increase the soil sorption properties such as CEC and decrease the soil Pb availability and plant Pb uptake which is explainable by the fact that the organic and inorganic fractions of manure impact the Pb availability. However, the influences of soil physico-chemical properties such as pH upon soil Pb availability should be taken into account.