Chemical fractions of zinc in the rhizosphere of corn in texturally different contaminated soils treated with chelators

  Rhizosphere processes have an important role in zinc (Zn) fractions in soils. Plant roots have the ability to transform metal fractions for easy uptake by root exudation in the rhizosphere. In the peresent study, the effects of EDTA, citric acid and poultry manure extract (PME) on fractionation of Zn in the rhizosphere of corn (hybrid (KSC.704)) were investigated in two contaminated soils with different texture. This research was conducted as factoriel in a completely randomized pattern with three replicates in greenhouse condition, and citric acid and EDTA were used at concentrations level 0, 0.5 and 1 mmol kg-1 soil and poultry manure extract at concentrations level 0, 0.5 and 1 g kg-1 soil. Three seeds of corn were planted in the rhizobox. After 10 weeks, the plants were harvested and rhizosphere and bulk soils were separated. Dissolved organic carbon (DOC), microbial biomass carbon (MBC) and Zn fractions were determined in the rhizosphere and bulk soils. Results showed that there is a difference between rhizosphere soils properties and bulk soils. In both soils, DOC and MBC in the rhizosphere were significantly (p≤0.05) increased, while, pH in the rhizosphere was significantly (p≤0.05) decreased comared with bulk soils. In sandy loam and clay loam soils, the average of exchangeable Zn and Zn associated with organic matter in the rhizosphere were significantly (p≤0.05) lower than those in the bulk soils, while, the average of Zn associated with iron-manganese oxides, Zn associated with carbonate and residual Zn in the rhizosphere were significantly (p≤0.05) higher than those in the bulk soils. In the rhizosphere and bulk soils of both soils, the maximum amounts of Zn fractions in different fractions were respectively, in the order of associated with iron-manganese oxides, residuals, associated with organic matter, associated with carbonates and exchangeable fractions. A significant correlation was found between Zn uptake by shoots with Zn associated with iron-manganese oxides in both soils (r = 0.71, p<0.05). In sandy loam soil, the highest Zn uptake by shoots was observed in the 1 g kg-1 PME treatment. In clay loam soil, the highest Zn uptake by shoots was observed in the 0.5 g kg-1 PME treatment. In sandy loam soil, 1 mmol kg-1 EDTA and in clay loam soil 1 mmol kg-1 citric acid treatments resulted in the release of the highest Zn concentrations in the iron-manganese oxides fraction. The results of average comparison showed that the average of the exchangeable Zn and Zn associated with iron-manganese oxides in the sandy loam soil were significantly (p≤0.05) higher than those in clay loam soil, while the average of the Zn associated with organic matter and residual Zn in clay loam soil were significantly (p≤0.05) higher than those in sandy loam soil, which can be attributed to different soil characteristics. The results of the present study showed that soil physical, chemical and biological changes due to the corn roots cause not only lead to Zn depletion in mobile soil Zn fractions, but also lead to change soil’s stable Zn fractions. Since excessive use of chelators can lead to increase more availability of soil’s zinc without increasing the plant’s absorption, so the using higher concentration levels is not recommended.