Assessment of Zinc Bioavailability in the Rhizosphere of Sorghum bicolor (L.) Moench

This study was carried out in rhizobox، with the aim of evaluating the effects of sorghum rhizosphere on changes in available zinc using 4 single-extractants. Two soil samples with relatively similar physical and chemical properties were used from two areas، one from a non-polluted area and the other from the vicinity of the zinc and lead concentrate factory. In both soil samples، zinc contents were raised to three levels (250، 375، and 500 mg kg-1soil) according to their initial Zn content. Inside space of the rhizoboxes were sectioned into 3 separate compartments using nylon filters (40 mesh) including S1 (rhizosphere area)، S2 (the adjacent area influenced by plant roots)، and S3 (away from the rhizosphere) and sorghum seeds were planted in the S1 area. The effects of plant root activity on zinc bioavailable forms including distilled water extractable، calcium nitrate، and DTPA were significant. The amount of zinc in the forms of DTPA extractable، calcium nitrate and distilled water extractable in the rhizosphere (S1) of non-polluted soil was more than the bulk soil by 8. 6%، 100%، and 130%، respectively، at three weeks. Although HNO3 extractable zinc in this zone was not different than bulk soil، it decreased significantly about 10% in the seventh week. This shows the influence of sorghum root activity on less bioavailable [S1] form of zinc at longer time interval (seven vs. three weeks). Increase of the bioavailability (p<0. 01) in the third week resulted in significantly more absorption (30%) by the plant in non-polluted soil. The results showed that plant concentration and uptake of zinc by sorghum had significant correlation with nitric acid extractable zinc in rhizosphere soil)p<0. 01). Therefore، the nitric acid method can be used as an appropriate assessment method of bioavailable zinc for sorghum with regard to increasing levels in soil.