Zinc phytotoxicity to oilseed rape grown on zinc-loaded substrates consisting of Fe oxide-coated and calcite sand

The risk of zinc (Zn) phytotoxicity in soils has increased in various regions following application of different anthropogenic materials. In order to assess the relative efficiency of Fe oxide and calcite in sorbing Zn and hence alleviating Zn phytotoxicity, we grew oilseed rape for 28 days in pots containing Zn-loaded model substrates consisting of Fe oxide (ferrihydrite)-coated sand (FOCS, 0.2–0.5 mm, 0.3 m2 ferrihydrite g−1 sand) and calcium carbonate (calcite) sand (CCS, 0.2–0.5 mm, 0.3 m2 calcite g−1 sand). Five substrates containing 5, 10, 20, 40, and 80% FOCS and supplied with ZnSO4 at a rate of 30, 100, 300, and 1000 mg Zn kg−1 were used in the cropping experiment and in an in vitro study of Zn desorption for 62 days. Plants exhibited good growth and a similar dry matter yield (DMY) at the 30 and 100 mg Zn kg−1 rates. On the other hand, DMY was markedly reduced at the 300 and, especially, at the 1000 mg Zn kg−1 rate, particularly for the substrates with the higher FOCS proportions. Symptoms of phytotoxicity (viz. chlorosis, purple colouration due to P deficiency) were apparent at such rates and were accompanied by high Zn concentrations in both shoot (average values >1000 and >1500 mg Zn kg−1 dry matter for the 300 and 1000 mg Zn kg−1 rate, respectively) and root (average values >2500 and >6000 mg Zn kg−1 dry matter for the 300 and 1000 mg Zn kg−1 rate, respectively). Total Zn uptake was maximal at 300 mg Zn kg−1. The results of water extractable Zn in the substrate after cropping and the dissolved Zn concentrations measured in substrate–water systems (desorption experiment) suggest that, on a surface area basis, calcite is more effective than Fe oxide to retain Zn and thus alleviate phytotoxicity at high Zn loadings. However, the Zn-sorption capacity of the Fe oxide cannot be neglected, particularly at low Zn loadings, where Fe oxide seems to exhibit a higher affinity for Zn – but not a higher Zn-sorption capacity – than does calcite.