Absorption and translocation of sludge-borne zinc in field-grown maize (Zea mays L.)

Abstract

The absorption and translocation of sludge-borne zinc (Zn) in field-grown maize was studied in a long-term sewage sludge field trial on an acid sandy soil at Bordeaux, France. Since 1974, Zn-contaminated sludge had been applied at two levels of 10t dry matter (DM)ha−1year−1 (SS 10) and 100t DMha−1 per 2 years (SS 100) on annually cropped maize plots. Treatment with farmyard manure (FYM) at a rate of 10t DMha−1year−1 served as control. Plants were examined at six different growth stages from seedling to grain maturity. At each stage, the whole plant was separated into its different organs and Zn concentration and total Zn amount was determined in at least 12 different plant parts. The Zn transfer coefficient (plant Zn concentration/total soil Zn concentration) was highest in young FYM plants, followed by SS 10-amended ones, and was relatively low in SS 100 plants. Zn repartition between the different plant parts was more uniform in FYM-treated plants compared with SS 10 and SS 100 plants. High application of sewage sludge caused significantly raised Zn concentrations in all plant parts over the whole growing season. We determined an initial Zn accumulation with concentrations above 400μg Zng−1 DM in the roots and above 500μg Zng−1 DM in the lower stalks of the young maize plant. At silage stage, we measured Zn values between 300 and 500μg Zng−1 DM in the upper leaf and stalk parts of SS 100-treated plants. The increased Zn concentrations in sludge-treated plants coincided with a dry matter reduction of total shoots in SS 100-treated plants. The upper leaves and stalk parts occurred as the most important concentration and storage sites. In contrast, in the grains we determined low Zn concentrations of about 50μg Zng−1 DM until harvest without significant treatment effects or yield reductions. These observations provide evidence for a raised Zn exposure to animals and humans by vegetative aerial maize plant parts used as fodder.