Remediation of soils contaminated with molybdenum using soil amendments and phytoremediation

For ruminants, there is a narrow span between nutritional deficiency of Molybdenum and its potential toxicity. Molybdenosis occurs among cattle feeding on forage with Mo concentrations above 10 mug g(-1) or a Cu:Mo ratio <2. In the area under investigation forage Mo contents in the valley are as high as 180 mug g(-1) due to industrial pollution, while the alpine pastures, where cattle graze during summer, are nutrient (e.g. Cu) deficient. When driven to the valley pastures in fall, animals often fell ill with molybdenosis, and several died. The aim of the study was to remediate this 300 ha area. Two approaches were attempted: (1) to reduce the Mo contents of the forage by immobilizing soil Mo, and (2) to increase plant Mo contents by mobilizing soil Mo to increase plant Mo which may then be removed from the system (phytoremediation). In a soil extraction experiment we demonstrated that phosphate fertilization, ammonium sulfate amendment, vermiculite, humic acid and sewage sludge increased Mo leaching by 30 to 110%. Fe-humate and Mn-humate application decreased Mo in the leachate from 96 mug L-1 to 55 and 7 mug L-1, respectively. Plant Mo contents were increased up to 170% by P-fertilizer and up to 150% by vermiculite. Conversely, sewage sludge, Fe- and Mn-humate decreased plant Mo contents by 60, 40 and 75% in the greenhouse. In the field study, the effects were smaller, but Mo export through plant harvest increased by 88% after P-fertilization and 84% after vermiculite amendment. Mn-humate and sewage sludge decreased plant Mo content by 25 and 40%, respectively, rendering the forage suitable for feeding of ruminants. P-fertilization and vermiculite may thus be recommended for the severely contaminated sites to enhance phytoremediation through Mo export, and Mn-humate and sewage sludge application appear suited to remediate the less severely contaminated sites.