Remediation of soils contaminated with molybdenum using soil amendments and phytoremediation

For ruminants, there is a narrow span between nutritional deficiency of Mol ybdenum and its potential toxicity. Molybdenosis occurs among cattle feedin g 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 hig h 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 dr iven to the valley pastures in fall, animals often fell ill with molybdenos is, and several died. The aim of the study was to remediate this 300 ha are a. Two approaches were attempted: (1) to reduce the Mo contents of the fora ge by immobilizing soil Mo, and (2) to increase plant Mo contents by mobili zing soil Mo to increase plant Mo which may then be removed from the system (phytoremediation). In a soil extraction experiment we demonstrated that p hosphate fertilization, ammonium sulfate amendment, vermiculite, humic acid and sewage sludge increased Mo leaching by 30 to 110%. Fe-humate and Mn-hu mate application decreased Mo in the leachate from 96 mug L-1 to 55 and 7 m ug L-1, respectively. Plant Mo contents were increased up to 170% by P-fert ilizer 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 har vest increased by 88% after P-fertilization and 84% after vermiculite amend ment. Mn-humate and sewage sludge decreased plant Mo content by 25 and 40%, respectively, rendering the forage suitable for feeding of ruminants. P-fe rtilization and vermiculite may thus be recommended for the severely contam inated sites to enhance phytoremediation through Mo export, and Mn-humate a nd sewage sludge application appear suited to remediate the less severely c ontaminated sites.