Molybdenum Mobility and Bioavailability in Soil 25 Years After Treatment with a Mo-Contaminated Sewage Sludge.

Understanding long-term phytoavailability of sewage sludge-borne Mo will be helpful to set a limit for soil Mo loadings that is protective for the food chain and the environment without being unnecessarily restrictive. This long-term study evaluated the mobility and bioavailability of sludge-borne Mo after application at excessive rates. Sewage sludges were applied in 1982 to a Metea loamy sand soil (mixed, mesic, Arenic Hapludalfs) at 0, 42, 94, 194, and 200 Mg ha^-1, which supplied 0, 63, 141, 291, and 300 kg Mo ha^-1 loadings, respectively. The treatments were replicated four times in a randomized complete block design. Various crops were grown annually between 1982 and 2003 to study long-term bioavailability of added Mo. Molybdenum concentrations in surface soil samples taken from sludge treated plots in 1999 and 2003 were significantly reduced, compared to concentrations found in 1982 samples. At the high sludge and Mo rates, only 50% of soil Mo applied in 1982 was accounted for by Mo concentrations in 2003 surface soil samples. Deep core soil samples taken in 2008 suggest downward movement of the soil Mo. Molybdenum concentrations (17 - 671 mg kg^-1) in diagnostic leaf tissue of crops grown between 1997 and 2003 increased with increased sludge rate. While some plant Mo concentrations would be considered excessive, no phytotoxicity occurred and plant growth and grain yields were not reduced. Molybdenum concentrations in crops grown in 2003 (over 20 years after treatments) strongly correlated (r = 0.84*** - 0.94***) with soil Mo loadings applied in 1982. Crops with relatively high Mo accumulations due to high Mo loadings can result in low Cu:Mo ratios in the plants. We conclude that Mo bioavailability at elevated soil Mo loadings will persist several decades after sewage sludge application, and loadings above 40 kg Mo ha^-1 can result in low Cu:Mo ratios in the plants that may be unsafe for animal consumption.