Effect of alkaline-stabilized biosolids on alfalfa molybdenum and copper content

Agricultural utilization of biosolids poses a potential risk to ruminant animals due to transfer of Mo from biosolids to forage to the animal in amounts large enough to suppress Cu uptake by the animal. Alkaline-stabilized biosolids (ASB) must be given particular consideration in assessment of Mo risk because the high pH of these biosolids could increase Mo and decrease Cu uptake by forage legumes. In this 3-yr field experiment, ASB and ground agricultural limestone (AL) were applied based on their alkalinity at rates equivalent to 0, 0.5, 1.0, and 2.0 times the lime requirement of the soil and alfalfa (Medicago sativa L.) was grown. Alfalfa yield was similar with AL and ASB except in the second year when ASB produced larger yields, apparently due to increased B availability with ASB. Application of ASB did not detectably increase extractable soil Mo (0- to 15-cm depth), but increased alfalfa Mo uptake in all cuttings with yield-weighted uptake coefficients (UCs) of 8.07 and 7.11 following the first and second ASB applications, respectively. Although ASB increased extractable soil Cu, and alfalfa Cu content was greater with ASB than with AL, yield-weighted alfalfa Cu to Mo ratio was decreased by ASB to levels near 3. These results suggest that ASB may have a greater effect on Mo uptake and Cu to Mo ratio of forage legumes than do other biosolids. Additional research is needed to determine implications of larger Mo cumulative loading with ASB for Mo risk, particularly in the soil pH range of 7 to 8.