ACCUMULATION AND INTERACTIONS OF ARSENIC, SELENIUM, MOLYBDENUM AND PHOSPHORUS IN ALFALFA

Arsenic (As), selenium (Se), and molybdenum (Mo) have become increasingly important in the terrestrial and agrobiological environment in some parts of the western USA. A four factorial [3 As x 3 Se x 2 Mo x 2 P concentrations] completely randomized glasshouse study was conducted to evaluate the competitive interactions between As(V), Se(VI), Mo(VI) and P using nutrient solution concentrations representative of soil solutions and agricultural drainage waters. Alfalfa [Medicago sativa L., Germain WL 512] was grown in sand culture and irrigated five times per day with half-strength Hoagland's solution containing 0 (background), 0.05 and 0.10 mg L-1 As as Na2HAsO4; 0, 0.05, and 0.10 mg L-1 Se as Na2SeO4; and 0.01 (background) and 1.0 mg L-1 Mo as Na2MoO4. Instead of 16 mg L-1 P and 16 mg L-1 S commonly used in the half-strength Hoagland solution, 1.0 or 4.0 mg L-1 P added as KH2PO4 and one somewhat lower level (8.0 mg L-1) S added as MgSO4 were used. The results indicated no adverse effect of As and Se on shoot yield. Selenium and P strongly depressed shoot As concentrations, and their combined effects were multiplicative rather than additive (significant Se x P interaction). Uptake of Se was decreased by increasing P, but was stimulated by increasing As, and the combined effects of P and As were approximately additive (non-significant As x P interaction). Tissue concentrations of Mo and P were less dramatically influenced by solution concentrations of the other elements.