ARS | Publication request: THE EFFECTS OF PH AND REDOX ON SELENATE REDUCTION RATES IN CALIFORNIA AGRICULTURAL SOILS

Submitted to: Agronomy Society of America, Crop Science Society of America, Soil Science Society of America Meeting
Publication Type: Abstract
Publication Acceptance Date: July 25, 2005
Publication Date: November 1, 2005
Citation: Herbel, M., Suarez, D.L., Goldberg, S.R. 2005. The effects of ph and redox on selenate reduction rates in california agricultural soils. Agronomy Society of America, Crop Science Society of America, Soil Science Society of America Meeting. Paper No. 206-8.

Technical Abstract: Selenium is a potentially hazardous trace element in many agricultural soils. The most oxidized form of selenium, selenate [Se(VI)], is also the most soluble and mobile form in aerobic, aqueous environments. Reactions that transform Se(VI) to less soluble or less bioavailable forms are dominated by chemical and microbial reduction processes. Most investigations on Se(VI) reduction rates in soils and sediments have been conducted using gas (air, N2, or H2) control, and attainment of anaerobic redox equilibrium under such conditions is unlikely. Hence, our objectives were to find suitable methods to control pH and Eh under a constant gas composition, and to determine Se(VI) reduction rates in a variety of agricultural soils using these methods. Good buffers, such as MES and CHES, were used to constrain pH to target levels after initial adjustment using HCl or NaOH. Redox modifiers, such as NO3, soluble Fe(III), cysteine-HCl, etc., were tested to constrain the Eh of soil-water suspensions between +300 and -300 mV. Preliminary data indicated that under very reducing (-200 mV) conditions, maximum Se(VI) reduction rates in 1:5 (soil:water) Broadview Water District soils amended with 10 mg Se(VI)/L ranged from 0.11 to 0.26 mg Se(VI)/Lh over a buffered pH range of 6.2 to 9.0, with highest rates occurring at neutral pH conditions or after a lag period. In Hanford soil suspensions maintained at discrete Eh conditions, Se(VI) reduction was similarly constrained by microbial adaptation to the presence of Se and followed Monod kinetics. At +70 mV maximum Se(VI) reduction rates ranged from 0.12 to 0.19 mg while at -30 mV rates were slightly higher and ranged from 0.11 to 0.25 mgLh. This study demonstrates that soil suspensions can be maintained under constant gas headspace while pH and Eh are controlled by chemical amendments, and provides a viable alternative for investigating trace element redox transformations in agricultural soils under specific pH and Eh conditions.