[14C]Pentachlorophenol mineralization in the rice rhizosphere with established oxidized and reduced soil layers

Flooded soils with rooted aquatic macrophytes have adjacent aerobic and anaerobic zones at the soil-water interface and rhizosphere where many common soil constituents undergo sequential oxidation and reduction reactions. To investigate whether pentachlorophenol (PCP) mineralization would also be enhanced under these conditions, a laboratory study was conducted to determine the conversion of [14C]PCP to 14CO2, 14CH4 and [14C]humic substances in soil microcosms with established aerobic-anaerobic zones at the soil-water interface and rice (Oryza sativa) rhizosphere. Contrary to what was expected, PCP was least rapidly degraded in rhizosphere-soil microcosms that contained the most extensive amounts of aerobic-anaerobic interfaces (63% PCP loss in 82 d) and was most rapidly degraded in soil microcosms that lacked redox interfaces in the soil profile (94% PCP loss in 82 d). Decreased PCP mineralization in the presence of aerobic-anaerobic interfaces was explained by (i) lack of sufficient O2 for aerobic PCP mineralization, due to the oxidation of other soil constituents in aerobic zones, and (ii) lack of an adequate supply of electron equivalents for reductive dechlorination of PCP, due to the reduction of other alternate electron acceptors in anaerobic zones. It was concluded that PCP mineralization is inhibited in flooded soils that contain extensive amounts of aerobic-anaerobic interfaces, due to redox cycling of other soil constituents that occur in these zones.