Pentachlorophenol dissipation and ferrous iron accumulation in flooded paddy soils with contrasting organic matter contents and incorporation of legume green manures

Abstract

The effects of different amendment rates (1 and 3%) of Chinese milk vetch (Astragalus sinicus L.) and bird vetch (Vicia cracca L.) on the dissipation of extractable pentachlorophenol (PCP) residues were investigated in two flooded paddy soils with contrasting soil organic matter (SOM) contents. Following incorporation of the legume green manures, whether acetate-extractable ferrous iron [Fe(II)NaOAc] is useful for revealing the reductive dechlorination mechanism of PCP in flooded paddy soils was verified. The kinetic parameters of PCP dissipation and Fe(II)NaOAc accumulation were estimated using logistic curve fitting. Correlation and regression analyses were performed on PCP, Fe(II)NaOAc, water-soluble organic carbon (WSOC), pH, and oxidation-reduction potential data. The kinetic parameters of PCP dissipation and Fe(II)NaOAc accumulation varied significantly with the amendment rate of legume green manure. The changes in pH value and WSOC content varied significantly with the level of SOM and with the amendment rate of legume green manure. At a low amendment rate of green manure, the pH increase and WSOC consumption greatly enhanced Fe(II)NaOAc accumulation and contributed to PCP dissipation. The rate of PCP dissipation decreased with decreasing pH and WSOC accumulation, especially in the high-SOM soil amended with the higher rate of green manure. Legume green manure species had no effect on PCP dissipation. In terms of soil chemistry, Fe(II)NaOAc was found to be the key variable that could explain the mechanisms involved in the reductive dissipation of PCP in flooded paddy soils with contrasting SOM contents and incorporation of legume green manures.