Effects of iron and nitrogen-coupled cycles on cadmium availability in acidic paddy soil from Southern China

Purpose: Microbially mediated nitrate (NO) reduction coupled to ferrous Fe (Fe[II]) oxidation (termed NRFO) and anaerobic ammonium (NH) oxidation coupled to ferric Fe (Fe[III]) reduction (termed Feammox) represent two known processes driving Fe redox cycle, and the associated interactions between nutrients and contaminants in paddy soils. Thus, we hypothesized that NRFO and Feammox may play important roles in Cd availability, especially in paddy soils characterized by dynamic redox processes. Materials and methods: An acidic paddy soil from a red soil region of southern China was amended with NO or NH in strictly anoxic incubation experiments. The microcosm technique was used with some modification to achieve the conditions that would facilitate the gradual change in the magnitude of redox. The pH and redox potential (Eh) of soil suspension were measured in situ. Gas samples were withdrawn to analyze nitrous oxide (NO) concentration. Soil slurries were sampled to determine NO, nitrite (NO), NH, and Fe(II) concentrations and to study Fe and Cd distributions among soil liquid and solid phases. Stoichiometric calculations of redox reactions were carried out at the time point of maximum levels of NO production and NO emission in all soil slurries during the anaerobic incubation. Results and discussion: Our study demonstrated the occurrence of NRFO in soil slurries. Of the Fe(II) produced, 30% was contributed by Fe(II) oxidation via NRFO. The occurrence of Feammox in soil slurries was also observed, in which 17% of Fe(II) produced was attributed to Fe(III) reduction via Feammox. Both aqueous and solid-phase Fe(II) oxidation by NO and NO during NRFO lowered Fe concentrations in soluble, MgCl-extractable, and NaOAc-extractable fractions, and promoted the formation of amorphous Fe oxides, which provided reactive surfaces for Cd adsorption, thus decreasing the soluble Cd fraction. In contrast, enhanced Fe(III) reduction due to Feammox and subsequent Fe(II) adsorption and precipitation accelerated Fe transformation from MgCl-extractable to NaOAc-extractable fractions, which triggered Cd transformation from soluble to NHOH·HCl-extractable fractions. Conclusions: All NRFO and Feammox affected the Cd distribution by controlling the Fe redistribution among the soil liquid and solid phases. In the context of high N inputs and high Fe content in paddy soils of southern China, we suggest that NRFO and Feammox could be two potentially important mediating pathways for manipulating the bioavailability of Cd-contaminated soils.