Changes and relations of photosynthesis and iron cycling in anoxic paddy soil amended with high concentrations of sulfate.

Sulfate contamination is an increasingly serious environmental problem related to microbial reduction processes in anoxic paddy soil. This study revealed the changes and interrelations of ferric iron [Fe(III)] reduction, ferrous iron [Fe(II)] oxidation, and oxygenic photosynthesis in an anoxic paddy soil (Fe-accumuli-Stagnic Anthrosols) amended with a range of high sulfate concentrations. Soil slurries mixed with 0 (control), 50, 100, 200, and 400 mmol kg NaSO were incubated anaerobically under dark and light conditions. The changes in chlorophyll a (Chl a), Fe(II), pH levels, and the chlorophyll absorption spectrum were determined over a 42-day period. Fe(II) concentrations increased with the addition of sulfate under dark conditions, while Fe(III) reduction potential was enhanced by increasing sulfate addition. The effect of light on Fe(II) concentration was observed after 16 days of incubation, when Fe(II) started to decrease markedly in the control. The decrease in Fe(II) slowed with increasing sulfate addition. The concentrations of Chl a increased in all treatments after 16 days of incubation under light conditions. There was a reduction in Chl a accumulation with increasing sulfate at the same incubation time. The absorption peaks of chlorophyll remained shorter than the 700-nm wavelength throughout the incubation period. The pH of all treatments decreased in the first week and then increased thereafter. The pH increased with sulfate addition and light conditions. In conclusion, contamination with high concentrations of sulfate could accelerate Fe(III) reduction while inhibiting oxygenic photosynthesis, which correspondingly slows chemical Fe(II) oxidation in an anoxic paddy soil.