Localization of iron-reducing activity in paddy soil by profile studies

Profiles of iron speciations (porewater Fe(II) and Fe(III), solid-phase Fe(II) and Fe(III)) have been studied to localize both iron reduction and oxidation in flooded paddy soil. Sulfate and nitrate were determined to analyze interactions of redox reactions involved in the iron cycle with those of the sulfur and nitrogen cycle. The development of the iron(II) and iron(III) profiles was observed in microscale over a time period of 11 weeks. After 11 weeks the profiles were stable and showed lowest concentrations of solid-phase iron(II) on the soil surface with increasing concentrations to a soil depth of 10 mm (approximate to 100 mu mol/cm(3)). Profiles of iron(III) showed a maximum of iron(III) at a depth of 2 to 4 mm (approximate to 100-200 mu mol/cm(3)). Porewater iron(II) concentrations were three orders of magnitude lower than extracted iron(II) and indicated that most iron(II) was adsorbed to the solid-phase or immobilized as siderite and vivianite. Diffusive loss of iron from the soil was indicated by iron recovery (0.3 mu mol gdw(-1)) in the flooding water after 12 weeks. The organic content of the soil influenced the concentrations of solid-phase iron(II) in deeper soil layers (> 6 mm); higher Fe(II) concentrations in soil with limiting amounts of electron donors may indicate lower consumption of CO2 by methanogenic bacteria and therefore a higher siderite precipitation. Soil planted with rice showed similar iron(II) profiles of fresh paddy soil cores. However, maximal iron(III) concentrations (approximate to 350 mu mol/cm(3)) were present in planted soil at a depth of 1 to 2.5 mm where oxygen is provided by a mat of fine roots. Sulfate and nitrate concentrations in the porewater were highest on the soil surface (10 mu M NO3-, 40 mu M SO42-) and decreased with depth. Similar profiles were detected for malate, acetate, lactate, and propionate, the concentrations decreased gradually from the surface to a depth of 4 mm. Profiles of oxygen showed highest concentrations at the surface due to photosynthetic production and a depletion of oxygen below 3 mm depth. Methane production rates measured from soil layers incubated separately in closed vessels were zero at the soil surface and increased with depth. In soil depths below 4 mm where iron(III) concentrations decreased higher methane production rates were found.