Effects of iron(III) reduction on organic carbon decomposition in two paddy soils under flooding conditions

Iron oxidation and reduction have important effects on soil organic carbon conversion in paddy soil during flooding and dry conditions. This study selected two paddy soil samples, one from the city of Yueyang of Hunan Province and one from the city of Haikou of Hainan Province, that differ significantly in iron content. During a 25-day incubation, the effects of Fe(II) and Fe(III) contents and changes in the levels of several major iron forms on soil dissolved organic carbon (DOC) levels and emission of CH₄ and CO₂ were observed. The ratio of Fe(II) content to all active Fe increased with an increase in Fe(II) content after soil flooding, and the proportion of all active Fe was significantly higher in the soil samples from Yueyang than in those from Haikou. In only 5 days, 92% of Fe(III) was converted to Fe(II) in Yueyang soil samples, and almost all Fe(III) had been transformed into Fe(II) by the end of incubation. Similar behaviors occurred in soil samples collected from Haikou, but Fe(II) represented only 59% of the active Fe by the end of incubation. In total, 2.2 g kg^-1 of organic carbon in the Yueyang soil sample was converted to CO₂ and CH₄, and the DOC content increased to 410% of its initial value by the end of incubation. In the Haikou soil, only 0.7 g kg^-1 of organic carbon was converted to CO₂ and CH₄, and its DOC content increased to 245% of its initial value by the end of incubation, which was a much smaller increase than observed for the Yueyang sample. Decomposition of organic carbon in the soil was closely related to iron reduction, and reduction of iron in soil significantly affected the conversion rate of organic carbon in soil.