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Iron stable isotopes in bulk soil and sequential extracted fractions trace Fe redox cycling in paddy soils.

In paddy soils, iron (Fe) forms are highly influenced by the seasonal redox changes, and leave detectable isotope signals due to fractionation between different Fe forms. Here we present Fe concentrations and Fe isotope compositions (expressed as δ56Fe values) in a paddy soil profile from Suzhou, China. Light Fe isotopes were enriched in two iron-accumulation layers (Br3 and G1) with high Fe concentrations. In particular, large shifts in both Fe concentrations and δ56Fe values were found at the Br2 and Br3 boundary, showing fast and efficient transformation between these horizons. With sequential extraction, we show that Fe isotopes in the short-range-ordered Fe minerals and crystalline Fe oxides were lighter than those in the residual silicate minerals. Iron enriched in light isotopes was leached from the Ap horizon, and subsequently moved to Br horizon, quickly precipitating there as Fe oxides.