Iron dynamics in paddy soil chronosequence in view of stable iron isotope signature and iron speciation

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Paddy ecosystems are artificial wetlands that feed almost 50% of the world population. The ecosystem experiences long-term dry-wet cycles with specific redox reactions and subsequent biogeochemical process. Paddy soils are thus mainly characterized by unique dynamics in iron (Fe). However, a reconstruction of Fe cycling rates has not yet been achieved. This project is designed to test the hypothesis that Fe isotope signatures and Fe speciation of paddy soils may be scaled as a function of land-use duration. In particular, we assume that i) Fe isotope signatures of paddy soil profiles are different from that of adjacent non-paddy soils, ii) paddy soils of different age show different Fe isotope signatures, likely with increasing age and degree of paddy soil development, the subsoil is increasingly enriched in Fe light isotopes. Furthermore, we assume that iii) speciation and Fe isotope signature of plaques will show a constant offset to rice roots and aboveground tissues, irrespective of the degree of soil development, and that iv) microbial community composition contributes significantly to the plaque formation and function, which in turn influences Fe isotope composition of the Fe plaque. To test these hypotheses, we will study a paddy soil chronosequence up to 2000 years with adjacent non-paddy soils (up to 700years), as well as rice, upland and dry season plants. 1) Archived soil cores, newly taken flooded topsoils and plant samples will be analyzed for their Fe isotope composition using multicollector inductively coupled plasma mass spectrometry (by the German partner) to reveal the impact of repeated redox cycle on Fe isotope fractionation. 2) Partial soil and plant samples as well as Fe plaques will be analyzed using synchrotron radiation X-ray absorption spectroscopy (by both partners together) to illustrate Fe species evolution along paddy soil development. 3) Microbial community of Fe oxidizing and reducing bacteria across the paddy soil profiles and Fe plaques will be investigated (by the Chinese partner) to reveal their roles in Fe isotope fractionation.