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

稻田生态系统是人工湿地，养活了世界近50%的人口。生态系统经历了长期的干湿循环，伴随着特定的氧化还原反应和随后的生物地球化学过程。因此，水稻土具有独特的铁(Fe)动态特征。然而，铁循环速率的重建尚未实现。这个项目的目的是检验这样一个假设，即水稻土的铁同位素特征和铁形态可以作为土地利用期限的函数来衡量。特别是我们认为水稻土剖面的Fe同位素特征与邻近的非水稻土不同，不同年龄的水稻土表现出不同的Fe同位素特征，可能是随着水稻土发育的年龄和程度的增加，下层土壤中Fe轻同位素的丰度越来越高。此外，我们假设，无论土壤发育程度如何，菌斑的形态和铁同位素特征将显示出对水稻根部和地上组织的恒定偏移量，以及iv)微生物群落组成对菌斑的形成和功能有重要贡献，进而影响铁斑的铁同位素组成。为了验证这些假设，我们将研究2000年前的水稻土时间序列和邻近的非水稻土(长达700年)，以及水稻、旱地和旱季植物。1)存档的土芯、新采集的淹水表土和植物样品将使用多收集器电感耦合等离子体质谱(由德国合作伙伴)进行铁同位素组成分析，以揭示反复氧化还原循环对铁同位素分馏的影响。2)利用同步辐射X射线吸收光谱仪对部分土壤和植物样品以及铁斑块进行分析(由双方共同进行)，以阐明铁在水稻土发育过程中的形态演化。3)将(由中方合作)调查水稻土剖面和铁斑中铁氧化和还原细菌的微生物群落，以揭示它们在铁同位素分馏中的作用。

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.