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，养活了近50%的世界人口。生态系统经历了长期的干湿循环，并伴随着特定的氧化还原反应和随后的生态地球化学过程。因此，水稻土的主要特点是独特的动态铁（Fe）。然而，重建的铁循环速率尚未实现。本项目旨在验证水稻土的铁同位素特征和铁形态可以作为土地利用持续时间的函数进行缩放的假设。特别是，我们假设i）水稻土剖面的Fe同位素特征与邻近的非水稻土不同，ii）不同年龄的水稻土显示不同的Fe同位素特征，可能随着年龄的增加和水稻土发育程度的提高，底土中Fe轻同位素越来越富集。此外，我们假设，iii）物种和铁同位素签名的斑块将显示一个恒定的偏移到水稻根和地上组织，无论土壤发育的程度，和iv）微生物群落组成有助于显着的斑块的形成和功能，这反过来又影响铁同位素组成的铁斑块。为了验证这些假设，我们将研究一个水稻土的年代序列长达2000年与邻近的非水稻土（长达700年），以及水稻，高地和旱季植物。1)将使用多收集器电感耦合等离子体质谱法（由德国合作伙伴）分析存档的土芯、新采集的淹水表土和植物样品的铁同位素组成，以揭示重复氧化还原循环对铁同位素分馏的影响。2)部分土壤和植物样品以及铁斑块将使用同步辐射X射线吸收光谱分析（由双方合作伙伴一起），以说明铁物种演变沿着水稻土的发展。3)中国合作伙伴将调查水稻土剖面和铁斑块中的铁氧化和还原细菌微生物群落，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.