Biogeochemical mechanisms of C and P cycles in paddy driven by the FeIII-FeII redox wheel in dynamic redox environment

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Paddy rice fields are periodically flooded and drained, which leads to strongly fluctuating redox conditions in the soils. Under reducing conditions, ferric iron minerals in the bulk soil are subjected to microbial dissolution by iron reduction, while ferric iron plaques are formed by iron oxidizers around oxygen-transporting rice roots. These iron phases have distinctive interactions with P and C compounds, which control the cycling of both of these elements. Thus, the objective of the proposed project is to gain further knowledge on the processes and mechanisms that govern the formation of associations between redox-sensitive iron minerals and P as well as C. The proposal is based on three major hypothesises: (1) The dissolved P concentration in the soil solution controls whether ferrous iron that is released from microbial dissolution is built into ferric iron plaques around roots or into the ferrous phosphate mineral vivianite. Each of the iron phases forms hot spots for iron reducing and oxidizing bacteria. (2) The microbial dissolution of ferric iron/P complexes leads to formation of vivianite, so that most of the released P is not plant accessible. Low P-availability due to ferric/P complexes or vivianite formation stimulate the activity of P-solubilizing and P-mineralizing microorganisms. (3) Rhizodeposited organic C is directly associated with ferric iron plaques and vivianite minerals. Both the root-derived organic C and the steep oxygen gradient at plaques provide the habitat for active iron and methane cycling associated microbes. The hypotheses will be tested by a Sino-German consortium, integrating experiences in isotope labelling techniques of Fe, P, and C to elucidate biogeochemical interactions from soil mineralogical, biological, and microbiological perspectives. The specific processes will be studied with isotope labeled Fe, P, and C for hypothesises (1), (2), and (3), respectively. Greenhouse pot experiments to test (1) and (2) will be conducted in Germany, and greenhouse pot experiments as well as field experiments adressing (3) will be conducted China. To assess the mechanisms concerning the formation and transformation of mineral phases, including the associated P and C, MC-ICP-MS, XPS, NanoSIMS, 33P radioimaging, Mössbauer spectroscopy, XRD, and ICP-OES will be applied. Microbial processes and key taxa will be determined with 16S rRNA, 16S rRNA gene and functional P-cycle associated gene amplicon high throughput sequencing, MPN enrichments and isolation, taxon-specific quantitative PCR, gene expression analyses, as well as in situ-like RNA- and PLFA-based stable isotope probing. This Sino-German research team envisions a better understanding on the impact of redox processes, associated ferric and ferrous iron phases, as well as microbial key players on C accumulation and P availability in paddy soils.