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FERRY: Biogeochemical mechanisms of C and P cycles in paddy soils driven by the Fe(III)-Fe(II) redox wheel in a dynamic redox environment

Supervisor:Prof. Dr. Georg Guggenberger, Dr. Jannis Florian Carstens
Researcher:M.Sc.-Geowi Stephanie Günther
Funded by:Deutsche Forschungsgemeinschaft (DFG)

The cultivation of Asian rice on paddy fields is a major contributor to the global food supply. Typically, the cultivation techniques involve flooding the fields during the vegetation period. This creates very distinct properties in these soils, as the anthropogenic impact leads to the rapid onset of reducing conditions that last until the fields are drained prior to harvesting) The changing redox conditions significantly influence the characteristics of redox-sensitive Fe minerals, which in turn affects the cycling of important elements such as P and C in the soils by a multitude of redox, sorption and (co)precipitation processes.

With that, iron phases in paddy soils are of utmost importance for two environmental constraints associated with rice cultivation. Like many other agricultural soils, paddies receive high rates of P fertilization. In China, the release of P from nonpoint agricultural sources and the subsequent eutrophication of freshwater lakes caused disruptions of drinking water supply for cities and billions of yuan in economic damage. Hence, a better knowledge of the P cycling processes, and their controlling mechanisms is of significant importance to prevent the overuse of P fertilizers. Paddy soils are also playing a prominent role in the global C cycling. On the one hand, they are a potent source of the greenhouse gases CO2 and particularly CH4. In fact, flooded rice fields emit between 9-19% of the total anthropogenic release of methane into the atmosphere. On the other hand, paddy soils significantly sequester organic C and should be considered in the global C accounting scheme.

The reducing conditions in a submerged paddy soils are locally modified by the transport of oxygen by rice roots. This complex distribution of redox conditions in paddy soils affects redox-sensitive iron minerals, i.e. the microbially mediated dissolution of ferric oxides, the formation of ferric iron plaques, and of ferrous secondary minerals such as vivianite. Many open questions on specific mechanisms and processes that control the associations between ferric and ferrous iron phases with both P and C in paddy soils remain, which are jointly addressed by a group of Chinese and German soil scientists (Institute of Microbiology, LUH) with a synergistic expertise.

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