Simultaneous minimization of arsenic mobilization and N2O emission in rice paddy soils

• Applicants: Professor Dr. Andreas Kappler; Professorin Dr. Christiane Zarfl
• Subject Area: Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
• Term: from 2020 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 431072007

Rice represents a major food source for millions of people in Asia. The food quality has often been debated due to accumulation of toxic arsenic that accumulates in rice grains after irrigation with contaminated water. In many areas, arsenic is a natural contaminant originating from out-washing of arsenic rich mountains. Traditional water-logged rice cultivation prevents oxygenation of the soil and provides reducing conditions under which mobile arsenic (III) can enter the rice plant tissue and accumulate in rice grains. This cultivation method also has implications for greenhouse gas production. Under anoxic condition N2O can be produced via chemical and biotic denitrification processes. Fertilization of the rice field shifts the redox conditions towards denitrification. Microbial denitrification causes the formation of nitrite as intermediate and N2 or N2O as final reaction products. In iron rich reduced paddy soils dissolved iron(II) reacts with nitrite during the process of chemodenitrification and produces a tremendous amount of the greenhouse gas N2O. However, during chemodenitrification iron(III) minerals get formed which serve as highly reactive sorption template. Iron(III) minerals have been shown to actively immobilize toxic arsenic, and thus, prevent accumulation in the plant and grains. The application of less or no fertilizer in water-logged rice cultivation will shift redox conditions further along the redox ladder towards iron(III) reduction. Microbial iron(III) reduction partly dissolves iron(III) minerals, and thus, re-mobilizes toxic arsenic, but prevents the formation of N2O via (chemo)denitrification. The proposed research project aims to find an optimum balance between N2O production and As immobilization controlled by the application of fertilizer in traditional water-logged rice cultivation. Our goal is to decipher under which fertilizing conditions optimum food quality and low climate impact can be achieved. Along these lines we will combine experimental data collection with environmental system analysis in order to extrapolate from our experimental conditions to large scale systems.

• DFG Programme: Research Grants