Master Thesis

Impact of nickel-sulfide formation and aging on methanogenesis in paddy soils

José Miguel León Ninin (09/2018-09/2019)

Support: Britta Planer-Friedrich, Judith Forberg

Paddy fields are a major source of methane (CH4) since the flooded conditions create an ideal environment for methanogens. They currently produce 10-25% of the annual global CH4 emissions, which are predicted to increase with climate change. In wetlands, S-rich soils seem to have a natural resilience against CH4 emission increases, related to the high binding capacity of sulfide phases for Nickel (Ni), a trace element that has a key role in CH4 production. A potential presence and importance of such resilience in paddy fields remain to be studied. Here, paddy soils with different Ni and S contents were sampled and their Ni-binding forms and capacities were studied, showing that Ni is not directly available for methanogens under anoxic conditions. Nonetheless, results indicate that paddy soils are less Ni-limited than wetlands. Nickel binding to sulfide phases showed to be relevant under anoxic conditions, indicating Ni immobilization. During incubation experiments, no direct effect between Ni2+ spikes and CH4 production was observed, which could indicate that methanogens in paddy soils may not be Ni-limited. However, methodological problems avoid the confirmation of such statement, and experiments require repetition. Incubations with different NiS forms show that methanogens are capable of scavenging Ni from these particles, regardless of their crystallinity, which implies NiS particulate forms could act as middle- and long-term Ni source for methanogens on rice paddies. Thus, natural content of S in paddy soils may be too low to exert a strong resilience effect and increasing sulfate fertilization might produce phases that are still available for methanogens.