[B-CG05] Methane in terrestrial and aquatic ecosystems: from microbes to the atmosphere

[BCG05-P07] The effect of diel light cycle on the greenhouse gasses dynamics in a wetland rice field soil

*Bella Tegarwati Simamora¹, Jun Murase¹ (1.Nagoya University)

Keywords:Greenhouse gas, Diel light cycle, Paddy field, Oxygen

Rice cultivation in flooded paddy fields creates distinct physical, chemical, and biological conditions of the soil that significantly influence the microbial communities and biogeochemical processes. The surface layer of flooded rice soil undergoes diel fluctuations in oxygen levels due to photosynthetic activity, generating a dynamic redox gradient that supports diverse microbial metabolisms. This study investigates the impact of diel light cycle on biogeochemistry in the top layer of wetland rice soil through a microcosm experiment. Water-saturated rice soil slurries mixed with chemical fertilizer and rice straw powder were incubated under flooded conditions at 25°C in a semi-open bottle, allowing gas exchange with the atmosphere. The incubation was conducted under three different light conditions: 1) a 12-hour light (approx. 300 μmol m^-2 s^-1)/12-hour dark cycle, 2) continuous light, and 3) continuous dark. Dissolved oxygen profiles of the surface soil, Fe(II) content in the soil, and gas fluxes (CO₂ and CH₄) at the soil surface were determined weekly for up to four weeks. The diel light cycle induced dynamic temporal shifts in oxygen profiles. The oxygen concentration exceeded 100% air saturation in the top 500-700 μm in the light period due to the photosynthesis of soil algae and at the same depth exhibited a precipitous drop of oxygen, lower than 10% of air saturation, in the dark. The oxygen gradients in the diel light cycle were steeper than those observed in the continuous light and dark conditions. The Fe(II) content peaked in the first week and then gradually decreased, demonstrating active iron reduction during the initial stage of incubation followed by re-oxidation of reduced iron. The decreasing rate of Fe(II) content in the soil was higher under continuous light compared to continuous dark and diel light cycle conditions, which implies oxidation-reduction cycles of iron driven by the diel light cycle. CO₂ was emitted from the soil surface under the dark conditions. Under the light conditions, there was no CO₂ emission, even CO₂ uptake from the atmosphere was recorded, synchronized with the increased oxygen concentration in the soil. The CO₂ emission rate in the continuous dark conditions decreased with the time of incubation, while the diel light cycle kept the same CO₂ emission rate until the end of the incubation period, which suggested the re-mineralization of photosynthesized carbon. CH₄ emission from the soil was minimal under light conditions, probably due to the enhanced methane oxidation supported by oxygen produced by photosynthesis. In the dark conditions, CH₄ emission was observed during the first three weeks. CH₄ emission in the dark period of the diel light cycle showed a delay against CO₂ emission, which coincided with decreased oxygen in the soil profile. The results of this study demonstrate that diel light cycle promotes active photosynthesis with subsequent re-mineralization of photosynthesized carbon and plays a pivotal role in regulating the biogeochemical cycles, including CH₄ and likely iron, in wetland rice soils, offering broader implications of a diel light cycle for the ecosystem functioning.