Light inhibition of carbon mineralization associated with iron redox processes in calcareous paddy soil

Abstract

The iron redox cycle is closely tied to the fate of carbon in terrestrial ecosystems, especially paddy soils. Varies diurnally and seasonally, light—the crucial environmental factor—may be a fundamental factor elucidating temporal and spatial variabilities of carbon-containing gases emission. The role of sunlight in the iron-mediated carbon cycle, however, has not been fully elucidated. We conduct this study to test the role of light in the iron-mediated carbon cycling. In this study, we conducted anaerobic incubation experiments of a calcareous paddy soil in serum vials under alternating dark and light conditions. The dynamic evolution of the carbon and iron contents was evaluated by measuring the CO2, CH4, and O2 concentrations in the headspace of the vials, as well as the water-soluble inorganic carbon, microbial biomass carbon, and HCl-extractable ferrous iron contents in soil slurries. We also analyzed the soil microbial community structure by high-throughput 16S rRNA gene sequencing. The results highlighted the positive correlation between carbon mineralization and ferric iron reduction under dark conditions. Under light conditions, however, ferrous iron was oxidized by the O2 generated via oxygenic photosynthesis of phototrophic bacteria such as Cyanobacteria, along with a decreased production of CO2, CH4, and water-soluble inorganic carbon. The abundance of Cyanobacteria positively correlated to O2 levels and MBC content significantly. Light-induced periodic variations in the redox conditions facilitated carbon fixation in microbial biomass and up to 31.79 μmol g−1 carbon was sequestrated during 30 days light incubation. These results indicate that light inhibits the emission of carbon-containing greenhouse gases associated with the iron redox cycle in calcareous paddy soil. Assimilation of inorganic carbon by phototrophs may responsible for the inhibition of carbon mineralization. Our study suggests that procedures allowing more light to reach the soil surface, for instance, reducing the planting density, may mitigate greenhouse gas emissions and promote carbon sequestration in paddy soils.