Comparison of the bacterial and methanotrophic diversities between an Italian paddy field and its neighboring meadow

Methane is a potent greenhouse gas that contributes to global warming. However, under certain conditions, its release into the atmosphere can be mitigated by methane-oxidizing microorganisms. Typically, cultivated wetlands (i.e., paddy fields) are a major source of methane (CH4) while forests and meadow uplands are considered to be CH4 sinks. As the global need for rice production increases each year, more uplands are converted to inundated paddy fields. To investigate soils that may be converted into productive land for rice production, we investigated a paddy field and adjacent meadow in Northern Italy. Using a combination of 16S rRNA gene amplicon sequencing to analyze the bacterial community, and gas flux measurements to quantify CH4 emissions, we looked for differences between classically defined CH4 sinks (meadow soils) and CH4 sources (paddy fields). Analysis of the total bacterial community revealed that the family Fimbriimonadaceae, which belongs to the phylum Armatimonadetes, was significantly higher in paddy field soils driving the difference between paddy and meadow soils. Whereas, we found that the methylotrophic families Methyloligellaceae and Methylomirabilaceae were also present in higher relative abundance in the paddy field. Despite these major differences, CH4 fluxes were highly variable between the two sites with no significant differences observed. Furthermore, we found the Methylomonaceae family to be more abundant at the center of a neighboring paddy field compared to the edge of the paddy field from the current study, hinting at methanotrophic variation based on location. Taking these results into account, we propose a conceptual model to explain possible scenarios that may result in paddy and meadow fields not exhibiting classical source/sink properties. These findings call for caution when including paddy and meadow areas separately into global CH4 flux calculations, and urge further research to discern drivers of CH4 cycling under a range of environmental conditions rather than relying on assumptions.