Water Management Affected GHG Emissions and Microbial Community Pattern in a Clayey Soil of Central Italy.

Reducing CH₄ and N₂O emissions from rice cropping systems while sustaining productivity with less irrigation water requires a better understanding of the key processes involved. Also, the role of microbial communities driving CH₄ and N₂O production in soils under different management is still unclear. To close this knowledge gap, we assessed how alternate wetting and drying (AWD) irrigation practice influenced fluxes and annual budgets of CH₄ and N₂O emissions in Italian rice systems in a field trial over a 2-year period and the influence of water management on microbial community structure and adaptation in a laboratory experiment.

Overall, a larger global warming potential (GWP) was observed under AWD, as a result of huge N₂O peaks which offset reductions in CH₄ emissions. This trend was more evident with harsher water reduction, in fact the transition from anaerobic to aerobic soil conditions resulted in the highest N₂O fluxes under AWD. The water management strongly affected microbial community structure also, which was itself tightly linked to CH₄ and N₂O production. N₂O production was the highest in aerobic soil, which also exhibited the strongest evidence for active nitrifying communities (NirK). Drying and rewetting aerobic soil enhanced the production of nitrate, which was further reduced to N₂O through incomplete denitrification. As expected, CH₄ production was the lowest in aerobic soil, which showed a less abundant archaeal community.

The duration of flooding, transition to aerobic conditions, water level above the soil surface, and the relative timing between fertilization and flooding were the main drivers affecting GHG mitigation potential under AWD and should be carefully planned through site-specific management options. Microbial communities in paddy soils progressively adapt to water management practices, thereby reinforcing potential differences in GHGs production.