The Impact of Flooding on Soil Microbial Communities and Their Functions: A Review

Soil microorganisms provide multifaceted benefits, including maintaining soil nutrient dynamics, improving soil structure, and instituting decomposition, all of which are important to soil health. Unpredictable weather events, including flooding from heavy rainfall, flash floods, and seawater intrusion, profoundly impact soil ecology, which is primarily challenged by flooding stress, and imbalances these microbial communities and their functions. This disturbance impairs the symbiotic exchanges between microbes and plants by limiting root exudates and habitats for microbes, as well as nutrient acquisition efficiency for plants. Therefore, this review comprehensively examines the changes in soil microbial communities that occur under flooding conditions. Flooding reduces soil oxygen (O₂) levels, limiting aerobic microbes but promoting anaerobic ones, including potential pathogens. In flooded soil, O₂ deficiency indirectly depends on the size of the soil particles and water turbidity during flooding. O₂ depletion is critical in shaping microbial community adaptation, which is linked to variations in soil pH, nutrient concentrations, and redox status, and fresh and saline water vary differently in terms of the adaptation of microorganisms. Wet soil alters soil enzyme activity, which influences microbial community composition. Notably, three-month post-flooding conditions allow microbial communities to adapt and stabilize more effectively than once-weekly flooding frequency. Based on the presence of aboveground species, fungi are found to reduce under flooding conditions, while nematode numbers, surprisingly, increase. Direct and indirect impacts between soil microbes and physio-chemical properties indicate positive or negative feedback loops that influence the soil ecosystem. Over the years, beneficial microorganisms such as plant-growth-promoting microbes (PGPMs) have been identified as important in regulating soil nutrients and microbial communities in wetland environments, thereby enhancing soil health and promoting better plant growth and development. Overall, understanding the mechanisms of belowground ecosystems under flooding conditions is essential for optimizing agricultural practices and ensuring sustainable crop production in flood-prone areas.