Response of bacterial communities and soil chemistry to flood durations and recovery phases.

Flooding profoundly impacts soil bacterial communities; however, the underlying mechanisms remain poorly understood. This study investigated how flooding (3, 8, and 16 days) and post-flooding (immediately and 2, 5, and 30 days) durations affect soil physicochemical properties, bacterial communities, and their interactions-crucial factors in floodplain nutrient and carbon cycling. The results showed that bacteria constituted 99.9% of the total microbial composition, while archaea, contributing only 0.1%, had a negligible impact on soil traits. At 2-5 days after flooding (DAF), elevated soil electrical conductivity (EC) and pH enhanced soil bacterial abundance and activity, leading to increased water-extractable dissolved organic carbon (DOC), water-extractable total dissolved nitrogen, and biological production (BIX), accompanied by the degradation of soil organic matter (SOM) and aromatic compounds (SUVA254). These changes indicated robust interactions between soil bacterial communities and physicochemical properties affected by flooding events. However, these relationships weakened at 30 DAF, suggesting potential transitions from anaerobic to aerobic conditions in post-flooding soils after 5 DAF. Structural equation modelling indicated that an extended post-flooding duration increased BIX, accompanied by SOM and DOC degradation, providing nutrients and energy to soil microbes and consequently leading to increased bacterial diversity. This study underscores the significant impact of flooding and post-flooding durations on soil bacterial community composition and diversity, mediated by changes in EC, pH, SOM, and DOM, potentially influencing nutrient cycling in floodplains.

Tracing the pathways by which flood duration impacts soil bacteria through soil properties and water-extractable dissolved organic matter: A soil column experiment.

Soil microbial communities control biogeochemical processes, nutrient cycling, and organic carbon storage and release in wetlands, which are influenced by flooding. To predict soil nutrient function in wetland ecosystems, understanding the effect of flooding on soil biogeochemical cycling and energy flux, including soil properties, dissolved organic matter (DOM), and microbial communities is essential. This study investigated how different flood durations (1, 3, 8, 16, and 30 d) affect the interactions between physicochemical properties and bacterial communities in a river wetland. The DOM composition was measured using ultraviolet/visible spectrophotometry coupled with fluorescence spectroscopy, and the bacterial communities were identified using 16S rRNA sequencing. Simpson's diversity index varied from 0.92 to 0.94, indicating high bacterial diversity throughout the treatments; the highest and lowest bacterial diversities were found at 1 and 8 flooding days, respectively. The abundance of Desulturomonadales, Clostridiales, Bacteroidales, and Gaiellales was positively correlated with pH, electrical conductivity, water-extractable dissolved organic carbon (WEOC), and water-extractable total dissolved nitrogen (TDN) but negatively correlated with dissolved oxygen (DO) and soil organic matter (SOM), suggesting complex interactions among these factors in response to flooding. Structural equation model revealed that flooding directly increased TDN but indirectly increased WEOC through increasing soil pH; and directly decreased DO and SOM, leading to decreases in total protein-like fraction. Three significant pathways were identified, showing the impacts of flooding on bacterial diversity: (1) flood duration decreased DO, resulting in decreased bacterial diversity; (2) flood duration decreased SOM, leading to increased bacterial diversity; and (3) flood duration decreased DO and SOM, leading to increased bacterial diversity via decreased total protein-like fraction. This study indicated that prolonged flooding has both positive and negative impacts on bacterial diversity, depending on environmental factors. It highlights the importance of flooding in shaping soil bacterial communities, with implications for nutrient cycling and carbon storage in wetlands.