Monthly dynamics of microbial community structure and their controlling factors in three floodplain soils

• Three different floodplain soils can be discriminated via microbial community.
• Stable soil features determine microbial community stronger than dynamic properties.
• Long-term flooded soil shows lowest fungi–bacteria-ratio and aerobe–anaerobe-ratio.
• Mollic Fluvisol has highest fungi–bacteria-ratio and aerobe–anaerobe-ratio.

Seasonal dynamics of microbial community under frequently fluctuating oxidized and reduced conditions in floodplain soils are poorly understood, but are considered to be important for understanding microbial community and carbon cycling dynamics in these ecosystems. We determined the microbial community structure using phospholipid fatty acid analysis (PLFA) of three different floodplain soils (Eutric Gleysol = GLe, Eutric Fluvisol = FLe, and Mollic Fluvisol = FLm) at the Elbe River, Germany, for 17 months. Flood duration, soil moisture, soil temperature were also monitored, and hot and cold water extractable carbon (CHWE, CCWE) were determined. Flood duration seems to have a negative impact on total PLFA biomass which increased in the order GLe < FLe << FLm. All PLFA profiles were dominated by Gram-positive bacteria (GPB) and actinomycetes, respectively, and a low content of fungi and arbuscular mycorrhizal fungi (AMF). In the briefly flooded relatively quickly drained soils (FLe and FLm) Gram-negative bacteria (GNB) were abundant compared to the longer flooded, relatively slow drained soil (GLe). This was also obvious in the significant lowest fungi-bacteria ratio and aerobe-anaerobe ratio of GLe. Non-metric dimensional scaling (NMDS) and canonical discriminant analysis (CDA) as multivariate statistical procedures reveal that FLm could be separated from GLe and FLe probably due to aerobic conditions and available soil organic carbon. The GLe can be discriminated from FLe and FLm mainly due to different flooding durations. The GNB, fungi and AMF were more affected by changes of soil moisture and extractable carbon than the GPB, actinomycetes and anaerobes. We conclude that more stable properties of bulk soil such as the magnitude of soil organic carbon, soil texture, and associated flood duration had a stronger impact on soil microbial community than monthly fluctuations of more dynamic properties, such as soil moisture, soil temperature, and CHWE, CCWE in our soils.