Disturbances such as flooding affect both above- and below-ground ecosystem processes. Although often ignored, changes in below-ground environments following flooding are no less important than those that occur above-ground. Flooding changes soil properties by reducing oxygen availability, altering the mobility and availability of mineral ions, and increasing the production of toxic substances. Soil microorganisms are sensitive to disturbance, and shifts in soil microbial community structure are expected when anaerobic conditions develop from flooding. The primary objective of this study was to determine the effect of flooding on soil microbial communities. Simulated floods were established under greenhouse as well as field conditions. In the greenhouse, four flood treatments (control, intermittent, flowing and stagnant) were paired with four residue treatments (control, grass residue, legume residue and tree residue). Under field conditions at the Flood Tolerance Laboratory at the Horticulture and Agroforestry Research Center in New Franklin, MO (USA), four flood treatments (control, 3-week-flowing, 5-week-flowing and 5-week stagnant) were established. Pre- and post-flood soil samples were collected and examined using phospholipid fatty acid (PLFA) analysis. In addition, total organic C and total N were determined for each sample using a dry combustion technique (LECO TruSpec CN analyzer).

Results/Conclusions

Microbial community responses to flood treatments were observed for soils under simulated flood conditions in the greenhouse but not in the field. Stagnant flood conditions in the greenhouse resulted in the decrease of microbial biomass as well as decreases in the responses of aerobic bacteria, Gram-negative bacteria, Gram-positive bacteria, and mycorrhizal fungi. These changes in soil microbial community structure may have implications for nutrient cycling. Stagnant conditions also resulted in an increased response of stress indicators and a decreased response of monounsaturated fatty acids. Effects of flood treatments in the field were masked by effects due to sampling depth and date. General decreases in microbial biomass and microbial marker response with depth were observed. Changes in pre- and post-flood samples were more likely due to seasonal influences (spring vs. summer) rather than specific flood treatments. Regardless of setting (i.e. greenhouse or field) the simulated floods had no affect soil total organic carbon or total nitrogen. Under greenhouse conditions, soil microbial community structure and soil C and N were not correlated. However, under field conditions, the responses of aerobic bacteria, Gram-negative bacteria and Gram-positive bacteria were all positively correlated with soil C and N.