Redox-driven shifts in soil microbial community structure in the drawdown zone after construction of the Three Gorges Dam

Soil redox is a critical environmental factor shaping the microbial community structure and ultimately alters the nutrient cycling. However, the response of soil microbial community structure to prolonged or repeated redox fluctuations is not yet clear. To study the dynamic effects of prolonged redox disturbances to the soil microbial community structure, soil samples experiencing 8, 5 and 0 alternating oxic-anoxic cycles within approximately 6 months each year were collected and the microbial community structure were evaluated using phospholipid fatty acid analysis (PLFA) profiles. Prolonged redox disturbances had significant effects on soil physiochemical properties and soil microbial community structure. The relative abundance of straight chain saturated PLFAs, cyclopropyl, and terminal- and mid-branched chain saturated PLFAs increased due to prolonged redox disturbances, but there was a consistent decrease in linear monounsaturated PLFAs and polyunsaturated PLFAs in the fluctuating zone. Prolonged redox disturbances had a negative impact on the total PLFA content (a proxy for biomass). Both the fluctuating zone (8-cycle and 5-cycle plots) and the never flooded zone (0-cycle plots) were dominated by Gram-positive bacteria and a low content of fungi, actinomycetes and protozoa. The fungi and protozoa abundance decreased significantly with an increase in the occurrence of alternating flooding-dry events, suggesting that the prolonged redox disturbance leads to high stress on the fungi and protozoa populations. Moreover, total organic matter (TOC) and C:N ratio, environmental factors that can be influenced by recurring redox fluctuations, also influenced the microbial community structure.