Flooding forested groundwater recharge areas modifies microbial communities from top soil to groundwater table

Subsurface microorganisms are crucial for contaminant degradation and maintenanceof groundwater quality. This study investigates the microbial biomass andcommunity composition [by phospholipid fatty acids (PLFAs)], as well as physicaland chemical soil characteristics at woodland flooding sites of an artificialgroundwater recharge system used for drinking water production. Vertical soilprofiles to c. 4m at two watered and one nonwatered site were analyzed. Themicrobial biomass was equal in watered and nonwatered sites, and considerablefractions (25–42%) were located in 40–340 cm depth. The microbial communitystructure differed significantly between watered and nonwatered sites, predominantlybelow 100 cm depth. Proportions of the bacterial PLFAs 16:1o5, 16:1o7,cy17:0 and 18:1o9t, and the long-chained PLFAs 22:1o9 and 24:1o9 were moreprominent at the watered sites, whereas branched, saturated PLFAs (iso/anteiso)dominated at the nonwatered site. PLFA community indices indicated stressresponse (trans/cis ratio), higher nutrient availability (unsaturation index) andchanges in membrane fluidity (iso/anteiso ratio) due to flooding. In conclusion,water recharge processes led to nutrient input and altered environmentalconditions, which resulted in a highly active and adapted microbial communityresiding in the vadose zone that effectively degraded organic compounds.