Enhanced activity of oligotrophic endogenous bacteria in clay‐rich sediments by nutrient injection

A controlled field experiment was performed in which the microbiology and geochemistry of clay‐rich fluvial‐deltaic sediments were characterized both before and after nutrient injection into a shallow well. Acetate addition (with nitrogen and phosphate) initially increased the heterotrophic bacteria population in the groundwater within 21 days after nutrient addition. Consumption of oxygen and injected nutrient resulted in an expected stimulation of copiotrophic bacterial growth (31–48 days), then a noticeable “trough phase”; reflecting minimal or no bacterial growth followed by a secondary peak reflecting another bacterial population growth surge (62–85 days). This secondary surge was apparently supported by the nutrients generated by the decomposing biomass (initial population peak), and by oxygen replenishment supplied by continual ground water flow. During the intervening trough phase, bacterial counts by most‐probable‐number analysis of soil samples indicated that the denitrifying population increased to a greater degree than the remainder of the heterotrophic population. An increase in methane gas was detected in the head space of water samples collected during the trough phase, suggesting an increase in bacterial methanogenesis. There was no evidence of a concomitant increase in bacterial sulfate‐reducing activity. This rapid progression from aerobic to microaerophilic and anaerobic growth probably resulted from the microbial biodiversity present in clay‐rich sediments, which in turn is related to the development of microhabitats. These microhabitats developed because of the hydrogeologic character of the clay‐rich sediments in which flow is controlled by macropores while fluid in the clay matrix (micropores) is relatively isolated from the groundwater in the macropores.