Effects of physical and chemical factors on denitrifier community composition and denitrification rates in agriculturally impacted streams

Agricultural fertilization leads to excess nitrogen (N) in many waterways; one way in which N is removed is through the microbial process of denitrification.  This study, which will clarify the relationship of key chemical and physical factors to efficiency of denitrification, is being conducted in an agricultural watershed in central Indiana. Sediments were collected from the center, sides, and riparian zones of a first and a second order stream within the watershed. The acetylene block method was used to examine denitrification rates. Molecular methods, focused on the nosZ gene (nitrous oxide reductase), were used to examine the bacterial community and included quantitative PCR and terminal restriction fragment length polymorphism to investigate genetic diversity. DOC in the streams ranged from 1.7 to 26.3 mg/L and total dissolved N ranged from 0.12 to 4.0 mg/L. PicoGreen quantification of DNA extracts yielded concentrations ranging from 1.25 to 52.45 ng/µl. To date, PCR has recovered nosZ in all samples examined despite undetectable or very low (<18 µg N/m²/h) denitrification rates when nitrate concentrations are low.  Even in agricultural streams, environmental conditions are not always conducive to denitrification, but the denitrifier community appears poised to respond as conditions become favorable.