The Development of Denitrification and of the Denitrifying Community in a Newly-Created Freshwater Wetland

Abstract

Agricultural growth and urban development have increased nutrient loading into streams to unnatural levels, negatively affecting estuaries downstream. Wetland creation along streams has the potential to mitigate watershed nutrient loading, providing the necessary conditions to encourage bacterial denitrification. We compared denitrification in two wetland systems, one recently created and one well established, along a branch of the St. Martin River, the largest and most degraded tributary flowing into the Maryland Coastal Bays. Ambient and potential denitrification rates were measured using membrane inlet mass spectrometry, and the denitrifying bacterial communities were compared using qPCR with genes nirK, nirS, nosZ-I, and nosZ-II. For all sites, denitrification rates were highly variable, but similar between the two wetlands, while potential rates increased almost 10-fold after a NO3− amendment, indicating that the denitrifying communities can respond to pulses of NO3− delivered via episodic flow events. Overall, the denitrifying community was more abundant at the created wetland, but the reference site had a higher abundance of the nosZ-I gene and, therefore, a lower genetic potential for N2O production. The created wetland has developed denitrification rates and a denitrifying community similar to those of the established wetland and could successfully remove excess nutrients entering the system.