Unearthing a stream-wetland floodplain system: increased denitrification and nitrate retention at a legacy sediment...

Nitrogen (N) retention is a common goal of stream-wetland restoration projects in systems with excess nitrate ( $\text{N}{{\text{O}}_3}^{-}$ ), however N retention depends on habitats with high denitrification and uptake rates that interact with $\text{N}{{\text{O}}_3}^{-}$ . Legacy sediments deposited along formerly impounded streams bury and disconnect historic floodplain-wetland systems. This disconnection limits sediment-water interactions, decreases N retention and increases N delivery. Restoration with legacy sediment removal should lead to greater N retention due to the reestablishment of wet habitats that interact with $\text{N}{{\text{O}}_3}^{-}$ -rich water, but the formation of biogeochemically retentive soils under modern conditions of high $\text{N}{{\text{O}}_3}^{-}$ , N retention rates, and recovery time are unclear. An experimental restoration approach undertaken at Big Spring Run in Lancaster, PA, USA was used to test the hypothesis that reconnection of a stream to its historic floodplain with legacy sediment removal enhances N processing and retention. We describe changes in sediment and water concentrations of N and organic carbon (C) along with the changes in sediment biogeochemical processing rates of denitrification, nitrification, and C mineralization, before and for five years following restoration. Our results show that biogeochemical processing increased and higher $\text{N}{{\text{O}}_3}^{-}$ retention developed following stream-wetland restoration. $\text{N}{{\text{O}}_3}^{-}$ retention improved after several years as organic matter accumulated to ultimately support higher rates of denitrification that transitioned from organic C limitation to $\text{N}{{\text{O}}_3}^{-}$ limitation. We conclude that, in systems with high contemporary $\text{N}{{\text{O}}_3}^{-}$ , restoration via legacy sediment removal and floodplain reconnection can lead to the accumulation of organic matter and improved biogeochemical $\text{N}{{\text{O}}_3}^{-}$ retention over time.