The fate of particulate nitrogen in fluvial systems

Abstract

Large storms can erode, transport, and deposit large amounts of allochthonous particulate organic matter (POM) and particulate nitrogen (PN) to the fluvial network. The role of storm-driven POM in the processing of fluvial C and N and its impact on water quality is still poorly understood. This study investigates the fate of C and N from storm-driven POM deposition using a 56-day incubation experiment of five known POM sources from a 79-ha forested watershed. Incubation columns were treated with one of two moisture treatments; Moisture Regime 1: sediment subjected to frequent rewetting treatments, Moisture Regime 2: sediments subjected to dry-wet cycles. Sediment and porewater samples were collected throughout the incubation and analyzed to characterize and compare the solid and solution pool chemistries and the abundances of nitrifying and denitrifying microbial populations. Key findings from this study are: (1) C and N rich sources experienced decomposition, mineralization, and nitrification and released large amounts of dissolved N, but the amount of N released varied by POM source and moisture regime. Drying and rewetting stimulated nitrification and suppressed denitrification in most POM sources. (2) Fluvial Storm Deposits released large amounts of porewater N regardless of the moisture conditions, indicating that they can readily act as N sources under a variety of conditions. (3) Forest Floor Humus was the only POM source to exhibit mineralization and denitrification when frequently rewetted. Under these conditions, this POM source acted as the strongest source of dissolved organic nitrogen. Under the drying and rewetting moisture conditions, Forest Floor Humus became a N source, specifically exhibiting intense decomposition, mineralization, and nitrification, resulting the release of large amounts porewater nitrate. The inputs and processing of large storm-driven PN inputs becomes increasingly more important as the frequency and intensity of these large storms are predicted to increase due to global climate change. Gaining a better understanding of the fate of the N derived from particulate materials is critical to the development and implementation of effective management practices for controlling water pollution and maintaining healthy waterways.