Size Fractionation of Dissolved Organic Nitrogen in Peatland Fluvial Systems

Understanding the nature and fate of nitrogen (N) in freshwater systems is crucial for assessing the risk of eutrophication. However, there is a paucity of information on the characterization of fluvial N in upland peat-dominated environments. Here, we employ a combination of field sampling and tangential flow ultrafiltration (TFU) to investigate the concentrations and fluxes of low molecular weight (LMW) and high molecular weight (HMW) dissolved organic N (DON) in a peatland stream-reservoir system in the south Pennines (UK). Our TFU results show that ∼26% of DON concentration is LMW DON and represents an estimated fluvial flux of 3.07 ± 22 kg N ha^-1 during the study period. Our mass balance results reveal that the reservoir retains 71% of LMW DON input, which accounts for ∼25% retention of bioavailable (dissolved inorganic N + LMW DON) N. Our study suggests that current understanding of inorganic N as the sole source of bioavailable N with eutrophic significance in upland freshwaters requires a reappraisal. Evaluation of ecosystem response to increased loading of N needs to include a consideration of LMW DON.