Hydrilla-derived organic matter dominates sediment nitrogen mineralization in Poyang Lake

The unique hydrological regime of Poyang Lake creates pronounced spatial heterogeneity in sediment organic matter. Variation in source contributions and composition drives differences in nitrogen mineralization and ammonium immobilization, yet the mechanisms remain poorly understood. This study quantified gross nitrogen mineralization (GNM), gross ammonium immobilization (GAI), and net nitrogen mineralization (NNM) across Lake, Inlet, and Mudflat zones during the dry season. Strong spatial variation was observed: GNM ranged from 0.10 to 2.59 mg N kg^-1 d^-1, GAI from 0.08 to 2.19 mg N kg^-1 d^-1, and NNM from -0.11 to 1.28 mg N kg^-1 d^-1. GNM and GAI were highest in the Mudflats, whereas NNM peaked in the Inlet. A Bayesian isotope mixing model indicated that hydrilla-derived organic matter was the primary contributor to nitrogen mineralization in Mudflats. Multiple regression and structural equation modeling identified distinct controls for each transformation rate: sediment temperature governed GNM under cold conditions, while DOM quality indicators (e.g., C3, C:N, a(254)) were strongly associated with all three rates. Notably, a multiple regression model including C3, TOC, and a(254) explained 93.3 % of the variance in GNM. These results highlight the central role of autochthonous macrophyte-derived organic matter in regulating sediment nitrogen cycling, offering insights into internal nutrient dynamics and guiding targeted lake management.