Mechanisms of high ammonium loading promoted phosphorus release from shallow lake sediments: A five-year large-scale...

The unprecedented global increase in the anthropogenic-derived nitrogen (N) input may have profound effects on phosphorus (P) dynamics and may potentially lead to enhanced eutrophication as demonstrated in short-term mesocosm experiments. However, the role of N-influenced P release is less well studied in large-scale ecosystems. To gain more insight into ecosystem effects, we conducted a five-year large-scale experiment in ten ponds (700-1000 m² each) with two types of sediments and five targeted total N concentrations (TN) by adding NH₄Cl fertilizer (0.5, 1, 5, 10, and 25 mg N L^-1). The results showed that: (ⅰ) The sediment P release increased significantly when TN exceeded 10-25 mg N L^-1. (ⅱ) The most pronounced sediment P release increase occurred in summer and from sediments rich in organic matter (OM_Sed). (ⅲ) TN, algal biomass, fish biomass, non-algal turbidity, sediment pH, and OM_Sed were the dominant factors explaining the sediment P release, as suggested by piecewise structural equation modeling. We propose several mechanisms that may have stimulated P release, i.e. high ammonium input causes a stoichiometric N:P imbalance and induce alkaline phosphatase production and dissolved P uptake by phytoplankton, leading to enhanced inorganic P diffusion gradient between sediment and water; higher pelagic fish production induced by the higher phytoplankton production may have led increased sediment P resuspension through disturbance; low oxygen level in the upper sediment caused by nitrification and organic decomposition of the settled phytoplankton and, finally, long-term N application-induced sediment acidification as a net effect of ammonium hydrolysis, nitrification, denitrification; The mechanisms revealed by this study shed new light on the complex processes underlying the N-stimulated sediment P release, with implications also for the strategies used for restoring eutrophicated lakes.