The impact of the unharvested reeds on the nitrogen and phosphorus cycles in Lake Dongting

In recent years, large areas of reeds were abandoned for harvesting in Lake Dongting, adding unique hydrological conditions, which may alter the biogeochemical cycle of nitrogen (N) and phosphorus (P). To identify the influence of unharvested reeds on N and P cycles in Lake Dongting, areas near the reeds and central area of East Lake Dongting in August 2024 were investigated. Total and labile organic matter, P fractionation and sorption behaviors, extracellular enzymatic activities, dehydrogenase activity, and iron in sediment, different forms of N and P in surface and interstitial water as well as chlorophyll a (Chl a) were analyzed. In areas near the reeds, organic matter decomposition caused by reeds decay led to a higher anaerobic degree, yet the release of iron bound P from the sediment was relatively low due to the extremely low content. Chl a of the two areas was similar, but P required for algae in areas near the reeds mainly came from the hydrolysis of organic P in terms of higher organic P content and alkaline phosphatase activity, while in central area mainly from nutrients deposited and high P release potential in terms of high equilibrium phosphorus concentration. Although high ammonium in interstitial water caused by decomposition of protein, low dissolved inorganic N existed in the surface water of the areas near the reeds due to few N migration from interstitial water caused by short-term flooding and the absorption of N by reed leaves. The decline of unharvested reeds would not result in eutrophication and algae blooms due to unique hydrological conditions, and both hydrology and vegetation cause differences in the migration and transformation of sediments nutrients in Lake Dongting.