Insight into the nutrient change in freshwater ecosystem under ferrous addition: Revealed by phosphorus, nitrogen, and microbial community

Restoration of submerged macrophytes in freshwater ecosystems is a widely utilized and effective way for controlling eutrophication, but it's not enough for the endogenous nutrient removal. Incubation experiments were conducted in this study to explore the effects of ferrous addition on the nutrients and rhizosphere microorganisms. The ferrous addition with different dosages has different effects on the phosphorus (P), nitrogen (N) and other properties in the freshwater ecosystem with submerged macrophyte growth. The ferrous addition can affect the nitrogen cycling of water and sediments significantly by affecting the functional bacteria related to the nitrogen cycling in rhizosphere sediment. The genera related to denitrification (e.g., Thiobacillus), ammonification (e.g., Chitinophaga) and nitrification (e.g., Nitrospira) were enriched with the ferrous addition above 0.5 mg/L. On the contrary, the relative abundance of Clostridium_sensu_stricto_1 which was related to dissimilarity nitrate reduction to ammonium (DNRA) decreased after ferrous addition. The relative abundance of functional genera associated with iron cycle were enriched, especially the Ferritrophicum and Thiobacillus. Ferrous addition can successfully control phosphorus levels in the water at a low concentration (0.02 mg/L) and help with the stabilization of phosphorus in sediment. Considering the effects of iron on nutrients and microorganisms at different dosages, 5.0 mg/L is recommended to promote endogenous nitrogen removal and sediment phosphorus stabilization in freshwater ecosystems with submerged macrophytes. These findings contribute to the ecological evaluation of ferrous addition, and provide a reference or strategy for ecological restoration with submerged macrophyte especially V. natans in freshwater ecosystems.