Effects of ecological restoration on microbial-mediated ammonia removal in polluted rivers: Microcosm experiment and physical modelling

Quantifying the effects of ecological restoration on microbial-mediated NH 3 -N removal is essential for effective river management. However, the conventional hydrodynamic-water quality models treat NH 3 -N removal by microorganisms as a constant absorption quantity, underestimating the important contribution of microorganisms, ignoring the dynamic changes of NH 3 -N removal rate under different restoration conditions. To address this gap, we developed a novel microbial-mediated NH 3 -N transformation model through microcosm experiments to quantify the impact of ecological restoration on NH 3 -N removal, employing a piecewise function to represent the NH 3 -N removal rate variation with dissolved oxygen concentration and submerged macrophyte coverage. Microcosm experiments demonstrated that artificial aeration promoted nitrifiers yet impairs denitrification efficiency and sustainability . Additionally, the submerged macrophyte restoration increased the abundance of nitrification and denitrification genes, indicating enhanced potential for NH 3 -N transformation. In these ecological restoration simulation systems, the abundance of specific functional genes (AOA _amoA , AOB _amoA , nirS , nosZ and hzo ) emerged as key rate-limiting factors for NH 3 -N removal. Overall, microbial action contributed to 53.7% of NH 3 -N removal, a proportion that remained stable ( R 2 = 0.91). Compared to models before microbial integration, our model showed greater reliability in simulating NH 3 -N concentrations based on field observations, with improved R 2 (0.76–0.80) and Nash-Sutcliffe efficiency coefficient (0.64–0.70) values in both time and space. Multi-scenario simulation results demonstrated that both artificial aeration and submerged macrophyte restoration could significantly reduce NH 3 -N concentration, with higher coverage of submerged macrophytes, rather than increased oxygen concentration, leading to greater NH 3 -N removal efficiency. Combining the two restoration measures resulted in even lower NH 3 -N levels compared to using submerged macrophytes alone. This study advances the understanding and prediction of the complex microbial process underlying nutrient transformation in ecological restoration systems. • This study evaluates ecological restoration's impact on microbial ammonia removal. • Microcosms reveal the relationship between key functional genes and ammonia removal. • Key functional genes (AOA_ amoA , AOB_ amoA , nirS , nosZ , hzo ) limit ammonia reduction. • The dynamic removal rate model improves ammonia prediction in ecological restoration. • Combining aeration with submerged macrophytes boosts ammonia removal efficiency.