Harvesting reed (Phragmites australis) for wetland nitrogen removal: Productivity, microbial communities, and...

Non-point source nitrogen (N) pollution is a primary driver of aquatic eutrophication. While reed (Phragmites australis) wetlands effectively intercept N, the optimal harvesting strategy for maximizing N removal while maintaining ecosystem function remains unclear. This study investigated the effects of different harvesting frequencies on N removal, plant productivity, and associated microbial mechanisms in wetland microcosms over a three-year period. Four treatments were evaluated: unplanted control (CK), planted with no harvest (T0), annual harvest (T1), and biennial harvest (T2). Results demonstrated that all planted treatments significantly enhanced N removal compared to CK. Although not statistically significant among planted groups, T1 consistently achieved the highest average removal efficiencies for total nitrogen, ammonium-nitrogen, and nitrate-nitrogen. Furthermore, T1 produced the greatest aboveground biomass, facilitating the largest export of N and other nutrients. Metagenomic analysis revealed that reed planting shifted the microbial community, suppressing Cyanobacteria (e.g., Stanieria) and Nitrospirota (e.g., Nitrospira F), while enriching Proteobacteria and Chloroflexota. These compositional changes were coupled with a functional shift that key dissimilatory pathways (denitrification and dissimilatory nitrate reduction) were upregulated, while assimilatory nitrate reduction was suppressed. Additionally, annual harvesting fostered a more complex and stable microbial co-occurrence network. Structural equation modeling indicated that harvesting enhanced N removal primarily through plant-microbe interactions, with increased plant N accumulation promoting microbial N-functional gene abundance, and ultimately driving N removal. Overall, annual harvesting optimally coupled high biomass production with microbial N removal, presenting a sustainable management strategy for wetlands that balances water purification with resource recovery.