Seasonal adaptation and plant–microbe interactions governing nutrient removal in a long-term operated constructed wetland on the Loess Plateau

Constructed wetlands have been widely studied in humid regions, but their long-term treatment stability and plant–microbe interactions remain poorly understood in semi-arid seasonal rivers subject to strong temperature and hydrological fluctuations. This study investigated the Gaoping Two Rivers Constructed Wetland on the Loess Plateau using long-term monitoring data from 2019 to 2024 and intensive seasonal observations in 2025, focusing on four parallel vertical subsurface-flow (VSF) cells planted with reed, purple loosestrife, iris, and bulrush. Long-term monitoring showed that average downstream concentrations of NH₄ + –N, TP, and COD were 38.62%, 29.41%, and 31.20% lower than upstream concentrations, respectively. During May–October 2025, concentration-based removal efficiencies varied among vegetation units and months, reaching maxima of 87.35% for TN, 80.82% for NH₄ + –N, 78.95% for TP, and 61.51% for COD. Areal mass removal rates showed temporal patterns distinct from concentration-based efficiencies, indicating the influence of hydraulic loading and influent pollutant levels. Iris showed strong N and P removal performance, reed maintained relatively stable organic pollutant removal, and purple loosestrife supported high rhizosphere microbial richness. Plant nutrient assimilation increased during vegetation growth but represented only one component of nutrient removal. RDA and correlation analyses showed that microbial communities were associated with water chemistry and plant traits, with significant relationships among plant biomass, functional microbial groups, and nutrient removal. Treatment performance reflected the combined effects of vegetation, microbial processes, pollutant loading, and hydraulic conditions.