Effectiveness of bioretention system with saturated zone and different dry-wet alterations on nitrogen removal: performance and microbial community.

This study redefines bioretention systems (BRSs) by elucidating how saturated zone (SZ) depths (0‒480 mm) and antecedent dry days (ADDs: 0.5‒4 days) orchestrate microbial-driven nitrogen removal without organic carbon supplementation. Through lab-scale experiments with Pennisetum alopecuroides, we demonstrate that optimized hydrologic-microbial synergy shifts nitrogen elimination from passive filtration to a self-sustaining redox interface. Critically, coupling deeper SZs (> 320 mm) with moderate dry-wet cycles (> 3-day ADDs) activated autotrophic denitrification via Saccharibacteria and Bradyrhizobium, reducing NO-N accumulation to 7.59 ± 0.29 mg/L (31.7% removal) while achieving stable NH-N removal (> 61.4‒68.8%) across conditions. In contrast, shallow SZs (< 160 mm) disrupted microbial cooperation, favoring incomplete nitrification and nitrate leakage. Proteobacteria dominated functional guilds in optimal SZ scenarios (e.g., 480 mm SZ with 4 ADDs), where total inorganic nitrogen removal surpassed non-saturated systems by 25‒30%, proving carbon-independent pathways. The highest NH-N removal (68.8 ± 1.5%) occurred at 160 mm SZ with 2 ADDs, yet sustained efficiency required hydrologic thresholds that stabilized redox gradients. Microbial networks revealed deterministic assembly of nitrifier-denitrifier consortia under controlled hydrology, contrasting the stochasticity in suboptimal designs. These insights establish SZ-ADD-microbe codependencies as a design cornerstone, replacing ad hoc filtration metrics. By prioritizing microbial niche differentiation over soil adsorption, this work resolves the paradox of nitrate pollution in carbon-limited stormwater, offering a mechanistic blueprint for adaptive, carbon-neutral BRSs-a critical advance for eutrophication mitigation.