Insights into seasonal nitrogen dynamics and microbial responses in dual-mode stormwater-lake water biofiltration

Dual-mode biofiltration offers a sustainable solution for stormwater treatment in the face of climate change. To assess adaptability of dual-mode inflow, nitrogen dynamics, and microbial responses, six replicated dual-mode biofiltration columns were tested over a ten-month period under alternating stormwater and lake water inflows, with (Canna indica, Iris tectorum) and without vegetation, in Nanjing, China, a region characterized by a humid subtropical climate (Köppen Cfa). Sequential nitrogen fractionation revealed carbonate-bound nitrogen (CN) (12.8-55.1 %) and total ion-exchangeable nitrogen (IEN-TN) (18.7-43.7 %) as dominant forms across all treatments and seasons, indicating that high nitrogen mobility is a key feature of dual-mode operation. All biofiltration systems exhibited significantly enhanced retention capacities for the majority of nitrogen forms in August, driven by elevated temperatures and intermittent nitrogen inputs that intensified microbial community activity while accelerating mineralization processes. Microbial profiling identified Proteobacteria (32.64-55.88 %), Firmicutes (1.35-21.34 %), and Actinobacteria (4.65-18.58 %) as dominant phyla, with nitrogen-transforming genera (i.e., Azoarcus, Azospira, Dechloromonas, Vogesella and Brevundimonas) emerging as key biomarkers responsive to seasonal and inflow variations. Redundancy analysis (RDA) underscored IEN-NH₄⁺-N and nitrate reductase activity as main factors in shaping microbial community structure. PICRUSt and KEGG pathway analysis further identified seasonally distinct metabolic strategies corresponding to temperature-nutrient regimes: while diversified stress-response pathways dominated in the colder, oligotrophic January conditions, warmer months (May and August) with elevated temperatures and nutrient availability exhibited enhanced biosynthesis (amino acids, fatty acids, carbohydrates), particularly through branched-chain aminotransferase (BCAT, EC 2.6.1.42) activity mediating Valine, Leucine and Isoleucine metabolism under inflow fluctuations. These findings demonstrate the potential of dual-mode biofiltration as a robust, self-regulating system that enhances stormwater-lake water management through stable nitrogen cycling and plant-microbe interactions.