Estimating nitrogen fates and gross transformations in bioretention systems with applications of ¹⁵N labeling methods

Two batches of ¹⁵NH₄⁺ and ¹⁵NO₃^- labeling experiments were conducted to understand the complex nitrogen (N) fates and transformations in bioretention systems, respectively. The fates of ¹⁵NH₄⁺ were first traced in six bioretention systems with different wet-dry regimes and submerged zone settings during four months, indicating: (1) ¹⁵N was mainly leached during the second storm events following the ¹⁵NH₄⁺ addition during the first storm events, suggesting nitrification during the dry period; (2) the main ¹⁵NH₄⁺ fates after four-month exposure were: soil media 59.6%-80.0%, outflow 5.3%-16.4%, plants 2.3%-8.9%, denitrification losses 0-28.4%; (3) longer antecedent dry weather period and submerged zone could help alleviate outflow NO₃^- leaching. The occurrence time, positions and rates of major N transformation processes were later examined by the ¹⁵NO₃^- labeling experiment in a bioretention system over an 8 d wet-dry cycle, indicating: (1) during the brief wet period, hydraulic mixing of "old" water and "new" inflow mainly occurred; (2) during the subsequent dry period, gross rates of nitrification, denitrification and mineralization showed "pulse effects", i.e. peaking at 24-48 h and decreasing significantly within 72 h; (3) denitrification became more dynamic with soil media depth, especially in submerged zone. This study evidenced the feasibility of ¹⁵N labelling method in studying N dynamics in bioretention systems and would inform future engineering and stormwater management practices.