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

Two batches of NH and NO labeling experiments were conducted to understand the complex nitrogen (N) fates and transformations in bioretention systems, respectively. The fates of NH were first traced in six bioretention systems with different wet-dry regimes and submerged zone settings during four months, indicating: (1) N was mainly leached during the second storm events following the NH addition during the first storm events, suggesting nitrification during the dry period; (2) the main NH 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 NO leaching. The occurrence time, positions and rates of major N transformation processes were later examined by the NO 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 N labelling method in studying N dynamics in bioretention systems and would inform future engineering and stormwater management practices.