Migration and transformation of nitrogen in bioretention system during rainfall runoff

Bioretention systems have been extensively studied as a highly efficient technical measure to tackle the global threat of nitrogen pollution during global rainfall runoff. However, the migration and transformation of various forms nitrogen in bioretention system is unclear. So, in this paper, the bioretention systems with different flow regimes and planted configurations were designed to study the nitrogen removal performance and migration and transformation mechanism. The dynamic changes of NH₄⁺-N and NO₃^--N were continuously monitored within 60 h after rainfall, and the abundance of ¹⁵N isotopes in soil layer NH₄⁺-N was simultaneously measured. The results indicated that NH₄⁺-N was mainly intercepted in soil layer in four constructed bioretention systems with similar removal efficiencies (95.42-97.69%). However, NO₃^--N was retained in submerged layer with significant different removal efficiencies (43.03-83.00%). After fitting calculation, the nitrification rate of NH₄⁺-N (0.0626 mg kg^-1 h^-1) in soil was 5.31 times higher than that of the accumulation rate of NO₃^--N (0.0118 mg kg^-1 h^-1). During the elimination process of residual NH₄⁺-N in soil, 41.46% removed by denitrification and plant absorption assimilation, another 57.28% stored in the form of organic nitrogen or inorganic nitrogen, only 1.26% leaked out. Based on this, the content variation of TN, NH₄⁺-N and NO₃^--N could be analyzed by a system-wide and established the nitrogen balance model, which provides a new insight into the enhancement of nitrogen removal in the bioretention system.