Optimizing bioretention nitrogen control: woodchip mulching as vadose zone-focused amendment for pyrite systems.

Pyrite-based bioretention systems exhibit reduced pollutant removal efficiency during heavy rainfall due to insufficient electron donation. While woodchip mulching (WM) is known to supply organic matter, the specific mechanisms remain unclear. This study examined the role of WM in pyrite-based system over 13 months. Results show that WM primarily enhanced vadose nitrogen transformation, increasing NH4+-N removal by 24.19 % and raising the potential nitrification rate by 9.50 ng N·g-1·h-1. By improving cation exchange capacity and root activity, WM facilitated NH4+-N adsorption and plant uptake.A 1.97 % increase inNitrospiraabundance indicating synergistic plant-microbe effects enhancing nitrogen removal. WM alleviated submerged NH4+-N accumulation from dissimilatory nitrate reduction to ammonium by promoting nitrification. However, Long-term WM addition favoured heterotrophic over autotrophic denitrifiers, and carbon limitation ultimately constrained submerged denitrification. WM effectively enhances vadose nitrogen conversion, but strategies such as optimized electron donation in the submerged zone (SZ) are recommended.