Soil Nitrogen Speciation and the Relationship with Microbial Biomass Carbon in Bioretention Tanks

This outdoor research investigated the variations in soil ammonium (NH₃-N), nitrite (NO₂-N), nitrate (NO₃-N), total organic nitrogen (TON), and microbial biomass carbon (MBC) in bioretention tanks. Two biretention tanks (tank 1#: The depth of 0–20 cm was vacant aquifer layer; 20–90 cm, filled with the planting soil; 90–105 cm, filled with gravel. tank 3#: 0–20 cm was aquifer layer, 20–50 cm, filled with the planting soil; 50–90 cm, filled with blast furnace slag and sand; 90–105 cm, filled with gravel) were used with simulated rainwater discharge experiments to obtain soil samples at intervals of 1 h before the inflow and 24 h after the end of inflow. Results indicate that soil nitrogen (N) and MBC in two bioretention tanks were mainly captured at 10∼30 cm depth in soil; the content of soil NH₃-N exhibited a trend of initial decline but increase with time; the content of NO₂-N varied from 0.011 to 0.024 g/kg, and the change regularity was similar with the NH₃-N; different from the NH₃-N and NO₂-N, soil NO₃-N exhibited a trend of declining; while TON exhibited a trend of declining after slightly increase. Meanwhile, the content of NH₃-N and NO₃-N at 50 cm depth in tank 1# was slight lower than those at 10 and 30 cm; conversely, the discrepancy at the different depths in tank 3# was small. The contents of soil NH₃-N and NO₂-N before inflow were less than those after inflow, but it was adverse for NO₃-N. The NO₃-N leaching in bioretention system is a main reason for poor N removal in runoff. The content of MBC ranged from 1.055 to 1.847 g/kg and exhibited a trend of decline after increase. Furthermore, the content of MBC and TN has good linear correlation in bioretention tanks (R ² > 0.8), but it has general performance with TP (R ² > 0.5). The immobilization of NH₃-N, NO₂-N, and NO₃-N at the planting soil layer in tank 1# was greater than that in tank 3#. The N interception differences in the two tanks resulted from different infiltration rates of their underlying fillers.