Nitrogen Transformations and Microbial Characterization of Soils from Passive Nitrogen Removing Biofilters

To investigate spatial variability, regulation, and mass balances of N transformations in nitrogen-removing biofilters (NRBs), N2-N production and NO3-N consumption were measured in suboxic incubations of nitrified percolate combined with sand and woodchip samples collected at different depths from an excavated NRB. Potential N2-N production averaged 0.34 and 0.54 μg g-1 h-1 in slurries amended with 18 and 27 NO3-N mg L-1 and accounted for 82% (range 70%-100%) of NO3-N consumption in incubations. Production occurred despite suboxic (1-3 mg L-1) conditions in slurries and varied across sample depth intervals with the distribution of nirK genes. To identify the fate of residual NO3-, N2O was measured in two additional incubations (N2-N production: 0.33 and 0.57 μg g-1 h-1) but no net N2O gain was found. Bioassimilation may account for N mass balance deficits. Anoxic incubations of sand, methanol, and nitrified percolate (NO3- 18 mg L-1) produced 3.5 times greater N2-N production relative to incubations without methanol and suggested production rates were C limited. This study provides evidence that complete denitrification is the dominant pathway for N transformations in NRBs and can be enhanced by labile carbon.