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

Abstract

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.