Potential for Bioremediation of High Nitrate Irrigation Water Via Denitrification

Abstract

AbstractNitrogen fertilizer application to irrigated corn (Zea mays L.) can result in N leaching from the crop rooting zone into groundwater. Microbial denitrification, with ethanol as an energy source, was evaluated for prevention of groundwater contamination under irrigated corn in Nebraska where irrigation water NO−3‐N levels can exceed 30 mg L−1. A field study was conducted on a Hord silt loam (Pachic Haplustoll) in central Nebraska by installing plastic (PVC) cylinders (28.7‐cm diam. by 1.8 m long) in soil to a depth of 1.2 m and irrigating with 17.1 cm of water containing 30 mg L−1 isotopically enriched (76.6 atom% 15N) KNO3‐N equivalent to 51.8 kg N ha−1. Soil remediation treatment included addition of 10 mL ethanol (644 kg C ha−1) to three of six cylinders. Gas samples taken every 6 h for 4 d from the soil surface and soil profile indicated that ethanol addition stimulated microbial respiration and denitrification. Cumulative loss of gaseous N from the soil surface over this period was 12.7 kg N ha−1 or 24.5% of the added 15N‐NO3; an additional 27 kg N ha−1 NO3 was lost from the ethanol‐amended soil profile. In the laboratory, intact soil cores taken at five intervals to a 132‐cm soil depth adjacent to field cylinders were brought to 90% WFPS (water‐filled pore space) by applying treatments of ethanol (19.8 kg C ha−1) and NO−3 (0.88 kg NO−3‐N ha−1) and incubated at 25 °C, with 100 mL L−1 C2H2, for 7 d during which time headspace gas samples were analyzed for N2O and CO2. Soil respiration and denitrification increased with ethanol addition except in surface soil where C availability was not limiting. We conclude that ethanol addition to high NO−3 irrigation water may remediate groundwater NO−3 contamination.