POTENTIAL FOR BIOREMEDIATION OF HIGH NITRATE IRRIGATION WATER VIA DENITRIFICATION

Nitrogen fertilizer application to irrigated corn (Zea mays L.) can re sult in N leaching from the crop rooting zone into groundwater. Microb ial denitrification, with ethanol as an energy source, was evaluated f or prevention of groundwater contamination under irrigated corn in Neb raska where irrigation water NO3--N levels can exceed 30 mg L(-1). A f ield study was conducted on a Herd silt loam (Pachic Haplustoll) in ce ntral 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 o f water containing 30 mg L(-1) isotopically enriched (76.6 atom% N-15) KNO3-N equivalent to 51.8 kg N ha(-1). Soil remediation treatment inc luded addition of 10 mt ethanol (644 kg C ha(-1)) to three of six cyli nders. Gas samples taken every 6 h for 4 d from the soil surface and s oil profile indicated that ethanol addition stimulated microbial respi ration 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 N -15-NO3; an additional 27 kg N ha(-1) NO3 was lost from the ethanol-am ended soil profile. In the laboratory, intact soil cores taken at five intervals to a 132-cm soil depth adjacent to field cylinders were bro ught to 90% WFPS (water-filled pore space) by applying treatments of e thanol (19.8 kg C ha(-1)) and NO3- (0.88 kg NO3--N ha(-1)) and incubat ed at 25 degrees C, with 100 mt 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 a ddition to high NO3- irrigation water may remediate groundwater NO3- c ontamination.