Bioremediation of nitrate-contaminated shallow soils and waters via water table management techniques: evolution and release of nitrous oxide

Nitrate (NO3-N) commonly accumulates in soils because of fertilizer additio ns or when crop demand is much less than the rate of NO3-N production. Wate r table management (WTM) has been proposed to stimulate denitrifying bacter ia, thus removing the accumulated NO3-N by converting it to N2O (a greenhou se gas) and N-2. We studied the emission of N2O and N-2 as affected by wate r table depth. Undisturbed soil columns (30 cm dia by 90 cm long) from thre e soil series (Blount, somewhat poorly drained Aeric Ochraqualf; Clermont, poorly drained Typic Glossaqualf; and Huntington, well drained Fluventic Ha pludoll) were treated with 2.11 g N (as KNO3) applied as a band 10 cm below the surface. Two different WTM schemes were studied: static (WTM1) and dyn amic (WTM2). We repeated WTM2 using N-15 and this treatment, applied to the Huntington soil only, was designated WTM3. In general, N2O concentrations in a soil column responded to fluctuations in water table depth. Concentrat ions of N2O were usually higher in soils immediately below, as compared to above, the water table. The Clermont columns departed from this general tre nd. Maintaining the water table at 50 cm depth resulted in N2O emission rat es (1.8-44 mg N2O-N m(-2) d(-1)) comparable to those reported for cultivate d fields. A water table only 10 cm below the surface caused N2O emission ra tes to increase considerably (60-560 mg N2O-N m(-2) d(-1)). Four days after imposition of a water table 10 cm below the soil surface, N2O comprised 95 % of the N gas emitted (i.e. N2O mole fraction was 0.95). One week later, h owever, the N2O mole fraction was 0.35 which was significantly (P less than or equal to 0.05) lower than the mole fraction (0.68) measured prior to ra ising the water table. These results suggest that when using WTM practices, the best option to maintain high NO3-N removal rates and to reduce the pro portion of N2O in the emitted gases is to maintain a high water table for a prolonged period in the most biologically-active portion of the soil profi le. (C) 2000 Elsevier Science Ltd. All rights reserved.