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 additions or when crop demand is much less than the rate of NO3–N production. Water table management (WTM) has been proposed to stimulate denitrifying bacteria, thus removing the accumulated NO3–N by converting it to N2O (a greenhouse gas) and N2. We studied the emission of N2O and N2 as affected by water table depth. Undisturbed soil columns (30 cm dia by 90 cm long) from three soil series (Blount, somewhat poorly drained Aeric Ochraqualf; Clermont, poorly drained Typic Glossaqualf; and Huntington, well drained Fluventic Hapludoll) 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 dynamic (WTM2). We repeated WTM2 using 15N 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. Concentrations of N2O were usually higher in soils immediately below, as compared to above, the water table. The Clermont columns departed from this general trend. Maintaining the water table at 50 cm depth resulted in N2O emission rates (1.8–44 mg N2O–N m−2 d−1) comparable to those reported for cultivated fields. A water table only 10 cm below the surface caused N2O emission rates 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, however, the N2O mole fraction was 0.35 which was significantly (P≤0.05) lower than the mole fraction (0.68) measured prior to raising 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 proportion 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 profile.