Bioremediation of nitrate-contaminated shallow soils using water table management techniques: Nitrate removal efficiency

Nitrate (NO3--N), in excess of plant uptake during crop production, may mov e below the crop's rooting zone. In order to prefect groundwater quality, w ater table management (WTM) has been proposed as a way to remove, via stimu lation of denitrification, this excess NO3--N. We conducted a simulation ex periment for 130 days to evaluate the effectiveness of WTM for such purpose . In this experiment, undisturbed soil columns (90 cm long x 30 cm diameter ) from three soils were used. The soils were Blount (somewhat poorly draine d Aeric Ochraqualf), Clermont (poorly drained Typic Glossaqualf) and Huntin gton (well-drained Fluventic Hapludoll). A band (< 1 cm thick) of NO3--N (2 .11 g KNO3-N column(-1)) treated soil was created 10 cm below the soil surf ace and removal of that NO3--N was attempted using two different WTM scheme s: static (WTM1) and dynamic (WTM2). We also repeated WTM2 using N-15 to me asure N transformations and this treatment, applied to the Huntington soil only, was designated WTM3. Based on denitrification gases (N-2, N2O) emitte d, from 9 to 14% of the added NO3--N was removed from soil columns using WT M1 during the 130 day test period. With WTM2, 24 to 43% of the added NO3--N was removed. A comparable NO3--N removal estimate (40%) was obtained for t he N-15-treated (WTM3) Huntington columns using a mass balance approach. Re moval of NO3--N was faster when the water table was near the soil surface. Maximum rates of in situ NO3--N losses were generally 2 to 7 times lower th an soil denitrification potential determined in the laboratory. This experi ment shows that NO3--N removal from shallow soils and waters cart be achiev ed using WTM techniques. However; the need for long periods of flooding dur ing rimes when other climatic conditions (e.g., warm temperatures) are cond ucive to high rates of denitrification may limit successful field applicati on of these WTM techniques for NO3--N removal from cropland.