WATER-TABLE MANAGEMENT EFFECTS ON DENITRIFICATION AND NITROUS-OXIDE EVOLUTION

Previous research suggested that using controlled drainage to elevate the water table reduces NO3- contamination of surface water by enhancing denitrification. The C2H2 inhibition technique was used to study denitrification and N2O evolution using 56-cm-long, undisturbed cores of Cape Fear loam (clayey, mixed, thermic Typic Umbraquult) maintained in the field and subjected to three static water table levels (15, 30, and 45 cm). Results shelved that intermittent C2H2 exposure did not (i) affect soil inorganic N distribution between NH4+ and NO3-, (ii) diminish inhibition of N2O reduction during subsequent C2H2 exposure, or (iii) induce C2H2 decomposition. Denitrification from 1 Nov. 1993 through 21 Apr. 1993 (172 d) was 341 kg N ha(-1) for the 15-cm water table treatment, 260 kg N ha(-1) for the 30-cm water table treatment, and 86 kg N ha(-1) for the 45-cm water table treatment. Denitrification was maximum at the lowest monitored zone (36-54 cm) for each water table treatment. Total N2O evolution was 9 kg N ha(-1) for the 15-cm water table treatment, 4 kg N ha(-1) for the 30-cm water table treatment, and 2 kg N ha(-1) for the 45-cm water table treatment. Nitrous oxide evolution was positively correlated with mean soil temperature (10-cm depth) until low NO3- levels appeared to limit denitrification. Since steady-state diffusion was not reached, estimates of N-2 evolution, using N2O evolution in the presence of C2H2, were underestimated 12-fold. Evolved N2O-N represented only 2% of denitrification in the soil core (0-54 cm) for each water table treatment. Drainage control to elevate the water table enhanced denitrification and N2O evolution, reducing the potential for N transport with subsurface drainage to surface water.