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 enhanci ng denitrification. The C2H2 inhibition technique was used to study de nitrification and N2O evolution using 56-cm-long, undisturbed cores of Cape Fear loam (clayey, mixed, thermic Typic Umbraquult) maintained i n 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) dim inish inhibition of N2O reduction during subsequent C2H2 exposure, or (iii) induce C2H2 decomposition. Denitrification from 1 Nov. 1993 thro ugh 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 8 6 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 t able 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 evol ution was positively correlated with mean soil temperature (10-cm dept h) until low NO3- levels appeared to limit denitrification. Since stea dy-state diffusion was not reached, estimates of N-2 evolution, using N2O evolution in the presence of C2H2, were underestimated 12-fold. Ev olved N2O-N represented only 2% of denitrification in the soil core (0 -54 cm) for each water table treatment. Drainage control to elevate th e water table enhanced denitrification and N2O evolution, reducing the potential for N transport with subsurface drainage to surface water.