DENITRIFICATION ENZYME ACTIVITY IN A MARSH-POND- MARSH WETLAND USED FOR SWINE WASTEWATER TREATMENT AS INFLUENCED BY ALTERNATE WETTING AND DRYING CYCLES

Constructed wetlands with continuous vegetative cover (marsh) have been documented to be very effective for denitrification of nitrogen in swine wastewater. Generally, the limiting factor for denitrifica-tion in such wetlands is slow formation of nitrate via nitrification because anaerobic soil conditions are pre-valent and ammonia is the major nitrogen component in swine wastewater. It has been postulated that the shallow pond section of constructed wetlands with marsh-pond-marsh (M-P-M) designs would promote nitrate formation and subsequently denitrification. However, experimental results have shown the marsh-pond-marsh wetlands to be less effective for removal of nitrogen. Thus, we investigated enhancing soil aeration and the associated nitrification by short interruptions of wastewater application. The procedure was a one week drying cycle followed by two, three, or four weeks of wastewater applications vs. conti-nual application. The study was conducted in 2002 at North Carolina A&T State University on marsh-pond-marsh wetlands with flat bottoms and cattails vegetation in the marsh sections. We measured soil redox, nitrogen treatment efficiency, and denitrification enzyme activity. Soil redox condition was more oxidized in the 2:1 cycle, but the conditions were not sufficient to promote higher treatment efficiency. However, there were highly significant increases in DEA values from addition of nitrate. Furthermore, when nitrate was added, there was a good linear correlation between percentage of time in the drying cycle and the level of denitrification. Effective use of a pond section in constructed wetlands will likely require altered design