The effects of infiltration rates and carbon treatments on denitrifying bacteria populations and N-removal in soil columns intermittently flooded with secondary sewage effluent (SSE) were determined. At low infiltration rates of 10 to 15 cm/day, N-removal by denitrification was >80% and enrichment of the SSE with organic carbon did not further enhance N loss, even though the denitrifying bacteria populations were increased. However, at higher infiltration rates (15 to 40 cm/day), the physiologically active denitrifying bacteria populations were effective in net N-removal, and carbon enrichment of the SSE was essential for maintaining >80% N-removal. Continuous carbon treatments were more effective than 2-day pulse treatments for increasing the numbers of denitrifying bacteria and N-removal. With continuous carbon treatments of 200 mg/liter, >80% N-removal was obtained at infiltration rates of 40 cm/day. These results indicated that the activity of denitrifying bacteria and the N-removal capacity of high-rate land treatment systems could be enhanced by the timely additions of organic carbon to the SSE applied to the soil infiltration basins.; The effects of infiltration rates and carbon treatments on denitrifying bacteria populations and N-removal in soil columns intermittenly flooded with secondary sewage effluent (SSE) were determined. At low infiltration rates of 10 to 15 cm/day, N-removal by denitrification was 80% and enrichment of the SSE with organic carbon did not further enhance N loss, even though the denitrifying bacteria populations were increased. However, at higher infiltration rates (15 to 40 cm/day), the physiologically active denitrifying bacteria populations were effective in net N-removal, and carbon enrichment of the SSE was essential for maintaining 80% N-removal. Continuous carbon treatments were more effective than 2-day pulse treatments for increasing the numbers of denitrifying bacteria and N-removal. With continuous carbon treatments of 200 mg/liter, 80% N-removal was obtained at infiltration rates of 40 cm/day. These results indicated that the activity of denitrifying bacteria and the N-removal capacity of high-rate land treatment systems could be enhanced by the timely additions of organic carbon to the SSE applied to the soil infiltration basins.