Practices to Reduce Nitrate-Nitrogen Losses from Drained Agricultural Lands

Abstract

Two practices were studied to reduce nitrate-nitrogen (NO3--N) losses from drained agricultural lands - shallow subsurface drainage systems and in-stream constructed wetlands.

Data was collected between January 2001-September 2002 from two drainage systems near Plymouth, NC. Drains in Plot 1 were 1.5 m deep and 25 m apart, and drains in Plot 2 were 0.75 m deep and 12.5 m apart. Overall, decreased drain depth reduced drainage outflows by 42%. On average, NO3--N export from the shallow subsurface drains was 8 kg/ha in 2001 and 27 kg/ha in 2002. Nitrate export from the deeper drains was 6 kg/ha in 2001 and 37 kg/ha in 2002. Overall, an average of 8 kg/ha less NO3--N was exported from the shallow subsurface drainage system. Decreased export observed in 2002 from the shallow subsurface drainage system was significant at the 10% level, but not for the entire 21-month period. The model DRAINMOD was calibrated with these field observations. Long-term simulations indicated that shallow drains would reduce drainage outflows by 23% at this site, and based on observed drainage water NO3--N concentrations in 2002, NO3--N export could be reduced by as much as 16%.

A wetland mesocosm experiment was conducted to determine if organic matter (OM) addition to soils used for constructed wetlands would increase NO3--N treatment. Eight batch studies, with initial NO3--N concentrations ranging from 10-120 mg/L, were conducted in 2001 and 2002 in 21 surface-flow wetland mesocosms. The results indicated that increasing the organic matter content of a Cape Fear Loam soil from 5% to 11% enhanced NO3--N wetland treatment efficiency in 7 of the 8 batch studies. Wetlands constructed with dredged material from Wilmington, N.C., with initial OM of 12%, showed improvement in NO3--N treatment efficiency when increased to 22 %.