SHALLOW SUBSURFACE DRAINS TO REDUCE NITRATE LOSSES FROM DRAINED AGRICULTURAL LANDS

Abstract

Nitrate losses from subsurface drainage systems remain an important environmental concern. Data were collected from two drainage systems near Plymouth, North Carolina, to evaluate the effect of subsurface drain depth on nitrate-nitrogen (NO3 --N) losses. 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. Both plots received swine wastewater applications. Overall, the shallow drainage system had 42% less outflow than the deeper drainage system. Lower NO3 --N concentrations were observed in the shallow groundwater beneath the shallow drainage plots as a result of higher water tables and likely increased denitrification. However, NO3 --N concentrations in the drainage water from the shallow drains were not reduced. On average, NO3 --N export from the shallow subsurface drains was 8 kg ha-1 in 2001 and 27 kg ha-1 in 2002. Nitrate export from the deeper drains was 6 kg ha-1 in 2001 and 37 kg ha-1 in 2002. 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. Longer-term field studies, which incorporate variable climatological events, are needed to conclude whether shallower drain depth will reduce NO3 --N export from subsurface drainage systems.