The Effect of Controlled Drainage on Crop Yields and Nitrate Nitrogen Losses on Drained Lands in Eastern North Carolina

Research studies on a wide range of soils, crops, locations, and climates have shown that Controlled Drainage (CD) has the potential to substantially reduce the loss of nitrogen (N), from drained agricultural lands to surface waters. Controlled drainage is an integral part of Drainage Water Management (DWM), of which the adoption and widespread application depends on its impact on crop yields and overall performance on reducing nitrogen loading. This dissertation presents results from a long term field study on the effect of CD on crop yields and nitrate-N losses in a 3 crops in 2 years corn/wheat-soybean rotation.;Crop yields were measured on replicated field scale plots under CD and conventional or Free Drainage (FD) treatments for a total of 18 crops on two experimental sites during the period 1990 to 2011. Data were collected on 7 corn crops, 5 wheat crops and 6 soybean crops. Controlled drainage had no significant effect on yields of winter wheat, which in North Carolina is grown in the wettest, coolest part of the year. Controlled drainage significantly increased corn yields compared to FD in all 7 years. The average yield increase for corn was 11% with a range between 4 and 23%. Controlled drainage also significantly increased soybean yield in all years with an average increase of 10% compared to FD with a range between 2 and 20%. The data shows that small amounts of drainage water conserved by CD during critical periods of the growing season can have a substantial impact on crop yield.;Nitrate N losses were measured on replicated field scale plots under CD and FD treatments on a tile drainage system at the Tidewater Research station during 9 years between 1992 and 2012. Annual nitrate-N export was reduced by an average of 6.3 kg/ha or 30% compared with FD. Annual reductions ranged from 0.5 to 14.1 kg/ha. The observed reductions were a function of significant increases in NO3-N concentrations and significant decreases in drainage volumes with CD. Controlled drainage reduced annual drainage volumes by 33% on average. The reduction in drainage volume and increases in nitrogen removed by the crop under CD were the primary factors responsible for reducing NO3-N export. The reduction in the loss of NO3-N with CD is about the same as the increase in N taken up and harvested by the crop. The successful reduction in NO3-N is directly linked to continuous management and control settings. The results indicate that CD is an effective method for reducing nutrient loadings while increasing corn and soybean yields.