Impact of Controlled Drainage on Reducing Nitrogen Loss in the Red River Valley.

It is well known that when control drainage (CD) is applied to a field, it can retain the drainage flow, hold more water in the field, and thus reduce nitrogen loss through denitrification process. A two year study was conducted in the Red River Valley of western Minnesota that revealed some controversial results about the CD effectiveness. Drainage outflow and nutrient loads from a subsurface drained field with no drainage control - free drainage (FD) and a CD field were measured and compared in 2013 and 2014. The drainage outflows in 2013 were 398 m3/ha for FD and 103 m3/ha for CD, while the drainage outflows in 2014 were 802 m3/ha for FD and 1002 m3/ha for CD. This resulted in 12.0 kg/ha and -2.3 kg/ha more nitrate-nitrogen loss from the FD than that from the CD in 2013 and 2014. The results in 2014 were very different from the 2013 results, while CD retained much more flow and nutrient in 2013, but not in 2014.  Essentially, more water volume and longer length of time were measured from the CD field than that from the FD field in 2014. After examining the data, we found that a large amount of drainage outflow occurred in fall 2014 when CD ended and conventional subsurface drainage started in the CD field. In this region, CD baffles have to be opened in the fall to allow drainage otherwise they will freeze shut. Overall, the CD practice resulted in a longer drainage duration for the entire season in the CD field, which were 75 and 98 days of drainage in the FD and CD fields in 2014, respectively. The effectiveness of controlled drainage on reducing flow and nitrogen loss was therefore significantly reduced by the rainfall amount in the fall.