Drainage Water Management to Mitigate Nitrogen Leaching from a Tile Drained Agricultural Landscape.

Nitrogen (N) losses through subsurface drainage systems can be an important transport pathway for the movement of nutrients to surface waters. A number of management and technical mechanisms exist for increasing N retention in arable fields. One of the more promising practices is the implementation of controlled drainage. The objectives of this research were to quantify N loss from two drainage water management strategies: conventional free-drainage (FD) and controlled drainage (CD) in Minnesota, USA. A field study was conducted from 2006-2015 on a drained Havelock loam soil. The field site consisted of two independently drained management zones, 15 and 22 ha, respectively.  The project used a time-series analysis approach to evaluate treatments.  Daily values were used in all statistical analysis for ammonium-N (NH₄-N), nitrate-N (NO₃-N), and total-N (TN). Based on the non-parametric Mann-Kendall test, showed significant increasing trend (7.01x10^-6< Sen Slope <0.0233) for load of nitrogenous compounds from FD. No trend was detected from CD for load of most nutrients; load if NH₄-N showed significant trends, but it was associated with very low Sen Slope values (<2.78x10^-17).  Concentration of nutrients from the FD management zone were also significantly increasing (1.91x10^-5< Sen Slope <7.49x10^-3).  For the CD management zone, concentrations of NH₄-N, and TN were significantly increasing; however, the corresponding Sen Slope values were too small (<3.55x10^-15) to affect their overall trend.  Daily concentrations of NO₃-N for CD had no trend. Based on this data set, and the non-parametric time-series analysis,nutrient load and concentration were higher for FD.