The effects of subsurface drainage depth and intensity on nitrate loads in the northern Cornbelt

Nitrate loads to surface waters from croplands in the Mississippi River basin are considered a leading cause of hypoxia in the Gulf of Mexico. Subsurface (tile) drainage systems exacerbate nitrate loads to surface waters by providing a pathway for the transport of soluble compounds from the crop rooting zone. This work investigated the role of subsurface drainage depth and drainage intensity (theoretical drainage rate) on nitrate loads to subsurface drains through field experimentation over a five-year period (2001-2005). Subsurface drainage plots were installed on nine subfield-sized plots ranging in size from 0.8 to 2.5 ha. The nine plots comprised two drainage depths (90 and 120 cm) and two drainage intensities: a "conventional" intensity of 13 mm d ^-1 design drainage rate and a "high" intensity of 51 mm d ^-1, obtained by using one-half the drain spacing of the conventional system. Subsurface drainage volume and nitrate loads in subsurface drainage outflows were monitored with automated equipment over the five-year period. Reductions in annual drainage volume and nitrate loads were observed with shallow and less intense drainage as compared to conventional depth and more intense drainage, respectively, in every year of the study. However, annual differences among treatments were not statistically significant due to variability among the nine plots. An analysis of aggregated data across the five study years of data showed statistically significant reductions in both drainage volume and nitrate loads of 20% and 18%, respectively, for drains placed 30 cm shallower than the 120 cm conventional depth drains. Reductions in nitrate loads were attributed primarily to reductions in annual drainage volume rather than reductions in nitrate concentrations. Subsurface drains placed at shallower depths, without reducing design drainage rates, may provide environmental benefits compared to conventionally designed, deeper drainage systems.