Denitrification Within Saturated Riparian Buffers Re-Designed to Remove Nitrate From Artificial Subsurface Drainage.

Riparian buffers are a proven practice for removing NO₃ from both overland flow and shallow groundwater.  However, in tile drained landscapes, most of the subsurface flow leaving fields is passed through the buffers in drainage pipes leaving little opportunity for NO₃ removal.  We investigated the feasibility of re-routing a fraction of field tile drainage as subsurface flow through a riparian buffer for increasing NO₃ removal.  We intercepted an existing field tile outlet draining a 10 ha area of a row cropped field in central Iowa, USA and re-routed a fraction of the discharge as subsurface flow along 335 m of an existing riparian buffer.  Tile drainage from the field was infiltrated through a perforated pipe installed 75 cm below the surface by maintaining a constant head in the pipe at a control structure installed in-line with the existing field outlet.  During two years, over 18,000 m³ or 55% of the total flow from the tile outlet was redirected as infiltration within the riparian buffer.  The redirected water seeped through the 60-m wide buffer, raising the watertable approximately 35 cm.  The redirected tile flow contained 228 kg of NO₃.  Based on the strong decrease in NO₃ concentrations within the shallow groundwater across the buffer, we concluded that all of the NO₃ was removed within the buffer either by denitrification or sequestration into buffer vegetation and did not enter the stream.  Re-directing tile drainage as subsurface flow through a riparian buffer increased its NO₃ removal benefit.