Nitrogen Fate in Drainage Ditches of the Coastal Plain after Dredging

Drainage ditches are a key conduit of nitrogen (N) from agricultural fields to surface water. The effect of ditch dredging, a common practice to improve drainage, on the fate of N in ditch effluent is not well understood. This study evaluated the effect of dredging on N transport in drainage ditches of the Delmarva Peninsula. Sediments from two ditches draining a single field were collected (0–5 cm) to represent conditions before and after dredging. Sediments were packed in 10‐m‐long recirculating flumes and subjected to a three‐phase experiment to assess the sediment's role as a sink or source of ammonium (NH 4 ) and nitrate (NO 3 ). Under conditions of low initial NH 4 –N and NO 3 –N concentrations in flume water, sediment from the undredged ditch released 113 times more NO 3 –N to water than did sediment from the dredged ditch. When flume water was spiked with NH 4 –N and NO 3 –N to simulate increases in N concentrations from drainage and runoff from adjacent fields, NO 3 –N in flume water increased during 48 h compared with the initial spiked concentration, while NH 4 –N decreased. These simultaneous changes were attributed to nitrification, with 23% more NO 3 –N observed in flume water with undredged ditch sediment compared with dredged ditch sediment. Replacing the N‐spiked water with deionized water resulted in two times more NO 3 –N released from the undredged ditch sediment than the dredged ditch sediment. These results suggest that ditch sediments could represent significant stores of N and that dredging could greatly affect the ditch sediment's ability to temporarily assimilate N input from field drainage.