Nitrogen Fate in drainage ditches of the coastal plain after dredging
Received for publication June 14, 2008. 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 (NH4) and nitrate (NO3). Under conditions of low initial NH4–N and NO3–N concentrations in flume water, sediment from the undredged ditch released 113 times more NO3–N to water than did sediment from the dredged ditch. When flume water was spiked with NH4–N and NO3–N to simulate increases in N concentrations from drainage and runoff from adjacent fields, NO3–N in flume water increased during 48 h compared with the initial spiked concentration, while NH4–N decreased. These simultaneous changes were attributed to nitrification, with 23% more NO3–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 NO3–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.