Submitted to: Journal of Environmental Quality
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/10/2009
Publication Date: 9/11/2009
Publication URL: www.agronomy.org/publications/jeq/view/38-6/q08-0268.pdf
Citation: Shigaki, F., Schmidt, J.P., Kleinman, P.J.A., Sharpley, A.N., Allen, A.L. 2009. Nitrogen fate in drainage ditches of the Coastal Plain after dredging. Journal of Environmental Quality. Available: https://www.agronomy.org/publications/jeq/view/38-6/q08-0268.pdf Interpretive Summary: Nitrogen is strongly tied to eutrophication of brackish waters and is a key control of eutrophication in the Chesapeake Bay. Minimizing the loss of N from agricultural lands in the Chesapeake Bay watershed has therefore been a major water quality priority. In the heavily managed areas of the Coastal Plain setting, drainage ditches are a key conduit of nutrients from agricultural fields to surface water. Ditches are routinely dredged to improve their hydrologic function; however, this management practice causes severe disturbance. Dredging resulted in a greater potential for N transport in drainage ditches in a short-term evaluation. In addition, nitrification was implicated as a main component of N processes in drainage ditches. Results highlight how management practices such as dredging can impact the water quality protection.
Technical Abstract: 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 and 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 and NO3 to simulate increases in N concentrations from drainage and runoff from adjacent fields, NO3-N concentration in flume water increased during 48 h compared to the initial spiked concentration, while NH4-N concentration decreased. These simultaneous changes were attributed to nitrification, with 23% more NO3-N observed in flume water with undredged-ditch sediment compared to dredged-ditch sediment. Replacing the N-spiked water with deionized water resulted in two times more NO3-N released from the sediment in the undredged- than dredged-ditch sediment. These results suggest that ditch sediments could represent significant stores of N, and dredging could greatly affect the ditch sediment’s ability to temporarily assimilate N input from field drainage.