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TECHNICAL REPORTS

Wetlands and Aquatic Processes

Nitrogen Fate in Drainage Ditches of the Coastal Plain after Dredging

^a Center for Agrarian and Environmental Sciences, Federal Univ. of Maranhao, Chapadinha, MA, 65500, Brazil
^b USDA-ARS, Pasture Systems and Watershed Management Research Unit, Curtin Rd., University Park, PA, 16802
^c Dep. of Crop Soil and Environmental Sciences, 115 Plant Sciences Building, Univ. of Arkansas, Fayetteville, AR 72701
^d Dep. of Agriculture, Univ. of Maryland Eastern Shore, Princess Anne, MD 21853. Mention of trade names does not imply endorsement by the U.S. Department of Agriculture

^* Corresponding (francirose{at}yahoo.com.br; francirose{at}pq.cpq.br).

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.

Abbreviations: CEC, cation exchange capacity • S_w, uptake length • V_f, mass transfer coefficient