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TECHNICAL REPORTS

Landscape and Watershed Processes

Denitrification in the Shallow Ground Water of a Tile-Drained, Agricultural Watershed

^a Illinois State Geological Survey, 615 E. Peabody Dr., Champaign, IL 61820
^b Dep. of Geology, Univ. of Illinois at Urbana-Champaign, 1301 West Green St., Urbana, IL 61801
^c Illinois State Water Survey, 2204 Griffith Dr., Champaign, IL 61820

^* Corresponding author (mehnert{at}isgs.uiuc.edu)

Received for publication March 8, 2006. Nonpoint-source pollution of surface water by N is considered a major cause of hypoxia. Because Corn Belt watersheds have been identified as major sources of N in the Mississippi River basin, the fate and transport of N from midwestern agricultural watersheds have received considerable interest. The fate and transport of N in the shallow ground water of these watersheds still needs additional research. Our purpose was to estimate denitrification in the shallow ground water of a tile-drained, Corn Belt watershed with fine-grained soils. Over a 3-yr period, N was monitored in the surface and ground water of an agricultural watershed in central Illinois. A significant amount of N was transported past the tile drains and into shallow ground water. The ground water nitrate was isotopically heavier than tile drain nitrate, which can be explained by denitrification in the subsurface. Denitrifying bacteria were found at depths to 10 m throughout the watershed. Laboratory and push-pull tests showed that a significant fraction of nitrate could be denitrified rapidly. We estimated that the N denitrified in shallow ground water was equivalent to 0.3 to 6.4% of the applied N or 9 to 27% of N exported via surface water. These estimates varied by water year and peaked in a year of normal precipitation after 2 yr of below average precipitation. Three years of monitoring data indicate that shallow ground water in watersheds with fine-grained soils may be a significant N sink compared with N exported via surface water.

Abbreviations: DEA, denitrification enzyme activity • MPN, most probable number • org-N, organic N • TKN, total Kjeldahl nitrogen • TOC, total organic carbon • DO, dissolved oxygen