DENITRIFICATION AS REVEALED BY ISOTOPE RATIOS IN A TILE-DRAINED, AGRICULTURAL WATERSHED IN CENTRAL ILLINOIS

A tile-drained agricultural watershed in central Illinois was used to study the mass balance of nitrogen by a multidisciplinary team. The present study uses nitrogen and oxygen isotope ratios to determine the extent of denitrification, which may be an important component of the overall mass balance. Of the twenty monitoring wells installed across the 38 sq. mi watershed in northwest Champaign County, 11 shallow wells monitored the water table at depths ranging from 10 to 12.5 ft and 9 other wells searched deeper sand-and-gravel layers at depths ranging from 25 to 44 ft. The depth of the tile drain ranged from 3 to 4 ft.

In general, nitrate present at low concentrations in most of the shallow wells, but was rarely detected in the deep wells. Water samples were collected periodically for nitrate isotopic analysis from both tile drains and from a few of the shallow monitoring wells throughout 2001. The d¹⁵N of the NO₃^- of tile drain water ranged from 1.5 to 11.4 ‰ and the d¹⁸O from 3.1 to 10.0 ‰. Nitrate isotopic ratios of most tile drain water reflected nitrification of the anhydrous ammonia fertilizer and soil nitrogen. However, some data clearly revealed the influence of denitrification in tile drain water, which demonstrated that the denitrification process removes certain proportion of the applied nitrogen from the shallow soil zone. The d¹⁵N of the NO₃^- in the shallow groundwater ranged from 3.9 to 17.5 ‰ and the d¹⁸O from 4.6 to 14.8 ‰ in 2001. The extent of denitrification varied among the shallow wells, which may be due to the natural heterogeneity of the watershed’s geology. Overall, shallow groundwater showed a greater degree of denitrification than the tile drain water. All the isotopic data collected to date indicate that much of the nitrate that penetrates into the shallow groundwater is lost to denitrification, and this could be a significant sink in the nitrate budget of this watershed and others like it. Isotopic monitoring of the tile drains and shallow groundwater will continue until Spring 2002.