Nitrogen Balance in and Export from an Agricultural Watershed

Surface water nitrate (NO⁻₃) pollution from agricultural production is well established, although few studies have linked field N budgets, NO⁻₃ loss in tile drained watersheds, and surface water NO⁻₃ loads. This study was conducted to determine field sources, transport, and river export of NO⁻₃ from an agricultural watershed. The Embarras River watershed at Camargo (48 173 ha) in east-central Illinois was investigated. The watershed is a tile-drained area of fertile Mollisols (typical soil is Drummer silty clay loam, a fine-silty, mixed mesic Typic Haplaquoll) with primary cropping of maize (Zea mays L.) and soybean (Glycine max L.). Agricultural field N sources and sinks, tile drainage NO⁻₃ concentrations and fluxes, and river NO⁻₃ export were estimated for the entire watershed. Large pools of inorganic N were present following each harvest of maize and soybean (average of 3670 Mg N yr⁻¹ over a 6-yr period). The source of most of the inorganic N was divided between N fertilizer and soil mineralized N. High concentrations of NO⁻₃ were found in four monitored drainage tiles (5–49 mg N L⁻¹), and tile concentrations of NO⁻₃ were synchronous with Embarras River NO⁻₃ concentrations. High flow events contributed most of the yearly NO⁻₃ loss (24.7 kg N ha⁻¹ yr⁻¹) from tile drained fields in the 1995 water year (1 Oct. 1994 through 30 Sept. 1995) where high rainfall events occurred in a low overall precipitation year (in one tile 21% of the annual load was exported in 1 d). During the 1996 water year, NO⁻₃ export in tiles was much higher (44.2 kg N ha⁻¹ yr⁻¹) due to greater precipitation, and individual days were less important. On average, about 49% (average of 1688 Mg N yr⁻¹ over a 6-yr period) of the field inorganic N pool was estimated to be leached through drain tiles and seepage and was exported by the Embarras River, although depending on weather and field N balances this ranged from 25 to 85% of the field N balance over the 6-yr period. It seems likely that agricultural disturbance (high mineralization inputs of N) and N fertilization combined with tile drainage contributed significantly to NO⁻₃ export in the Embarras River.