NITROGEN-BALANCE IN AND EXPORT FROM AN AGRICULTURAL WATERSHED

Surface water nitrate (NO3-) pollution from agricultural production is well established, although few studies have linked field N budgets, N O3- loss in tile drained watersheds, and surface water NO3- loads, Thi s study was conducted to determine field sources, transport, and river export of NO3- from an agricultural watershed. The Embarrass River wa tershed at Camargo (48 173 ha) in east-central Illinois was investigat ed. The watershed is a tile-drained area of fertile Mollisols (typical soil is Drummer silty clay loam, a fine-silty, mixed mesic Typic Hapl aquoll) With primary cropping of maize (Zea mays L.) and soybean (Glyc ine max L.). Agricultural field N sources and sinks, tile drainage NO3 - concentrations and fluxes, and river NO3- export were estimated for the entire watershed, Large pools of inorganic N acre present followin g 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 betw een N fertilizer and soil mineralized N. High concentrations of NO3- w ere found in four monitored drainage tiles (5-49 mg N L-1), and tile c oncentrations of NO3- were synchronous with Embarras River NO3- concen trations. High flow events contributed most of the yearly NO3- loss (2 4.7 kg N ha(-1) yr(-1)) from tile drained fields in the 1995 water yea r (1 Oct, 1994 through 30 Sept. 1995) where high rainfall events occur red in a low overall precipitation year (in one tile 21% of the annual load was exported in 1 d). During the 1996 water year, NO3- export in tiles was much higher (44.2 kg N ha(-1) yr(-1)) due to greater precip itation, 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 inorga nic N pool was estimated to be leached though drain tiles and seepage and was exported by the Embarras River although depending on weather a nd field N balances this ranged how 25 to 85% of tile field N balance over the 6-yr period. It seems likely that agricultural disturbance (h igh mineralization inputs of N) and N fertilization combined with tile drainage contributed significantly to NO3- export in the Embarras Riv er.