MOVEMENT OF NITRATE FERTILIZER TO GLACIAL TILL AND RUNOFF FROM A CLAYPAN SOIL

Although water from 20 to 25% of shallow farmstead wells in northern M issouri has concentrations of nitrate (NO3-) exceeding 10 mg L(-1) as nitrogen (N), many potential sources for this NO3- are usually present . A field experiment was designed to trace and isolate the amount of a single application of N fertilizer lost to a glacial-till aquifer and runoff from a 400 m(2) corn (Zea mays L.) plot with bromide (Br-) and isotopically labeled (N-15) fertilizer. Soil at the plot is a Albaqui c Hapludalf of the Adco Series containing a 61 cm claypan beneath 41 t o 43 cm of topsoil. Groundwater levels ranged from 0.38 to 2.40 m belo w the land surface. Transport of water and NO3- to the saturated zone was not substantially retarded by the claypan. Labeled-N fertilizer ac counted for as much as 8.6 mg L(-1) of the NO3- (as N) in groundwater, but only in the top 1 to 2 m of the saturated zone. After two growing seasons (16 mo), <2% of the labeled-N fertilizer was lost to runoff, about 30% was in the saturated zone, 27.3% was removed with the grain, and about 5% remained in the unsaturated zone. A large part of the re maining labeled N may have been lost in gaseous N forms. The presence of labeled NO3- only in the top 2 m of the aquifer, slow horizontal tr ansport, and winter recharge indicate grass crops such as wheat (Triti cum aestivum L.) or rye (Secale cereals L.) might be used to extract n ear-surface N during the winter recharge period. Also, fall fertilizat ions can be expected to readily leach. Because groundwater concentrati ons of labeled NO3- were still increasing after two growing seasons, r otation of crops requiring small N inputs could be expected to limit t he cumulative effect of large annual fertilizer applications on ground water.