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 Missouri has concentrations of nitrate (NO − 3 ) exceeding 10 mg L −1 as nitrogen (N), many potential sources for this NO − 3 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 runoffrom a 400 m 2 corn ( Zea mays L.) plot with bromide (Br − ) and isotopically labeled ( 15 N) fertilizer. Soil at the plot is a Albaquic Hapludalf of the Adco Series containing a 61 cm claypan beneath 41 to 43 cm of topsoil. Groundwater levels ranged from 0.38 to 2.40 m below the land surface. Transport of water and NO − 3 to the saturated zone was not substantially retarded by the claypan. Labeled‐N fertilizer accounted for as much as 8.6 mg L −1 of the NO − 3 (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 remaining labeled N may have been lost in gaseous N forms. The presence of labeled NO − 3 only in the top 2 m of the aquifer, slow horizontal transport, and winter recharge indicate grass crops such as wheat ( Triticum aestivum L.) or rye ( Secale cereale L.) might be used to extract near‐surface N during the winter recharge period. Also, fall fertilizations can be expected to readily leach. Because groundwater concentrations of labeled NO − 3 were still increasing after two growing seasons, rotation of crops requiring small N inputs could be expected to limit the cumulative effect of large annual fertilizer applications on groundwater.