Submitted to: Iowa Water Monitoring Conference
Publication Type: Other
Publication Acceptance Date: February 20, 2003
Publication Date: February 20, 2003
Citation: Tomer, M.D., Burkart, M.R. 2003. Discerning effects of crop-management history on groundwater nitrate concentrations. Iowa Water Monitoring Conference. p. 4.
Agricultural management can be modified to minimize leaching of NO3-N. But the time needed to observe improvements in groundwater quality after management change is uncertain. This study was conducted in two small, first-order watersheds (30 and 34 ha) in the Loess Hills of southwest Iowa. Both were kept in continuous corn from 1964 through 1995, but one received large fertilizer-N applications, averaging 446 kg ha-1 y-1, between 1969 and 1974. In 1996 new crop rotations were initiated that are hypothesized to differ in NO3-N leaching. This study's objective was to determine if NO3-N from the large historical applications persisted in groundwater, and make it difficult to identify effects of the new rotations. Transects of piezometer/lysimeter nests were installed, deep cores collected, and water levels and NO3-N concentrations were measured each month. In June 2001, 33 water samples were collected and analyzed for tritium and stable isotopes. The watershed that received large N applications had greater NO3-N concentrations in groundwater and stream baseflow. Groundwater time-of-travel estimates and tritium data support persistence of NO3-N from fertilizer applied 30 years ago. "Bomb-peak" precipitation (1963-1980) most influenced tritium activity in groundwater beneath toeslope positions, and deep groundwater was dominated by pre-1953 precipitation. Data from analysis of deep cores suggest NO3-N may take 30 years to percolate to groundwater below the watershed's divide. Stable isotope data suggest runoff/infiltration processes contribute greater recharge and mixing of groundwater below the toeslope. Therefore historical and current practices affect NO3-N concentrations in groundwater near the stream. It may take many years to quantify impacts of management systems implemented in 1996 by monitoring groundwater. In many areas, changes in agricultural practices may take decades to fully impact groundwater quality.