Submitted to: Interagency Conference on Research in the Watersheds
Publication Type: Proceedings
Publication Acceptance Date: October 30, 2003
Publication Date: October 30, 2003
Citation: TOMER, M.D., BURKART, M.R. LONG-TERM EFFECTS OF N FERTILIZER ON GROUNDWATER IN TWO SMALL WATERSHEDS. INTERAGENCY CONFERENCE ON RESEARCH IN THE WATERSHEDS. 2003. CD-ROM. TUCSON, AZ. Technical Abstract: Changes in agricultural management can minimize leaching of NO3-N. But then how much time does it take to improve groundwater quality? This study was conducted in two small, first-order watersheds (30 and 34 ha) in 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. This study's objective was to determine if NO3-N from these large applications persisted in groundwater. In 1996, transects of piezometer nests were installed, deep cores were collected, then water levels and NO3-N concentrations were measured each month. In 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, by 8 mg L-1. 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 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 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.