Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/12/2003
Publication Date: 9/1/2003
Citation: TOMER, M.D., BURKART, M.R. LONG-TERM EFFECTS OF N-FERTILIZER USE ON GROUNDWATER NITRATE IN TWO SMALL WATERSHEDS. JOURNAL OF ENVIRONMENTAL QUALITY. 2003. V. 32. P. 2158-2171. Interpretive Summary: Changes in agricultural management can minimize leaching of NO3-N to groundwater. However the time needed to realize improvements in groundwater quality after management change is uncertain. In two small, first-order watersheds in southwest Iowa, we found large applications of fertilizer nitrogen that were applied in the early 1970s had a long-term effect on groundwater concentrations of nitrate that was still evident in 2001. Three independent lines of evidence supported this conclusion. The effects were most clear in groundwater beneath the upper landscape positions, while historical and current practices affected NO3-N concentrations in groundwater near the stream. Changes in agricultural practices may take decades to fully impact groundwater quality. These results are useful to producers, conservationists, EPA, and researchers working at the watershed scale, who should not always expect newly implemented best management practices to result in rapid improvements in groundwater quality.
Technical Abstract: Changes in agricultural management can minimize leaching of NO3-N to groundwater. However the time needed to realize 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. They were similarly managed from 1964 through 1995, except that one received 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. Transects of piezometer/lysimeter nests were installed, deep cores collected, and water levels and NO3-N concentrations were measured monthly. In June 2001, 33 water samples were collected and analyzed for **3H, stable isotopes, and major cations. The watershed that received large N applications had greater NO3-N concentrations in groundwater. Groundwater time-of-travel estimates and tritium data support persistence of NO3-N from the historical applications. "Bomb-peak" precipitation (1963-1980) influenced tritium concentrations, especially near toeslope positions, while 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 and inorganic solute data suggest that 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. Impacts of management systems implemented in 1996 will not be clearly discerned by monitoring groundwater for many years. In many areas, changes in agricultural practices may take decades to fully impact groundwater quality.