|Simpkins, W - IOWA STATE UNIVERSITY|
Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 29, 1998
Publication Date: N/A
Interpretive Summary: Contamination of rivers, lakes, and streams with nitrate-N has been linked to agriculture across the midwestern United States. Additionally, there is widespread public perception that application of excessive amounts of nitrogen fertilizer in the upper Midwest Corn Belt has contributed to a zone of low oxygen in the Gulf of Mexico. We conducted a four-year study to estimate the concentrations and amounts of nitrate-N in subsurface tile-drain water leaving a single field in central Iowa. Looking at a single field provides a great deal of information because subsurface tile drains in Iowa are the primary source of water entering the streams and rivers that ultimately flow into the Gulf of Mexico. We sampled the drainage water three times a week for four years and found that nitrate-N concentrations exceeded the public health standard critical value of 10 ppm for 12 of the 48 months. Most of the loss occurred when there were no crops growing in this field (November-May). We found no evidence to support the belief that N fertilizer application was directly controlling the loss of nitrate-N to the tile drains. Loss of nitrate-N through subsurface tile drains was controlled by the volume of water flowing into the drains, and the amount of water flowing into the drains was related to the amount and timing of rainstorms. This research suggests that simply reducing the amount of fertilizer N applied will not significantly lower nitrate-N levels in Midwest rivers and streams.
Technical Abstract: Nonpoint source contamination of surface and groundwater resources with nitrate-N has been linked to agriculture across the midwestern United States. A four-year study was conducted to assess the extent of nitrate-N leaching to subsurface drains and shallow groundwater in a central Iowa field. Nitrate-N concentrations in shallow groundwater exhibited temporal trends and were higher under Clarion soil than under Okoboji or Canisteo soil. Denitrification rates were two times higher in Okoboji surface soil than in Clarion surface soil and the highest denitrification potential among subsurface sediments was observed for deep unoxidized loess. Soil profile nitrate-N concentrations decreased with depth and were the same below 30 cm for fertilized corn and soybean. Nitrate-N concentrations in subsurface drainage water exceeded 10 mg L**-1 25% of the time and were between 6 and 9 mg L**-1 67% of the time during the 48 month study. The temporal pattern of nitrate-N concentrations in subsurface drainage water was not related to the timing of fertilizer N application or to the amount of fertilizer N applied. Total nitrate-N losses to subsurface drains were greatest in 1993 (51.3 kg ha**-1) and least in 1994 (4.9 kg ha**-1). The greatest relative amounts of nitrate-N were lost between November and May for all 4 years, when plant uptake of N was not significant. Our results indicate that nitrate-N losses to subsurface drainage water occur primarily as a result of asynchronous production and uptake of nitrate-N in the soil and the presence of large quantities of potentially mineralizable N in the soil organic matter.