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Nutrients National Synthesis Project

Estimation of Nitrate Contamination of an Agro-Ecosystem Outwash Aquifer Using a Nitrogen Mass-Balance Budget

By Larry J. Puckett, Timothy K. Cowdery, David L. Lorenz, and Jeffrey D. Stoner

Abstract
A mass-balance budget of N cycling was developed for an intensive agricultural area in west-central Minnesota to better understand NO₃^- contamination of ground water in the Otter Tail outwash aquifer. Fertilizer, biological fixation, atmospheric deposition, and animal feed were the N sources, and crop harvests, animal product exports, volatilization from fertilizer and manure, and denitrification were the N sinks in the model. Excess N, calculated as the difference between the sources and sinks, was assumed to leach to ground water as NO₃^-. The budget was developed using ground water data collected throughout the 212-km² study area. Denitrification was estimated by adjusting its value so the predicted and measured concentrations of NO₃^- in ground water agreed. Although biological fixation was the largest single N source, most was removed when crops were harvested, indicating that inorganic fertilizer was the primary source of N reaching the water table. It was estimated that denitrification removed almost half of the excess NO₃^- that leached below the root zone. Even after accounting for denitrification losses, however, it was concluded that the ground water system was receiving approximately three times as much N as would be expected under background conditions.

Table of Contents
Study area
Methods
  Study design
  Land use data
  Sample collection and analysis
  Mass-balance budget
Results and discussion
  Ground water chemistry
  Land use
  Nitrogen source and sink coefficients
  Mass-balance budget
  Estimated nitrate-nitrogen concentrations
  The nitrogen cycle
Conclusions
Acknowledgments
References

• Report (0.1 mb PDF)