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ORIGINAL RESEARCH

Long-Term Observations of Vadose Zone and Groundwater Nitrate Concentrations under Irrigated Agriculture

^a Dep. of Soil Science, North Dakota State Univ., Fargo, ND 58105-5638
^b USDA-CSREES, Washington, DC 20250
^* Corresponding author (nathan.derby{at}ndsu.edu).

Received 1 October 2007.

Proper N management for agricultural production is critical to minimize groundwater contamination with NO₃. For 18 yr, research was conducted to observe NO₃–N concentrations in the vadose zone, groundwater, and subsurface drainage under sprinkler-irrigated, primarily corn (Zea mays L.) production. Potato (Solanum tuberosum L.) and soybean [Glycine max (L.) Merr.] were also grown intermittently on the site. The lysimeter leachate NO₃–N concentration increased to 156 mg L^–1 under corn production within 1 yr after the initiation of irrigation (at the onset of the study), then decreased to <10 mg L^–1 during a 6-yr period of best-management N fertility management for corn. The average yearly lysimeter NO₃–N concentration fluctuated between 8 and 117 mg L^–1 during the study. Nitrate concentrations in the shallow groundwater followed a similar time series trend as leachate concentrations, but with lower concentrations and lagging about 1 yr. Subsurface drainage NO₃–N concentrations were much lower but followed the same trend as the shallow groundwater. An N balance indicated higher net N mineralization after the initiation of irrigation and the years after potato production. Fertilizer N application rates and yearly weather conditions, which affected crop vigor and N uptake, combined to affect the fall residual soil NO₃. Fall soil NO₃ from 0- to 1.8-m depth was the most significant factor influencing the leachate NO₃–N concentration each year during the study (r² = 0.76).

Abbreviations: GDD, growing degree days • NBAL, nitrogen balance