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

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 ( r 2 = 0.76).