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 NO 3 For 18 yr, research was conducted to observe NO 3 –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 NO 3 –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 NO 3 –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 NO 3 –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 NO 3 Fall soil NO 3 from 0‐ to 1.8‐m depth was the most significant factor influencing the leachate NO 3 –N concentration each year during the study ( r 2 = 0.76).