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

Proper N management for agricultural production is critical to minimize contamination with NO(3). For 18 yr, research was conducted to observe NO(3)-N concentrations in the vadose zone, groundwater, and subsurface 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 to 156 mg L(-1) under corn production within 1 yr after the initiation of irrigation (at the onset of the study), decreased to < 10 mg L(-1) during a 6-yr period of best-management N fertility management for corn. The average lysimeter NO(3)-N concentration fluctuated between 8 and 117 mg L(-1) during the study. Nitrate concentrations in shallow groundwater followed a similar time series trend as leachate concentrations, but with lower concentrations lagging about 1 yr. Subsurface drainage NO(3)-N concentrations were much lower but followed the same trend as shallow groundwater. An N balance indicated higher net N mineralization after the initiation of irrigation and the after potato production. Fertilizer N application rates and yearly weather conditions, which affected crop vigor N uptake, combined to affect the fall residual soil NO(3). Fall soil NO3 from 0- to 1.8-m depth was the most significant influencing the leachate NO(3)-N concentration each year during the study (r(2) = 0.76).