Root and Vadose Zone Nitrate Accumulation As Affected by N Management Practices for Irrigated Corn.

In 1986, a study was initiated at the University of Nebraska South Central Agricultural Laboratory to investigate the impact of various management practices on nitrogen (N) use efficiency of irrigated corn. Historic annual treatments have included tillage (conventional chisel/disk or no-till); nitrogen rate (0, 75, 150, 300 kg N ha-1 as anhydrous ammonia); nitrapyrin (0 or 0.5 kg ha-1), and N application timing (preplant or sidedress). After 21 years of treatment, detailed sampling of the root and vadose zones was conducted to investigate the cumulative effects of treatment on soil properties. Root zone samples were collected to a depth of 0.9 m and analyzed for moisture, bulk density, organic matter, particulate organic matter, pH, Bray-1 phosphorus, potassium and zinc. Same location vadose zone samples were collected to a depth of 9.1 m and analyzed for moisture, bulk density and nitrate-N. Results showed that continuous corn production can result in substantial vadose zone nitrate loading if N application rates exceed crop demand. More intense tillage resulted in greater nitrate-N loading. The use of nitrapyrin had no collective impact on vadose zone nitrate load, though in some years there were increased yields and N removal with the use of nitrapyrin. There were patterns of treatment effects on nutrient removal and/or redistribution in the root zone involving organic matter, phosphorus, potassium, pH, bulk density and moisture. There were no treatment effects on root zone particulate organic matter or zinc concentration.