Crop rotation systems may reduce inorganic N fertilizer needs because of increased N mineralization, but if that N is mineralized either too early or too late with respect to crop needs, it can also be susceptible to leaching. These concerns prompted an investigation of a long-term field study at Mead, NE on a Sharpsburg silty clay loam (fine, montmorillinitic, mesic, Typic Argiudoll) to determine effects of crop rotation and N fertilizer application on residual inorganic N levels to a depth of 150 cm after 4 yr. The study included continuous corn (Zea mays L.), continuous soybean [Glycine max (L.) Merr.], continuous grain sorghum [Sorghum bicolor (L.) Moench.], corn/soybean and grain sorghum/soybean 2-yr rotations, and corn/oat [Avena sativa (L.)] + clover (80% yellow sweetclover [Melilotus officinalis (L.) Lam.] + 20% red clover [trifolium pratense])/grain sorghum/soybean and corn/soybean/grain sorghum/oat + clover 4-yr rotations with three rates of N fertilizer application. High N application resulted in greater residual NO3-N concentrations for the continuous corn and grain sorghum systems than in any of the other cropping systems to a depth of 150 cm. Residual NO3-N concentrations were low (<4 mg kg-1 at depths below 30 cm) at all N application rates in continuous soybean and 2- and 4-yr cropping systems. Nitrogen removal by grain accounted for 50% of the applied N in continuous corn and grain sorghum systems at the low N application rate, but only 20 to 30% of the applied N at the high rate. Likewise, only 20 to 30% of the applied N was accounted for by N removal in the rotation systems at either N application rate. Indirect results from our study suggest immobilization by crop residues and soil organic matter, not leaching, is probably most responsible for apparent N losses in these cropping systems. Although the mechanisms were not specifically identified, our results do show that crop rotations can reduce inorganic N fertilizer needs and at the same time reduce the amount of N available for leaching both of which are important to farmers in the western Corn Belt.
