LONG-TERM TILLAGE AND CROP-ROTATION EFFECTS ON RESIDUAL NITRATE IN THE CROP ROOT-ZONE AND NITRATE ACCUMULATION IN THE INTERMEDIATE VADOSE ZONE

Tillage influences the physical and biological environment of soil. Ro tation of crops with a legume affects the soil N status. A furrow irri gated site was investigated for long-term tillage and crop rotation ef fects on leaching of nitrate from the root zone and accumulation in th e intermediate vadose zone (IVZ). The investigated tillage systems wer e disk plant (DP), ridge-till (RT) and slot-plant (SP). These tillage treatments have been maintained on the Hastings silt loam (Udic Argius toll) and Crete silt loam (Pachic Argiustoll) soils since 1976. Contin uous corn (CC) and corn soybean (CS) rotations were the subtreatments. Since 1984, soybeans have been grown in CS plots in even calendar yea rs. All tillage treatments received the same N rate. The N rate varied annually depending on the root zone residual N. Soybeans were not fer tilized with N-fertilizer. Samples for residual nitrate in the root zo ne were taken in 8 of the 15 year study while the IVZ was only sampled at the end of the study. In seven of eight years, root zone residual soil nitrate-N levels were greater with DP than RT and SP. Residual ni trate-N amounts were similar in RT and SP in all years. Despite high r esidual nitrate-N with DP and the same N application rate, crop yields were higher in RT and SP except when DP had an extremely high root zo ne nitrate level. By applying the same N rates on all tillage treatmen ts, DP may have been fertilized in excess of crop need. Higher residua l nitrate-N in DP was most likely due to a combination of increased mi neralization with tillage and lower yield compared to RT and SP. Becau se of higher nitrate availability with DP, the potential for nitrate l eaching from the root zone was greater with DP as compared to the RT a nd SP tillage systems. Spring residual nitrate-N contents of DP were l arger than RT and SP in both crop rotations. Ridge till and SP systems had greater nitrate-N with CS than CC rotations. Nitrate accumulation in IVZ at the upstream end of the field was twice as high with DP com pared to RT and SP. At the downstream end, it was 2.4 and 1.6 times gr eater with DP than RT and ST: respectively. Nitrate concentration was greater in the IVZ of DP compared to RT and SP tillage systems. Nitrat e accumulations in IVZ of RT and SP were not different. Continuous cor n had slightly higher nitrate levels in NZ than CS. The depth of nitra te penetration at the upstream end was greater than that of the downst ream end. Estimated rates of nitrate movement ranged from 0.87 to 0.92 m yr(-1) at the upstream end and 0.73 to 0.78 m yr(-1) at the downstr eam end.