Tillage influences the physical and biological environment of soil. Rotation of crops with a legume affects thesoil N status. A furrow irrigated site was investigated for long-term tillage and crop rotation effects on leaching of nitratefrom the root zone and accumulation in the intermediate vadose zone (IVZ). The investigated tillage systems were diskplant(DP), ridge-till (RT) and slot-plant (SP). These tillage treatments have been maintained on the Hastings silt loam(Udic Argiustoll) and Crete silt loam (Pachic Argiustoll) soils since 1976. Continuous corn (CC) and corn soybean (CS)rotations were the subtreatments. Since 1984, soybeans have been grown in CS plots in even calendar years. All tillagetreatments received the same N rate. The N rate varied annually depending on the root zone residual N. Soybeans werenot fertilized with N-fertilizer. Samples for residual nitrate in the root zone were taken in 8 of the 15 year study while theIVZ 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 nitrate-N amounts were similar in RT and SP in all years. Despite high residual nitrate-N with DP and the same N applicationrate, crop yields were higher in RT and SP except when DP had an extremely high root zone nitrate level. By applying thesame N rates on all tillage treatments, DP may have been fertilized in excess of crop need. Higher residual nitrate-N inDP was most likely due to a combination of increased mineralization with tillage and lower yield compared to RT and SP.Because of higher nitrate availability with DP, the potential for nitrate leaching from the root zone was greater with DPas compared to the RT and SP tillage systems. Spring residual nitrate-N contents of DP were larger than RT and SP inboth 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 compared to RT and SP. At thedownstream end, it was 2.4 and 1.6 times greater with DP than RT and SP, respectively. Nitrate concentration was greaterin the IVZ of DP compared to RT and SP tillage systems. Nitrate accumulations in IVZ of RT and SP were not different.Continuous corn had slightly higher nitrate levels in IVZ than CS. The depth of nitrate penetration at the upstream endwas greater than that of the downstream end. Estimated rates of nitrate movement ranged from 0.87 to 0.92 m yr1 at theupstream end and 0.73 to 0.78 m yr 1 at the downstream end.
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Katupitiya et al. (1997) studied this question.