TILLAGE SYSTEM EFFECTS ON 15-YEAR CARBON-BASED AND SIMULATED N BUDGETS IN A TILE-DRAINED IOWA FIELD

Tillage influences N fate and transport by changing soil structure, ae ration, macropore continuity, plant-residue placement, and organic-mat ter mineralization rates. Our objective was to use 15-year N budgets t o compare four primary tillage treatments for continuous corn (Zea may s L.) production on tile-drained Aquic Hapludolls (FAO: Haplic Phaeoze ms) in northeastern Iowa, USA. A carbon-based N budget used annual gra in yield, grain-N concentrations measured in 1992, changes in surface- soil C content between 1977 and 1988 or 1992, surface-soil C : N ratio s, and measurements of NO3-N lost in tile-drainage water. It accounted for 98, 104, 99, and 99% of the fertilizer N applied to moldboard-, c hisel-, ridge-, and no-tillage-treatments, respectively. Averaged for 1977 through 1992, increased soil organic matter, harvested grain, and tile drainage accounted for approximate to 42, 45, and 13% of the N b udget, respectively. Simulated N budgets were computed using version 3 .25 of the root-zone water quality model (RZWQM). The best grain-yield predictions for 13 of the 15 years were 9% higher than the measured v alues, and if extreme outliers were eliminated, the predicted values w ere correlated (r(2) = 0.75) with the average measured yield for the f our tillage treatments. Simulations for 1988 and 1989 failed completel y because RZWQM could not accurately describe hydrology associated wit h low rainfall seasons. Predicted total N accumulation was much higher than measured in 1990, 1991, or 1992. Estimates of profile NO3-N, min eralization, seepage loss, and denitrification were not satisfactory, presumably because the model failed to simulate an accurate hydrology for the different tillage practices. We conclude that the simulation r esults were not suitable for predicting the fate of fertilizer N. Howe ver, both approaches for computing N budgets suggested that adopting r idge tillage, without changing N rates and other management practices related to N application technology and crop sequencing, will not redu ce the potential for off-site water quality degradation. (C) 1998 Else vier Science B.V. All rights reserved.