SEASONAL-CHANGES IN FLOW AND NITRATE-N LOSS FROM SUBSURFACE DRAINS

Subsurface drainage from thirty-six, 0.4-ha plots was monitored for th ree years (1990 to 1992) from chisel plow, moldboard plow ridge till, and no-till systems with continuous corn and corn-soybean rotations. D ata were analyzed in four seasonal stages to determine variations in d rain flows and nitrate-N contents in drain effluent. The hypothesis of this study was that differences among tillage systems would change du ring the monitoring season as rainfall patterns varied and as plots we re fertilized and cultivated. Forty-five to 85% of the annual nitrate- N loss through subsurface drainage occurred in the spring and fall whe n crops were not actively growing. These losses, however, were not sig nificantly different among tillage systems. Relative changes in drain flows and nitrate-N concentrations before and after summer cultivation were similar among the four tillage systems even though no-till and r idge till systems were undisturbed before this time. Nitrate-N losses or concentrations did not increase during the stage following fertiliz er application. No-till plots had significantly higher subsurface drai n flow than moldboard plow plots only under continuous cent, possibly an effect of reduced yields from long-term no-till continuous corn. Ni trate-N concentrations in drain effluent from moldboard and chisel plo w systems, however, were significantly greater than concentrations fro m no-till and ridge till systems for all crop rotations. Lower nitrate -N concentrations from no-till and ridge till systems may have resulte d from greater bypass flow, denitrification, and immobilization under nonplowed systems.