Ridge, Moldboard, Chisel, and No-Till Effects on Tile Water Quality beneath Two Cropping Systems

Soil conservation tillage systems, including ridge-tillage, often reduce surface water contamination by pesticides because soil erosion and surface runoff are reduced. However, the effects on losses through subsurface drainage tile are somewhat uncertain. Our field study quantified the effects of four tillage practices in continuous corn (Zea mays L.) and corn-soybean [Glycine max (L.) Merr] rotations on herbicide and nitrate N losses in tile drainage water. Fertilizer and pesticide application methods were uniform for ridge, moldboard, chisel, and no-till systems. Pesticide and nitrate N leaching losses were significantly affected by crop rotation. Tillage practice had little influence on nitrate N and pesticide losses to the subsurface drainage water within a corn-soybean rotation. However, ridge-till and no-till resulted in larger losses of atrazine than the moldboard plow and chisel based systems under continuous corn. Tillage system did not affect the timings of peak tile flow occurrences, although peak tile flow volume was affected by tillage, presumably because each system bad its own macropore system related to preservation or annual destruction of biopores by tillage. Corn yields were significantly higher under corn-soybean rotation than with continuous-corn for all tillage practices. These results indicate that continuous corn production is not an environmentally sustainable practice for this area because it resulted in higher nitrate N leaching losses to groundwater, received higher N-applications, and resulted in lower corn yields than the corn-soybean rotation. The results also reinforce the need for studies on chemical placement, rate, and timing for various tillage practices to reduce tile drainage losses of agricultural chemicals. ProblemPesticide and nitrate losses into groundwater aquifers from some soil and crop management practices are decreasing the water quality. Literature SummaryAgricultural chemicals applied on or near the soil surface can be rapidly transported to deeper soil depths and into shallow groundwater. One process through which this occurs is movement through tile drainage water. The type of tillage and cropping system used can influence the amount of these materials that can be lost from agricultural fields. However, the effects of these practices, especially at the field scale are not well understood. A better understanding of soil management practices is therefore needed to protect surface and groundwater quality. Chemical leaching losses can be minimized by using practices like banding of herbicides and multiple N applications to decrease herbicide and nitrate concentrations in the soil water, but it is difficult to control the volume of tile drainage water. One exception is that ridge-till and no-till practices can be used to increase the amount of crop residue left on the soil surface. This influences rainfall partitioning between surface runoff and subsurface drainage, which can increase agrichemical movement with subsurface drainage as surface runoff would be decreased. Ridge-till can increase crop yield on poorly drained soils by creating ridges that are warmer and drier than soils that are not tilled. Ridge-till can also have economic advantages because it combines tillage and herbicides to control weeds. Comparison between ridge-till and other conservation tillage practices is needed to determine the effects of these practices on drainage water quality. The main objective for this study was to quantify the effects of four tillage practices in continuous corn and corn-soybean rotations on herbicide and nitrate N loss to tile drainage water. Study DescriptionThis study was conducted at Iowa State University's Northeast Research Center near Nashua, on the Kenyon-Clyde-Floyd soil association. The site has 36 1-acre plots that have had a subsurface drainage system installed for more than 14 yr. Tile lines are spaced 95 ft apart at an approximate depth of 4 ft. Each plot has tile lies along the edge and one in the middle that has been intercepted and connected to an individual sump to measure the volume of water flow. Approximately 0.2% of the tile drainage water is collected every time water is pumped for nitrate N and herbicide analyses. Tillage treatments were fall moldboard plow, fall chisel plow, ridge-till and no-till. Continuous-corn and corn-soybean rotations were compared for all tillage Full scientific article from which this summary was written begins on page 227 of this issue. treatments. Crops were planted in 30-in. rows with a six row planter. Because the combine had a 90-in. wheel base, four of every six rows had a wheel track during the season. Continuous corn plots received 180 lb N/acre each year, while rotated corn received 150 lb N/acre as anhydrous ammonia injected between rows. Continuous-corn treatment received alachlor (2.0 lb a.i./acre) plus atrazine (2.5 lb a.i./acre) plus Counter for rootworm control. Corn in the corn-soybean rotation received alachlor (2.0 lb a.i./acre) plus cyanazine (2.5 lb a.i./acre) and no insecticide. Soybean plots received alachlor (2.0 lb a.i./acre) plus metribuzin (0.4 lb a.i./acre). All herbicides and insecticides were broadcast. Cultivation, even with the no-tillage treatment, was used to help with weed control. Drainage water samples were analyzed for alachlor, atrazine, cyanazine, metribuzin, and nitrate N. Applied QuestionWill adoption of ridge-tillage for continuous corn or corn-soybean rotations reduce nitrate N and pesticide loss to tile drainage water? The years 1988 and 1989 were extremely dry, and the rainfall was well below normal. The years 1990 and 1991 were unusually wet, with total rainfall amounts of more than 41 and 38 in., respectively. The year 1992 had a dry spring and wet fall with a total rainfall of 29.2 in. These rainfall patterns caused all tile lines to flow through most of the growing seasons of 1990, 1991, and 1992. Tile flow had similar relationships among tillage systems in 1991 and 1992, indicating that tillage systems did not affect the total tile flow volumes. Larger tile flows occurred with continuous-corn than with the corn-soybean rotation. On the average, no-till and ridge-till systems had the highest peak tile flows for most growing season storms regardless of crop. Higher peak tile flows under the ridge-till and no-till probably occurred because macropores (worm or root holes and natural fractures) are not destroyed or disturbed by primary tillage. With continuous corn, average subsurface drainage from no-till was significantly higher than from moldboard plow plots, but for the com-soybean rotation, tile flows were not statistically different for various tillages. A different macropore system appears to be operative under each crop rotation and tillage treatment. Losses of nitrate N were much greater under continuous corn than under corn-soybean rotation. In 1991, they ranged from 26 to 68 lb/acre, while in 1990, the highest loss of 96 lb/acre was about 50% of the annual amount applied. Less nitrate N was lost from the corn-soybean rotation than from continuous corn because N application rates were lower and less frequent and yields were higher. Atrazine losses were greater than for cyanazine or metribuzin. The rapid appearance of atrazine, alachlor, cyanazine, and metribuzin at high concentrations in subsurface drainage water shortly after rainfall suggests that preferential movement of these herbicides occurred in this silty soil overlaying loamy glacial till. Ridge-till and no-till practices, especially with continuous corn, appear to preserve a macropore network more than chisel or moldboard plowing. We conclude that, to reduce nitrate N and herbicide losses to drainage water, ridge-till and no-till practices should be used in combination with 2-yr corn-soybean rotation.