TILLAGE, CROP AND CLIMATIC EFFECTS ON RUNOFF AND TILE DRAINAGE LOSSES OF NITRATE AND 4 HERBICIDES

A four-year (1987-1990) field study in NW Ohio on a poorly drained, fine-textured soil (Mollic Ochraqualf) examined the effect of tillage (no-till versus fall moldboard plow) and crop type (maize, Zea mays, and soybean, Glycine max) on runoff and tile drainage losses of NO3-N and four commonly used herbicides (atrazine and alachlor on maize, metolachlor and metribuzin on soybeans). Weather was a significant variable with two years of drought (1987-1988) followed by very wet years (1989-1990). Runoff has previously averaged 17% of precipitation, but was 3% in 1987 and 1988 and 4.6% and 11% in 1988 and 1989, respectively. Tile drainage, which previously averaged 40% of precipitation, was virtually zero in 1987 and 1988 and was 28% and 31% of precipitation in 1989 and 1990, respectively. Tile flow losses of NO3-N (0.1-86 kg ha-1 year-1) were greater than those from runoff (0.01-2.3 kg ha-1 year-1), while herbicide loss was greatest in runoff and was only detected to any extent in tile drainage in the wettest year (1990). Flow weighted mean annual NO3-N concentrations ranged from 0.3 to 4.0 mg l-1 in runoff and from 0.1 to 17.8 mg l-1 in tile drainage. Ranges of losses in runoff/tile drainage for atrazine, alachlor, metolachlor, and metribuzin (g ha-1 year-1) were: 0.09-9.82/0.00-31.3; 0.01-2.91/0.00-0.14; 0.01-13.6/0.00-4.25 and 0.00-15.6/0.00-3.98, respectively. Corresponding concentration ranges (mug l-1) were: 0.3-52.3/0.1-59.2; 0.1-20.1/0.1-3.6; 0.3-164/0.1-84 and 0.1-91.9/0.1-40. There was significant carryover of nitrate in tile drainage from one year to the next, but of the herbicides monitored only atrazine exhibited any carryover behavior. Runoff and tile drainage flow weighted mean annual concentrations of the four herbicides were in the order: metolachlor> atrazine > metribuzin > alachlor, and this rank-order was correlated with half-life for these compounds but not with water solubility or soil organic matter-water partition coefficient. About half of the tile flow events in the study exceeded the US Maximum Concentration Limit (MCL) for drinking water NO3-N (10 mg l-1) regardless of crop type, while 20% and 50% of the runoff events exceeded the MCL for atrazine (3 mug l-1) for soybean and maize, respectively.