Fertilizer, tillage, and dairy manure contributions to nitrate and herbicide leaching

Few studies have examined the water quality impact of manure use in no-tillage systems. A lysimeter study in continuous corn (Zea mays L.) was performed on Maury silt loam (line, mixed, semiactive, mesic Typic Paleudalf) to evaluate the effect(s) of tillage (no-till [NT] and chisel-disk [CD]), nitrogen fertilizer rate (0 and 168 kg N ha^-1), and dairy manure application timing (none, spring, fall, or fall plus spring) on NO₃-N, atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine), and alachlor [2-chloro-2′-6′-diethyl-N-(methoxymethyl) acetanilide] concentrations in leachate collected at a 90-cm depth. Herbicides were highest immediately after application, declining to less than 4 μgL^-1 in about two months. Manure and manure timing by tillage interactions had little effect on leachate herbicides; rather, the data suggest that macropores rapidly transmitted atrazine and alachlor through the soil. Tillage usually did not significantly affect leachate NO₃-N, but no-tillage tended to cause higher NO₃-N. Manuring caused higher NO₃-N concentrations; spring manuring had more impact than fall, but fall manure contained about 78% of the N found in spring manure. Nitrate under spring "only fertilizer" treatment exceeded 10 mg L^-1 38% of the time, compared with 15% for spring only manure treatment. After three years, manured soil leachate NO₃-N exceeded that for soil receiving only N fertilizer. Soil profile (90 cm) NO₃-N after corn harvest exceeding 22 kg N ha^-1 was associated with winter leachate NO₃-N greater than 10 mg N L^-1. Manure can be used effectively in conservation tillage systems on this and similar soils. Accounting for all N inputs, including previous manure applications, will be important.