LONG-TERM CONSERVATION TILLAGE AND LIQUID DAIRY MANURE EFFECTS ON CORN .2. NITRATE CONCENTRATION IN SOIL-WATER

Deterioration of ground and surface water quality has often been assoc iated with failure to properly account for nitrogen (N) from manure an d legumes in crop production. The objective of this study was to evalu ate the effects of tilage, N source, and frequency of liquid manure ap plication on NO3-N concentration of soil water under the root zone of corn (Zea mays L.). The experiment was conducted on a silt loam soil i n southeast Minnesota, USA. Tillage and N treatments were initiated in 1982 and remained constant during the study. Tillage systems were chi sel plowing plus secondary tillage with a field cultivator (CP) and no -tillage (NT). Nitrogen treatments were unfertilized control, inorgani c fertilizer applied annually at 235 kg ha(-1) for NT and 191 kg ha(-1 ) for CP, and manure application of 284 +/- 20 kg ha(-1) N (total N) a nnually and biennially (application every other year). Soil water for NO3-N analysis was sampled weekly from 1.5-m depth using suction sampl ers during the 1989 and 1990 growing seasons. For these two years, mea n soil water NO3-N concentrations were 66 mg l(-1) for annual inorgani c fertilizer, 50 mg l(-1) for annual manure and 11 mg l(-1) for bienni al manure treatments. Differences in NO3-N concentrations between annu al manure and annual inorganic fertilizer treatments, and between annu al manure and biennial manure treatments were statistically significan t (P=0.05). Mean NO3-N concentrations averaged over annual inorganic f ertilizer and annual manure treatments were 69 mg l(-1) for CP and 50 mg l(-1) for NT. With small supplemental fertilizer N, biennial manure application offers an alternative to annual application to minimize N leaching.