Long-term conservation tillage and liquid dairy manure effects on corn. II. Nitrate concentration in soil water

Deterioration of ground and surface water quality has often been associated with failure to properly account for nitrogen (N) from manure and legumes in crop production. The objective of this study was to evaluate the effects of tillage, N source, and frequency of liquid manure application 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 in southeast Minnesota, USA. Tillage and N treatments were initiated in 1982 and remained constant during the study. Tillage systems were chisel plowing plus secondary tillage with a field cultivator (CP) and no-tillage (NT). Nitrogen treatments were unfertilized control, inorganic 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) annually and biennially (application every other year). Soil water for NO₃-N analysis was sampled weekly from 1.5-m depth using suction samplers during the 1989 and 1990 growing seasons. For these two years, mean soil water NO₃-N concentrations were 66 mg l^-1 for annual inorganic fertilizer, 50 mg l^-1 for annual manure and 11 mg l^-1 for biennial manure treatments. Differences in NO₃-N concentrations between annual manure and annual inorganic fertilizer treatments, and between annual manure and biennial manure treatments were statistically significant (P=0.05). Mean NO₃-N concentrations averaged over annual inorganic fertilizer 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.