DISTRIBUTION OF ORGANIC-CARBON AND INORGANIC NITROGEN IN A SOIL UNDERVARIOUS TILLAGE AND CROP SEQUENCES

The interaction of various tillage and crop sequences on the distribut ion of soil organic carbon (C) and inorganic nitrogen (N) is important for predicting changes in soil physical and chemical properties and p otential environmental impacts of soil and crop management. This study was conducted to determine the distribution of organic C, nitrate (NO 3-N) and ammonium (NH4-N) in a soil with various tillage methods and c rop sequences. A tillage experiment was started in 1978 with treatment s of chisel, disk, no-till, plow, ridge-till, and sub-soil. In 1985, e ach tillage plot was divided into two subplots to which two crop seque nces of continuous corn [Zea mays L.] (C-C), and soybean [Glycine max (L.) Merr.]-corn (S-C) were assigned. Nitrogen fertilizer was applied at a rate of 100 kg N ha-1 y-1 when plots were planted to corn. In the spring and autumn of 1989, soil samples were taken from crop sequence s and tillage plots to a depth of 1.5 m in 0.3 m increments for determ ination of NO3-N, and NH4-N contents. Soil samples were also collected from 0-10, 10-20, 20-40, 40-60, 60-80, 80-100, 100-150, 150-200, 200- 250, and 250-300 mm soil depths for organic C determination. Multivari ate analysis of variance (MANOVA) was used to analyze the data taken f rom different soil depths. Organic C concentration was greatest at 0-1 0 mm soil and decreased with increasing soil depth. Organic C was grea test for no-till and was least for sub-soil. Greater residue in C-C wa s associated with greater soil organic C than S-C. Organic C quantity (kg ha-1 300 mm-1) followed the same trend as organic C concentration. The plow treatment had the greatest NO3-N quantity in the 0-0.3 m soi l depth. All tillage systems had similar NO3-N amounts in deeper soil. NH8-N content of soil was not influenced by tillage or crop sequences . Soil organic C was related to both the amount of residue produced an d left on the soil surface after harvest and also to the degree of soi l surface disturbance. Nitrate-N was affected more by tillage and crop sequence systems than was ammonium-N.