Carbon and Nitrogen Levels in a Subsurface Drained Silty Clay Soil

ABSTRACT Organic carbon and nitrogen content were determined for soil samples taken annually from 1978 through 1983 from four depth increments from all plots of a shallow subsurface drainage research project. Drainage treatments were 3, 6 and 12 m spacing and no drainage. Drain tubing was 75 m diameter perforated corrugated plastic. Soil water from buried porous cups above and below the drains and from drain outflow was analyzed for nitrogen content. The objectives were to determine the long-term changes in soil carbon and nitrogen content, determine nitrogen levels in soil water and drain discharge, and to improve our understanding of nitrogen fertilizer management under intensive drainage systems for poorly drained marine and lacustrine silty clay soils. Over time, intensive cropping and drainage caused significant decreases from initial values of both soil carbon and nitrogen content for all depths measured. The difference between initial and final soil carbon and nitrogen values for the surface 0.15-m depth of soil suggests the possibility of a drainage-caused decrease in soil carbon (P = 0.08) and shows a significant drain treatment effect on soil nitrogen (P = 0.013). Drainage-caused soil nitrogen depletion in the 0.15-m soil depth occurred over the first years of the study during corn production and stopped after alfalfa was grown. Nitrogen levels in drain water exceeded 10 ppm only in July of the second year of intensive cropping.