THE ROLE OF WETLAND SOILS IN NITROGEN AND PHOSPHORUS REMOVAL FROM AGRICULTURAL DRAINAGE WATER.

摘要

In recent years, much attention has been focused on nutrient losses in drainage water from agricultural lands. One of the approaches to nutrient removal from agricultural drainage water is the use of wetland retention/detention areas. A series of laboratory studies was conducted to evaluate the role of wetland soils in nitrogen and phosphorus removal from agricultural drainage water.; The extent of denitrification in soil in the absence of overlying water was determined to evaluate denitrification potential of the soils. At least 96% of added ('15)N-labeled nitrate was lost rapidly from the soil within a 2 to 6 day period. Nitrification and denitrification in simulated wetland systems were investigated by adding ('15)N-labeled ammonium or ('15)N-labeled nitrate to the overlying water of the soil-water columns. After 25 days, 43 to 84% of the added ammonium was removed from the overlying water. Ammonium removal rate was apparently influenced by soil characteristics, especially soil pH. Estimated ammonium removal in soil-water systems ranged from 0.31 to 0.63 (mu)g N/ml/day. Denitrification in soil-water systems appeared to follow first-order kinetics with the rate constants ranging from 0.02 to 0.062 day('-1). Nitrate removal rate increased with soil organic matter content and soil pH.; Phosphorus adsorption maxima ranged from 62 to 2,030 (mu)g P/g soil indicating that wetland soils have high phosphorus removal potential. For all soils except two, phosphorus sorption decreased under anaerobic conditions; however, considerable amounts of phosphorus were retained by most soils under either aerobic or anaerobic conditions. In laboratory column studies, underlying soil readily adsorbed phosphorus from the overlying water. Approximately 54 to 88% of the initial phosphorus in the overlying water was removed in 25 days.; The extent of nutrient release from decomposing plant material was evaluated in simulated wetland systems. The rate of plant decomposition in these systems decreased with decreasing pH. Aeration did not have measurable effect on rate of plant decomposition. Underlying soils, in two of three cases, retained most of the nitrogen and phosphorus released during the plant decomposition process.