NITRATE LOSSES THROUGH SUBSURFACE TILE DRAINAGE IN CONSERVATION RESERVE PROGRAM, ALFALFA, AND ROW CROP SYSTEMS

Subsurface drainage of gravitational water from the soil profile throu gh tiles is a common practice used to improve crop production on poorl y drained soils. Previous research has often shown significant concent rations of nitrate-N (NO3-N) in drainage water from row-crop systems, but little drainage research has been conducted under perennial crops such as those used in the Conservation Reserve Program (CRP), Pour cro pping systems (continuous corn, a corn-soybean rotation, alfalfa, and CRP) were established in 1988 to determine aboveground biomass yields, N uptake, residual soil N (RSN), soil water content, and NO3 losses t o subsurface tile drainage water as influenced by cropping system, Hyd rologic-year rainfall during the 6-yr study ranged from 23% below norm al to 66% above normal, In dry years, yields were limited, RSN accumul ated at elevated levels in all crop systems but especially in the row- crop systems, soil water reserves and RSN were reduced to as deep as 2 .7 m in the alfalfa (Medicago saliva L.) and CRP systems, and tile dra inage did not occur, Drainage occurred only in the corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] systems in the year of normal rai nfall, In gears of excess precipitation, drainage from the row-crop sy stems exceeded that from the perennial crops by 1.1 to 5.3X. Flow-weig hted average NO3-N concentrations in the water during the now period o f this study were continuous corn = 32, corn-soybean rotation = 24, al falfa = 3 and CRP = 2 mg/L. Nitrate losses in the subsurface drainage water from the continuous corn and cent-soybean systems were about 37X and 35X higher, respectively, than from the alfalfa and CRP systems d ue primarily to greater season-long ET resulting in less drainage and greater uptake and/or immobilization of N by the perennial crops.