CROPPING SYSTEM AND RESIDUE MANAGEMENT EFFECTS ON NITRATE LEACHING AND CROP YIELDS

The effect of cropping system and of crop residue management on crop y ield and N uptake and on NO3 leaching was tested in a 2-year lysimeter experiment with sandy loam soil. One cropping system included a 4-yea r rotation of spring barley, ryegrass, winter wheat and sugarbeet. In the other system, two test crops of spring barley followed cereal lye, ryegrass or spring barley, grown continuously for 9 years. In the fir st autumn, straw was incorporated or removed in lysimeters previously in cereals, and sugarbeet top was returned or removed in lysimeters pr eviously in sugarbeet. Lysimeters with and without straw incorporation received I g N-15 m(-2) Of 99 at.% (NH4NO3)-N-15 in order to estimate N immobilization caused by straw and its potential for remineralizati on. In the second year, above ground residues (except for stubbles) we re removed in all lysimeters. Straw incorporation caused more N-15 to be retained in the soil but the leaching of total N in the first winte r was not reduced significantly. The pooled NO3 loss over the two wint ers was not affected by the straw management. Between 12 and 48% of N- 15 immobilized by the straw was remineralized during the 3 years follo wing straw incorporation. Sugarbeet top increased the leaching of N in the two winter periods. Losses were small from lysimeters with ryegra ss but turnover of ryegrass residues appeared to enhance losses of NO3 in the second winter after its incorporation. Straw incorporation red uced the yield and N uptake of the first test crop of barley, indicati ng a prolonged N immobilization phase, Sugarbeet yields were unaffecte d by straw probably because of its longer growth period. Return of sug arbeet tops increased the yield and N uptake of the two succeeding cro ps. Spring barley grown after termination of permanent ryegrass yielde d considerably batter and had larger N uptakes than barley succeeding cereal crops. Despite the differential effects of residue management, the type of cropping system was much more decisive for total N turnove r. Amounts of N exported in the crop rotation by NO3 leaching and plan t uptake generally balanced the amounts of N applied in mineral fertil izer. More N was exported than applied, however, when spring barley wa s grown continuously. Incorpora tion of straw could only partly preven t the negative N balance. (C) 1998 Elsevier Science B.V.