AVAILABILITY OF N FROM N-15-LABELED ALFALFA RESIDUES TO 3 SUCCEEDING BARLEY CROPS UNDER FIELD CONDITIONS

Rotation of N2-fixing legumes and cereals is becoming increasingly popular to save N fertilizers and to improve soil physical and chemical properties. The availability of N from residues of alfalfa (Medicago sativa L.) to succeeding barley (Hordeum vulgarae L.) crops was examined in this work. Isotopic 15N-labeled alfalfa reisidues were mixed into 20cm of top soil (Manotick sandy loam soil, typic Dystrochrep) in micro plots at the Plant Research Centre, Ottawa, Ontario in autumn, 1984 and barley was grown in three successive springs, 1985-1987. Alfalfa residues (2.5 tonnes ha-1) and N fertilizer (40 kg N ha-1) increased barley dry matter and grain production. There was no significant effect of ploughed-in alfalfa residues or added N fertilizer on N concentration of barley, while total N yields increased significantly (P = 0.05). Calculation of N benefit from alfalfa residues revealed that 18% of N was absorbed by barley during 1985-1987. In the first, second and third years following application, residual alfalfa N provided average values of 15, 6 and 5% of the total N content in barley and its N use efficiency was at rates of 11, 4 and 3%, respectively. Application of N fertilizer resulted in an increased rate of availability of N from alfalfa residues. © 1990.