Utilization by wheat crops of nitrogen from legume residues decomposing in soils in the field

Ground 15N-labelled legume material (Medicago littoralis) was mixed with topsoils in confined microplots in the field, and allowed to decompose for 7 and 5 months in successive years (1979, 1980) before sowing wheat. The soil cropped in 1979 (and containing 15N-labelled wheat roots and legume residues) was cropped again in 1980. The results support evidence that ungrazed legume residues, incorporated in amounts commonly found in southern Australian wheat growing regions, contribute only a little to soil available N and to crop N uptake, even in the first year of their decomposition. Thus mature first crops of wheat, although varying greatly in dry matter yield (2.9-fold) and total N uptake (2.4-fold), took up only 27.8 and 20.2% of the legume N applied at 48.4 kg ha-1, these corresponding to 6.1 and 10.8% of the N of the wheat crops. The availability of N from medic residues to a second wheat crop declines to <5% of input. For both first and second wheat crops, uptake of N from legume residues was approximately proportional to legume N input over the range 24.2 to 96.8 kg ha -1. The proportional contributions of medic N to soil inorganic N, N released in mineralization tests, and to wheat crop N, differed between seasons and soils, but for a given crop did not significantly differ between tillering, flowering and maturity. In both years, grain accounted for 52-65% of the total 15N of first crops, roots for < 5-6%. In neither year did the amounts of N or 15N in the tops change significantly between flowering and maturity, despite a gain in tops dry matter in 1979; by contrast N and 15N of roots decreased significantly during ripening in both years. Wheat plants at tillering contained about 75% of the N and 15N taken up at flowering. The amounts of legume-derived 15N in mature first wheat crops were equivalent to 82-88% of the amounts of inorganic 15N in the soil profiles at sowing. Wheat straw added at the rate of 2.5 t ha-1, 2 months before sowing, decreased the uptake of N (15%) and 15N (18%) by wheat in a nitrogen responsive season. © 1983.