Contributions of nitrogen in soybean crop residues to subsequent crops and to soils

Abstract

Dried soybean leaves, stems and roots, containing respectively 1.9, 2.1 and 1.4 atoms % 15N were incorporated (May 1986) into the trash-cleared topsoil of 6 m2, unconfined plots in an irrigated block at Trangie, N.S.W. At intervals over the next year, 15N was determined in succeeding crops of oats (May to October) and soybeans (November to May), in weeds and in the soil. Nitrogen derived from soybean leaf residues contributed 33% of the N of winter oats grown immediately after residue incorporation, but N from roots and stems contributed only 0.6 and 3.5%, respectively, of oat N. The following soybean crop derived most of its N from N2 fixation, but residues (15N stems, 15N leaves and total soybean trash) increased plant N. Thus 13, 60 and 394 mg N m-2 of the crop N was derived from 15N -labelled root, stem and leaf residues, respectively. Weeds had higher atoms % 15N than soybeans, but removed only small proportions of residue N from the soil. In the soil, most 15N from the residues remained in the upper (0-10 cm) level of the profile, but 15N excess was detected to 30 cm in soil treated with 15N -stems and 15N -leaves. Mineral N (NO3- + NH4+ ), extracted from the soil with 2 M KCl, represented 0-5 to 2% of the total N present, being greatest in March and least in May, but 15N excess in these extracts was a greater proportion (1 to 13%) of the total 15N excess present. This indicated that the soybean residue N remained more readily mineralizable than native organic N for at least one year. Losses of 15N from the system are discussed in relation to experimental constraints.