YIELDS OF WHEAT IN ROTATION WITH MAIZE AND SOYBEANS IN ZAMBIA

Wheat (Triticum aestivum L.) was grown in rotation with maize (Zea mays L.) and soybeans (Glycine max L.) as first crops at the National Irrigation Research Station, Nanga, Zambia. Two nodulating soybean cultivars used in the first season of the rotation were compared for their ability to support symbiotic N 2 fixation by Bradyrhizobium japonicum using a nonnodulating cultivar as a reference crop and the 15 N isotope dilution technique. All first crops received two levels of P (0 or 30 kg P ha −1 ). The legumes received a blanket application of 20 kg N ha −1 , whereas maize received two rates of N (20 and 60 kg N ha −1 ). There were no benefits of residual P from the first crops to the succeeding wheat. However, the yields of wheat grown on plots where fixing legumes had been grown the previous season were significantly higher than those where maize had preceded. The superior wheat yield in the soybean-wheat rotation over the maize-wheat rotation was attributed to residual N from biological N 2 fixation by the preceding legume crop. The benefits were greater when the lower yielding soybean cultivar preceded wheat in the rotation presumably due to its lower soil N demand. Key words: 15 N isotope dilution, B. japonicum, N fixation, residual P, residual N, crop rotation