COMPLEMENTARY EFFECTS OF GRAIN LEGUMES TO SORGHUM (SORGHUM BICOLOR (L.) MOENCH) YIELDS, NITROGEN NUTRITION AND RESIDUAL SOIL MINERAL NITROGEN ACCUMULATION IN ROTATION SYSTEMS (GLYCINE MAX, ENZYME ACTIVITIES, CROP)

Four nitrogen rates were superimposed over sorghum-sorghum and grain legume-sorghum rotation systems to determine the effects of the legumes on grain sorghum Sorghum bicolor (L.) Moench yields, nitrogen requirements and residual soil mineral nitrogen accumulations. Estimates of the soil biological activity were also made. The levels of N used were 0, 20, 40 and 60 kg ha('-1) in Botswana and 0, 56, 112 and 168 kg ha('-1) in Nebraska, applied to the sorghum crop only.;Sorghum grain and stover yields after soybeans (Glycine max) in eastern Nebraska were significantly higher under the rotation system compared to the monoculture sorghum system when 56 kg N ha('-1) or less was applied. These grain yield increases were 41.0% in 1984 and 1985. Soil nitrogen levels in Nebraska studies were not influenced by the cropping system in 1984. In 1985, NO(,3)('-)-N content was significantly higher after soybeans than after sorghum in either continuous or rotation culture when 56 kg N ha('-1) or less was applied and tended to decrease at higher levels of N. The trends were exactly the opposite after the sorghum crop in both cropping systems. The absence of residual N accumulation in 1984 was attributed to leaching or denitrification or both during the April-June period when 443 mm of rainfall was received. However, it appeared that N mineralization later in the season was sufficient to cause significantly higher yields and N recovery in the rotation plots.;The effect of the cropping system in soil biochemical activity, assayed by dehydrogenase and urease enzyme activities, was in most cases insignificant. Nitrogen fertilizer reduced the activities of both enzymes. The decrease in dehydrogenase activity was attributed to the inhibition of TTC reduction by the NO(,3)('-) ion. The decrease in urease activity was attributed to possible C/N ratio shifts in the soil.;Sorghum grain yields were increased 48.0% and 54.4% after cowpeas (Vigna anguiculata) and peanut (Arachis hypogaea), respectively, in the Botswana experiments. The increases were no different from those obtained where fertilizer N had been applied in a season characterized by severe water stress. Nitrogen recovery from above ground sorghum parts indicated that the two legumes furnished as much N to the crop as the fertilizer.