Crop Management for the Greater Utilization of Symbiotically Fixed Nitrogen

113 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1981.Two field studies were conducted at Urbana, Illinois, to evaluate different crop management systems to increase the utilization of symbiotically fixed nitrogen. The first 3-year study of simulated aerial interseeding of forage legumes, alfalfa, red clover, and sweetclover into standing soybeans was undertaken to examine the potential nitrogen contribution from forage legumes under interseeding competition. Results indicated that light and moisture were the most limiting environmental factors. Early seeding dates (late July and early August), in maturity group II and III soybeans were not acceptable because growth was restricted by low light intensities beneath the soybean canopy, high moisture conditions under the canopy which led to increased seedling diseases, and low moisture conditions which limited germination and early seedling development. It was determined that suitable forage legume stands could be obtained by the simulated aerial seeding techniques used but growth potential of the interseeded legume was limited by early frost and cool wet springs which delay soybean and corn planting. Vigorous legume growth in the spring may result in poor seedling emergence in the following crops under moisture stress conditions. The nitrogen contributions from the interseeded legumes ranged from 20-50 kg N/ha in the above-ground portion of the forage legumes with a potential of 150-190 kg N/ha obtained from monocultures of the same legumes without competition from soybeans.The second study was designed to examine the variations in grain and non-grain nitrogen in selected soybean cultivars from three maturity groups (II, III, and IV) over a 2-year period. Included in these cultivars was a non-nodulating isoline of 'Harosoy' to monitor the nitrogen contribution from the soil. Soybean grain yields were significantly increased by the application of fertilizer nitrogen when averaged across the two growing seasons. Maturity group II soybeans were significantly different from the other maturity groups for the harvest components, as well as harvest index and harvest nitrogen index. Later maturing cultivars had a higher proportion of dry matter and nitrogen in the non-grain components, indicating that these later maturing cultivars are less efficient at mobilizing nitrogen into the developing seed.Dry matter and nitrogen content of fallen leaves and petioles were included in the calculations of harvest index and harvest nitrogen index to provide the best estimate of non-grain nitrogen. Group II maturity soybeans had the highest harvest nitrogen index (0.817) and group IV the lowest (0.722). The actual non-grain nitrogen content ranged from 42-76 kg N/ha and 42-85 kg N/ha for the 0 and 300 kg N/ha nitrogen treatments, respectively. Group IV maturity cultivars should make the largest contributions to soil nitrogen as they contain the largest total nitrogen in the non-grain components. The range of symbiotically fixed nitrogen was 94-161 kg N/ha (44-58% of the total above-ground nitrogen) compared to 152-219 kg N/ha being removed in grain, indicating a possible depletion of soil nitrogen.U of I OnlyRestricted to the U of I community indefinitely during batch ingest of legacy ETD