Effect of Biological Nitrogen Fixation and Plant Growth in two Soybean Cultivars using 15N Isotopic Dilution Technique

In calcareous soils with high calcium carbonate contents and pH, the growth and development of plant might undergo nutritional disorders. The process of biological nitrogen fixation in legumes cultivated and plant growth in these soils could be limite. The use of acid-producing materials like sulfur and a reduction in soil pH improves plant growth and increase the biological nitrogen fixation. A 2 × 5 factorial experiment consisting of two soybean cultivars (Sahar and Williams) and six levels of sulfur (0, 1, 2, 5, 10 and 15 ton/ha) arranged in a completely randomized design with three replicates was conducted under greenhouse conditions in Nuclear Center for Agriculture & Medicine Atomic Energy Organization, Iran. Results showed that the accumulation of sulfate resulted from sulfur additions decreased soil pH and increased EC (p<0.01). Sulfur additions up to 5 and 10 ton/ha respectively for Sahar and Williams cultivars increased the plant dry weight. However, further increase of sulfur levels affected the plant dry weights negatively, due most likely to the increase in soil EC values. In spite of the increased soil EC, increasing sulfur levels resulted in enhanced nitrogen fixation, probably due to favorable soil reaction caused by sulfur oxidation.