Graduate Thesis Or Dissertation
 

Effect of sulfur on dinitrogen fixation of alfalfa (Medicago sativa L.)

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  • Field experiments were established in 1982 and 1983 on a Woodburn silt loam soil (Aquultic Argixeroll) to examine the effect of S fertilizer on dinitrogen fixation and dry matter yield of 10 week-old seedling alfalfa (Medicago sativa L. cv. 'Apollo'). Six levels of S fertilizer (0 to 67.2 kg S/ha) were applied as powdered gypsum. The experimental design was a split-block with one half of the seeds inoculated with commercial inoculum, and the other half non-inoculated. Inoculation effects were highly significant in both years, indicating that the indigenous population of Rhizobium meliloti at the experimental sites was ineffective in N₂-fixation. Dry matter yields were higher in inoculated than in non-inoculated treatments. Inoculated plants also showed a significantly higher acetylene reduction rate, N concentration, and total tissue N and S. No significant differences in any of these parameters were detected for the different S treatments. Sulfur fertilization increased the S concentration of non-inoculated plants more than inoculated plants, and decreased the N:S ratio in the forage by increasing tissue S content. The highest value of N₂-fixed by the inoculated plants was obtained from the 44.8 kg S/ha treatment. Greenhouse experiments were performed to evaluate the effect of varied nutrient solution concentrations of sulfate on the yield, nodulation, dinitrogen fixation, N and S concentrations, and partitioning of N and S into shoots and roots of six week-old alfalfa seedlings. Four levels of S (0, 1, 2.5, and 25 mg S/L) were applied in a randomized complete block design, with three replications. Seeds were inoculated with commercial inoculum, planted in plastic containers of acid-washed sand, and irrigated with nutrient solution for one minute, at 2 h intervals. The addition of 2.5 mg S/L to the nutrient solution resulted in the highest total dry matter, acetylene reduction rate, total N content, percent S recovery, and percent increase in N due to dinitrogen fixation. N:S ratios were 50% higher in shoots (16:1) than roots (9:1), with S fertilization decreasing the N:S ratios. Data from field and greenhouse experiments support the conclusion that S fertilization will increase seedling alfalfa yield when S levels in the plant are below 2.5 mg S/g (0.25%). In inoculated plants S fertilization increased both total N and S, demonstrating the importance of S in symbiotic N₂-fixation and the quality of forage produced.
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