Effect of Phosphorus on the Effectiveness of Strains of Rhizobium japonicum1
- P. W. Singleton2,
- H. M. AbdelMagid3 and
- J. W. Tavares2
We examined the relationship between P nutrition of soybean [Glycine max (L.) Merr. cv. ‘Davis’] and N2 fixation by five strains of Rhizobium japonicum ranging from an ineffective (SM-5) to a highly effective (USDA 110) symbiosis with soybean. Phosphorus at 0, 50, 125, and 400 mg P kg−1 soil (P0, P50, P125, P400) as Ca(H2PO4)2 · H2O was applied to pots containing 2.7 kg of a P-fixing humoxic tropohumult free of R. japonicum. Treatments consisting of + N and five strains of R. japonicum and the four P treatments were arranged in a complete factorial replicated three times. With 400 mg P kg−1 soil treatments ranked + N > USDA 110 > USDA 31> USDA 123> USDA 33>SM-5 for shoot dry weight and accumulation of N in the shoot. There were no significant strain effects for shoot dry weight, shoot N or nodule activity at Po. Strain rankings were the same at the intermediate levels of P but differences were smaller. Although nodule number was somewhat enhanced by P, it bore no relationship to strain effectiveness and was not a factor limiting N2 fixation at low P. Nodule dry weight and nitrogenase activities significantly increased with P additions. The concentrations of N and P in the shoot also increased with P additions. The concentrations of shoot P and N were higher in plants provided mineral N at the low and intermediate P levels than in the inoculated plants. The results show that P nutrition is important in interpreting the N2 fixation capability of strains of Rhizobium. Synergism between P and Rhizobium inputs requires that to obtain maximal response to the application of one input necessitates employing the other at maximum levels.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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