Effect of Phosphorus on the Effectiveness of Strains of Rhizobium japonicum

Abstract

AbstractWe 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.