Calcium affects the competitiveness of acid-sensitive and acid-tolerant strains of Bradyrhizobium japonicum in nodulating and fixing nitrogen with two soybean cultivars in acid soil

Abstract

A glasshouse experiment studied the role of calcium and pH on competitiveness of acid-sensitive and acid-tolerant Bradyrhizobium japonicum strains with similar N 2-fixation effectiveness in nodulating two soybean ( Glycine max L. Merr) cultivars selected for tolerance of aluminium (PI416937) or for manganese (Manta). Liming provides calcium (Ca) as well as increasing soil pH. Thus the effect of Ca and pH of soil are difficult to separate. We examined the effects of Ca per se by comparing the response to gypsum and lime amendment on the competitiveness of acid-tolerant and acid-sensitive strains in nodulating soybean in an acid soil. Acid soil was treated with either CaSO 4 or CaCO 3 and incubated for 2 weeks before sowing soybean seed. Two acid-sensitive and two acid-tolerant B. japonicum strains were mixed with each other (one acid-sensitive plus one acid-tolerant) and were inoculated onto soybean seeds at the rate of 10 6cfu seed −1. Soil pH, as amended by lime addition, had more effect on nodulation than Ca addition in the form of gypsum. The response was affected by cultivar and strain in a complicated fashion with a marked strain × cultivar interaction. One acid-tolerant strain formed most nodules with both cultivars in the unamended soil of pH 4.36 in competition with one acid-sensitive strain. The same acid-tolerant strain was not competitive against the second acid-sensitive strain with Manta but was with PI416937. The second acid-tolerant strain was not competitive with either acid-sensitive strain in unamended and gypsum treated soils. It was only competitive with PI416937 in limed soil, a rather surprising result. Inoculation of this soil with no native soybean nodulating strains, increased shoot weight, %N, N uptake. N 2-fixation was greatly increased by inoculation and lime addition, and to a lesser extent by gypsum addition for Manta. This experiment indicates that addition of Ca per se as gypsum to an acid soil has little effect on symbiotic performance, but changing pH by liming has a major effect, that both soybean cultivar and B. japonicum strain influence the competitiveness of strains in acid soil and that acid-tolerance does not necessarily increase a strain's competitiveness.