Laboratory prescreening of Bradyrhizobium japonicum for low pH, Al and Mn tolerance can be used to predict their survival in acid soils

We examined whether strains of Bradyrhizobium japonicum selected for growth on acid media in vitro would also survive and grow better in acid soils. Four agar screening media for acid-tolerant rhizobia, which differed in the number of acid soil stresses imposed (pH, low calcium (Ca) and phosphorus (P), high aluminum (Al) and manganese (Mn)), were assessed for their effects on the survival of 14 Indonesian strains and two commercial strains of B. japonicum. Survival of B. japonicum in the agar media was compared with that in two acid soils. A repeat stab inoculation method which provided a declining range of inoculum cell number to 10 cells per stab was used to assess the daily growth of the strains on the screening media at 5 pH levels (3.8, 4.2, 4.5, 5.0, and 6.8). The growth and survival of the 16 strains were then measured at days 1, 8, 18, and 28 after inoculation in two acid soils (pH 4.24 and 4.35) sterilized using γ-irradiation at 5.0 Mrad. Selectivity of the agar media improved as more acid stress factors were incorporated in the media. Those strains of Bradyrhizobium identified as acid, Al and Mn-tolerant in acidic agar media, also had better survival in the low pH soils. There was no relationship between acid or alkali production on agar media and acid tolerance on agar or in soil. There was no apparent relationship between symbiotic performance and acid tolerance, and one acid-tolerant strain was as effective as the commercial inoculant strain CB1809. The most acid-tolerant strain was also the most ineffective