INHIBITORY EFFECTS OF ACIDIC MINESOIL ON THE SERICEA LESPEDEZA-BRADYRHIZOBIUM SYMBIOTIC RELATIONSHIP

Effects of acidic minesoil on sericea lespedeza [Lespedeza juncea (L.F .) var. sericea (Mig.)] and its nitrogen (N2)-fixing symbiotic relatio nship with Bradyrhizobium spp. were examined. Sericea lespedeza was gr own in pots with N fertilization, without N fertilization, or with com mercial Bradyrhizobium as a seed inoculant. Minesoil (pH 5.2) was fert ilized with calcium (Ca), phosphorus (P), molybdenum (Mo), and potassi um (K), and the pH level was adjusted to 4.8 or 4.5 with aluminum or i ron sulfate [Al2(SO4)3; Fe2(SO4)3]. Minesoil was also limed to pH 6.1. Shoot dry weights, shoot N concentrations, nodule dry weights, and no dule numbers were significantly lower (P less-than-or-equal-to 0.05) w hen inoculated plants were grown in soil at pH 4.5 and 4.8 compared to limed soil. Thus, the N2 fixation process was adversely affected belo w pH 5.0. Nitrogen-fertilized plants grew well in acidified soil, and there were no significant differences in shoot dry weights of such pla nts among the soil acidification treatments including limed soil. Thus , the N2-fixing symbiosis appeared to be more sensitive to acidified s oil than the plant host. The effects of Al toxicity versus other facto rs could not be determined because Al2(SO4)3- and Fe2(SO4)3-amended so ils contained similar levels of toxic Al at the highest pH (4.8) that prevented N2 fixation. Time periods required for cells of Bradyrhizobi um strains to multiply by a factor of 10(4) were significantly longer (P less-than-or-equal-to 0.05) in extracts of Al2(SO4)3-amended soil ( pH 4.8 and 4.5) than in extracts of calcium carbonate [CaCO3]-amended soil (pH 6.1). These increases suggested that reduced multiplication o f Bradyrhizobium in acidified minesoils may have been at least partial ly responsible for the large decreases in nodulation and N2 fixation o bserved in these soils. It was also reasoned that the inability of exi sting bacteria to infect and nodulate plant roots may also have been a factor, based on the high inoculation rates used and the abilities of Bradyrhizobium cells to survive and multiply (albeit at a reduced rat e) in extracts of acidified soil. Sericea lespedeza is known to tolera te soils of pH 4.5. However, results of this study suggested sericea l espedeza may not fix appreciable N2 in acidic soil below pH 5 when ino culated with commercial Bradyrhizobium, even after the establishment o f lespedeza plants tolerant of such conditions.