Gr. Cline et Zn. Senwo, INHIBITORY EFFECTS OF ACIDIC MINESOIL ON THE SERICEA LESPEDEZA-BRADYRHIZOBIUM SYMBIOTIC RELATIONSHIP, Journal of plant nutrition, 16(9), 1993, pp. 1867-1880
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.