Abstract
Effects of acidic minesoil on sericea lespedeza [Lespedeza juncea (L.F.) var. sericea (Mig.)] and its nitrogen (N2)‐fixing symbiotic relationship with Bradyrhizobium spp. were examined. Sericea lespedeza was grown in pots with N fertilization, without N fertilization, or with commercial Bradyrhizobium as a seed inoculant. Minesoil (pH 5.2) was fertilized with calcium (Ca), phosphorus (P), molybdenum (Mo), and potassium (K), and the pH level was adjusted to 4.8 or 4.5 with aluminum or iron 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 nodule numbers were significantly lower (P < 0.05) when 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 below pH 5.0. Nitrogen‐fertilized plants grew well in acidified soil, and there were no significant differences in shoot dry weights of such plants among the soil acidification treatments including limed soil. Thus, the N2‐fixing symbiosis appeared to be more sensitive to acidified soil than the plant host. The effects of Al toxicity versus other factors could not be determined because Al2(SO4)3‐ and Fe2(SO4)3‐amended soils contained similar levels of toxic Al at the highest pH (4.8) that prevented N2 fixation.
Time periods required for cells of Bradyrhizobium strains to multiply by a factor of 104 were significantly longer (P ≤ 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 of Bradyrhizobium in acidified minesoils may have been at least partially responsible for the large decreases in nodulation and N2 fixation observed in these soils. It was also reasoned that the inability of existing 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 rate) in extracts of acidified soil. Sericea lespedeza is known to tolerate soils of pH 4.5. However, results of this study suggested sericea lespedeza may not fix appreciable N2 in acidic soil below pH 5 when inoculated with commercial Bradyrhizobium, even after the establishment of lespedeza plants tolerant of such conditions.
Notes
This research was sponsored by a USDA, Cooperative State Research Service grant to Kentucky State University under agreement KYX10–89–11P.
Department of Agronomy, Iowa State University, Ames, IA 50011.