Influence of lead additions on arbuscular mycorrhiza and Rhizobium symbioses under soybean plants

The influence of the arbuscular mycorrhizal fungus (AMF) Glomus macrocarpum Tul. and Tul. on Bradyrhizobium-inoculated soybean (Glycine max L. Merrill var. IAC-14) growth, nutrients and Pb uptake was studied in soils with different levels of Pb. Mycorrhizal and non-mycorrhizal plants were grown in a soil amended with 0, 150, 300 and 600 mg dm(-3) of Pb, using Pb(NO3)(2), and plants were harvested at two different growth stages: flowering and maturity. Soybean shoot dry weights were not affected by increasing doses of lead in the soil, but the number of pods decreased significantly. Nodule dry weights of mycorrhizal roots were reduced by soil Pb additions, although the mycorrhizae stimulated plant nodulation significantly. The inoculation of AMF in soybeans provided higher rates of nutrients uptake, mainly P, inducing greater mycorrhizal-soybean growth. Thus, mycorrhizae improved Pb uptake, produced shoots with Pb concentrations 30% lower than those of non-mycorrhizal plants, at the highest Pb concentration added to the soil. AM fungus was more susceptible to the higher Pb doses added to the soil than the soybean plants, decreasing both root AM colonisation and spore production. This work indicated that a concentration of 600 mg dm(-3) of Pb in the soil interfered with the establishment of double symbioses between AMF and Bradyrizhobium, and with the fungus perpetuation in the soil. (C) 2004 Elsevier B.V All rights reserved.