The soil population of Bradyrhizobium japonicum, the microsymbiont of soybean [Glycine max (L.) Merr.], is reduced following land application of sewage sludge. Previous research on the ability of B. japonicum to survive in soil recently amended with sewage sludge has been conflicting as to whether sludge-borne salts or heavy metals inhibit survival. To examine this question, sewage sludge compost was added to a rhizobia-free, Sassafras sandy loam soil (Typic Hapludult, fine loamy mixed mesic) amended with various rates of sludge compost, inoculated with B. japonicum USDA 110 ARS and the mixture was incubated for 42 d. The population of B. japonicum was reduced to nearly undetectable levels when inoculated into soil amended with sludge compost at a rate of 25%. The sludge compost was subsequently leached with 3-, 6-, 9- and 12-column void volumes of distilled water resulting in sludge compost samples with equal concentrations of heavy metals but decreasing concentrations of soluble salts. Leached and unleached compost samples were added to soil at a rate of 25% and the mixture inoculated with B. japonicum. Soil amended with sludge compost leached with six void volumes water exhibited the highest rhizobial population, while soil amended with unleached sludge compost contained the lowest rhizobial population. In soil amended with sludge compost leached with 3-, 9-, and 12-void volumes water, populations were not significantly different. Results demonstrate that sludge-borne soluble salts, and not heavy metals, are primarily responsible for short-term reductions in bradyrhizobial populations following the application of very high rates of sludge to soil.
Scientific Article no. A-6285 and Contribution no. 8454 of the Maryland Agric. Exp. Station, Dep. of Agronomy, University of Maryland, College Park, MD 20742.