Sewage Sludge and Heavy Metal Effects on Nodulation and Nitrogen Fixation of Legumes

Abstract

AbstractEffects of sludge‐borne heavy metals on plant growth, nodulation, and nitrogen (N2) fixation of alfalfa (Medicago sativa L.), white clover (Trifolium repens L.), and red clover (Trifolium pratense L.) were examined. Plants were grown in the greenhouse in pots containing soil (Typic Paleudults) obtained from plots where heat‐treated and Nu‐Earth sludges were applied in 1976 and 1978, respectively. Two soil pH regimes (low and high) were examined for each of the treatments. Nitrogen fertilizer (NH4NO3) was applied to one‐half the pots and each legume was inoculated with the appropriate Rhizobium culture. Soil pH and sludge type significantly affected uptake of metals with phytotoxicity observed in the Nu‐Earth, low pH soil. Nodulation was reduced, but not always completely eliminated in all low pH treatments, including the controls. Symbioses were generally ineffective in low pH treatments. In soils where pH was above 6.0, there was a significant increase in shoot weight and total shoot N with sludge addition. Plants derived significant quantities of N from soil, where sludge was applied many years ago. When pH was maintained at 6.0 or higher the results show that heavy metals in soil, and the resulting concentration in plants, had no effect on any of the parameters examined.