Enumeration and N 2 fixation potential of Rhizobium leguminosarum biovar trifolii grown in soil with varying pH values and heavy metal concentrations

Abstract

The potential risk to Rhizobium leguminosarum bv. trifolii and white clover ( Trifolium repens cv. ‘Regal’) from biosolids-induced heavy metal toxicity is of great concern because of their symbiotic association and capacity for N 2 fixation. A greenhouse experiment was conducted to assess the effects of heavy metals from biosolids on the population and N 2 fixing potential of Rhizobium leguminosarum bv. trifolii under two pH regimes. In 1994, soils (Typic Paleudults) were collected from plots that had previously received 224 Mg ha −1 heat-treated and 100 Mg ha −1 Nu-Earth biosolids (applied in 1976 and 1978, respectively). Six soil treatments were used for the study: a control with low and high pH and two biosolids treatments, each with low and high pH. Soil pH and biosolids application significantly affected uptake of metals with phytotoxicity observed in the low pH soil amended with biosolids. The number of Rhizobium was significantly reduced in all low pH treatments. This resulted in no or ineffective nodulation by plants grown in these treatments. High numbers of Rhizobium were found in all high pH treatments, irrespective of metal content. Heat-treated biosolids-amended soil had higher numbers of Rhizobium than the control, but Nu-Earth biosolids-amended soil had lower numbers than the control. Nitrogen fixation, as measured by acetylene reduction activity, was greater in all high pH treatments compared with low pH treatments. When soil pH from the acidic plots was adjusted above 6.0, most of the isolates remained ineffective. Shoot yield and the number of Rhizobium did not show any significant increase with the increase in soil pH. Adjustment of high pH soil to low soil pH significantly reduced the number of Rhizobium irrespective of whether biosolids were applied. In conclusion, few significant effects of biosolids-borne heavy metals on plants, N 2 fixation, and on numbers of Rhizobium leguminosarum bv. trifolii were observed at concentrations of metals studied, as long as soil pH was maintained near 6.0. Where reductions in rhizobial number and plant parameters were observed, the decrease was primarily attributed to low soil pH and to a lesser extent heavy metal toxicity from biosolids.