ENUMERATION AND N-2 FIXATION POTENTIAL OF RHIZOBIUM-LEGUMINOSARUM BIOVAR TRIFOLII GROWN IN SOIL WITH VARYING PH VALUES AND HEAVY-METAL CONCENTRATIONS

The potential risk to Rhizobium leguminosarum by. trifolii and white c lover (Trifolium repens cv. 'Regal') from biosolids-induced heavy meta l toxicity is of great concern because of their symbiotic association and capacity for N-2 fixation, A greenhouse experiment was conducted t o assess the effects of heavy metals from biosolids on the population and N-2 fixing potential of Rhizobium leguminosarum by. 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 w ith phytotoxicity observed in the low pH soil amended with biosolids. The number of Rhizobium was significantly reduced in all low pH treatm ents. 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-ame nded soil had higher numbers of Rhizobium than the control, but Nu-Ear th 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 rem ained ineffective. Shoot yield and the number of Rhizobium did not sho w any significant increase with the increase in soil pH. Adjustment of high pH soil to low soil pH significantly reduced the number of Rhizo bium irrespective of whether biosolids were applied. In conclusion, fe w significant effects of biosolids-borne heavy metals on plants, N-2 f ixation, and on numbers of Rhizobium leguminosarum by. trifolii were o bserved at concentrations of metals studied, as long as soil PH was ma intained near 6.0. Where reductions in rhizobial number and plant para meters were observed, the decrease was primarily attributed to low soi l pH and to a lesser extent heavy metal toxicity from biosolids.