Impact of biosolids and co-utilization wastes on rhizobia, nitrogen fixation and growth of legumes

The application of co-utilization products to soil, especially biosolids contaminated with high concentrations of heavy metals, can potentially affect soil microbial populations. Organic matter and micro and macronutrients can all enhance the number and activity of microbes in soil. Alternatively, toxic organics, heavy metals and soluble salts can have deleterious effects on number and activity. Some reports from Europe indicate that biosolids-borne heavy metals can reduce the number, diversity and metabolic activity of rhizobia present in metal-amended soil. Metal toxicity is believed to select for ineffective mutants of rhizobia that fix little or no nitrogen. Studies at the University of Maryland, College Park, MD, and the Environmental Chemistry Lab of the USDA, ARS, Beltsville, MD, have shown that many of the parameters noted above are enhanced with application of biosolids to soil. Numbers of rhizobia capable of nodulating alfalfa, white clover and soybeans were found to increase following application of biosolids to soil. This was observed despite the fact that the metal content of the soil was high enough to produce visible signs of toxicity in the respective macrosymbiont. Rhizobia isolated from plants experiencing severe phytotoxicity were capable of fixing nitrogen at rates similar to rhizobia isolated from unamended plots. Diversity of rhizobia was also increased in metal contaminated soil compared to rhizobia isolated from control soils. Enhanced diversity was observed using both serological analysis of rhizobial isolates and REP-PCR genetic fingerprints from soil isolates. When we examined survival of rhizobia and growth of white clover in chelate-buffered nutrient solution, zinc and cadmium were found to be approximately ten times more toxic to the plant compared to the rhizobial microsymbiont. At zinc and cadmium activities where plants experienced severe phytotoxicity, most plants were well nodulated and nodules were capable of fixing adequate amounts of nitrogen. The only observed metal related effect was a delay in nodulation. Nodulation was typically delayed for 24 to 48 hours compared to plants from control plots. Observation of young roots revealed that delayed development of root hairs was responsible for the observed difference in nodulation. Once root hairs were produced, nodulation was found to proceed at a normal rate even in the presence of high concentrations of heavy metals. It is our belief that where adverse effects of biosolids on rhizobia are reported, effects were related to past reductions in soil pH. Numerous reports indicate that when soil pH declines below 5.5, rhizobia that survive lack capacity to fix atmospheric nitrogen. It is therefore our opinion that unless metal loading rates to soil greatly exceed permissible levels, no adverse effects on clover will result from metals alone.