LONG-TERM EFFECTS OF METALS IN SEWAGE-SLUDGE ON SOILS, MICROORGANISMSAND PLANTS

This paper reviews the evidence for impacts of metals on the growth of selected plants and on the effects of metals on soil microbial activi ty and sail fertility in the long-term. Less is known about adverse lo ng-term effects of metals on soil microorganisms than on crop yields a nd metal uptake. This is not surprising, since the effects of metals a dded to soils in sewage sludge are difficult to assess, and few long-t erm experiments exist. Controlled held experiments with sewage sludges exist in the UK, Sweden, Germany and the USA and the data presented h ere are from these long-term field experiments only. Microbial activit y and populations of cyanobacteria, Rhizobium leguminosarum by. trifol ii, mycorrhizae and the total microbial biomass have been adversely af fected by metal concentrations which, in some cases, are below the Eur opean Community's maximum allowable concentration limits for metals in sludge-treated soils. For example, N-2(-), fixation by free living he terotrophic bacteria was found to be inhibited at soil metal concentra tions of (mg kg(-1)): 127 Zn, 37 Cu, 21 Ni, 3.4 C4 52 Cr and 71 Pb. N- 2-fixation by free-living cyanobacteria was reduced by 50% at metal co ncentrations of (mg kg(-1)): 114 Zn, 33 Cu, 17 Ni, 2.9 Cd, 80 Cr and 4 0 Pb. Rhizobium leguminosanrum by. trifolii numbers decreased by sever al orders of magnitude at soil metal concentrations of (mg kg(-1)): 13 0-200 Zn, 27-48 Cu, 11-15 Ni, and 0.8-1.0 Cd. Soil texture and pH were found to influence the concentrations at which toxicity occurred to b oth microorganisms and plants. Higher pH, and increased contents of cl ay and organic carbon reduced metal toxicity considerably. The evidenc e suggests that adverse effects on soil microbial parameters were gene rally found at surpringly modest concentrations of metals in soils. It is concluded that prevention of adverse effects on soil microbial pro cesses and ultimately soil fertility, should be a factor which influen ces soil protection legislation.