Toxicity of lead to soil respiration: mediation by clay minerals, humic acids, and compost.

The natural background concentration of lead (Pb) in soils in the United States ranges from 10 to 700 ppm, with an average of 20 ppm (Demayo et al., 1982). However, as a result of anthropogenic activities, soils may be contaminated with elevated levels of Pb. For example, gasoline combustion releases Pb into the atmosphere, and this Pb can be deposited· onto roadside soils. 1,225 ppm Pb ~s noted in soil 0.3 m from a highway along which 8,10D vehicles/day traveled, whereas at 100 and 200 m from the highway, the level of Pb in the surface soil was only 13 ppm. For a highway on which 550 vehicles/day traveled, levels of 35 and 13 ppm Pb were noted 0.3 and 25 to 200m, respectively, from the highway (Wheeler and Rolfe, 1979). Another source of Pb in soil is from deteriorating Pb-based paints: soil sampled within 2 feet of old wooden-frame houses painted with Pb-based paints contained 1,586 to 2,349 ppm Pb, whereas samples from within 2 feet of brick-veneer houses contained 351 to 501 ppm Pb (Demayo et al., 1.982). Extremely high levels of Pb occur in soils near smelters: e.g., the level of Pb in soil samples was 28,000, 8,333, 4,800, 3,654, and 703 ppm at 15, 90, 150, 180, and 1,000 m, respectively, frau a secondary Pb smelter (Bisessar, 1982). Levels greater than 24,000 ppm Pb occurred in soils in the immediate vicinity of a Pb mine and smelter, whereas noncontaminated soil contained from 1 to 37 ppm Pb (Djuric et al ., 1971). As Pb has no known biological function, elevated levels of Pb in ~fls and in other natural environments may adversely affect the hdigenous biota, including the microbiota. For example, populations of bacteria, actinomycetes, and fungi were decreased in ~ls surrounding a secondary Pb smelter (Bisessar, 1982). ~evated levels of Pb in soil may also adversely affect microbe~diated ecologic processes. For example, amendments of soils with Pb resulted in reductions in carbon (C) mineralization (Jikkelsen, 1974; Doelman and Haanstra, 1979a), nitrogen (N) •1neralization, nitrification (Bhuiya and Cornfield, 1974; Rother et al., 1982) denitrification (Bollag and Barabasz, 1979), df!composition of animal (Doelman and Haanstra, 1979b) and plant lStrojan, 1978} litter, and activities of soil enzymes (Doelman · and Haanstra, 1979a}.