Abstract. The most important sources of heavy metal contamination in their order of significance are air pollution, river sediments, sewage sludges, town waste composts, agricultural chemicals, and industrial waste.Lead accumulation in plants near highways and roads has decreased considerably since the law controlling the lead content of petrol came into force. Hazardous amounts of lead, however, are still concentrated 10-20 m along the sides of highways and roads, and in the strip of up to 5 m there are also higher concentrations of cadmium.

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The utilization of plant‐ and soil‐C by the microbial biomass in the rhizosphere of maize plants was investigated as a function of root proximity. The plants were cultivated in pots with divided root chambers and their shoots supplied with 14CO2 for 23 days. Subsequently the individual soil zones were analyzed for organic C, 14C, biomass C and biomass 14C. Plant roots induced a 197% increase in microbial biomass and a 5.4% decrease in soil organic C compared with an 1.2% decrease in the unplanted control soil. The contributions of plant‐ and soil‐C to this increased microbial growth amounted to 68% and 32% respectively. Biomass‐14C corresponded to 1.6% of the total photosynthetically fixed 14C, to about 15% of the organic 14C‐input into the rhizosphere and to 58% of the plant carbon remaining in soil after the removal of roots. 20% of this biomass‐14C was found outside the immediate root zone. These results demonstrate that growing roots are a significant C‐source for the microbial biomass and render an additional fraction of soil‐C available to microbial utilization. The efficiency of C‐utilization by the rhizosphere biomass is lower than values obtained with liquid cultures in laboratory experiments. The supply of plant‐C to the microbial biomass outside the immediate root vicinity indicates that the overall volume of the maize rhizosphere is greater than what has been supposed so far.