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Water-soluble ~4C.labelled organic material, released into soil from roots of wheat and maize plants, was recovered with a mild percolation technique, without disturbing the root-soil interface. Extraction yields were relatively high for the 14 C materials (up to 11% of residual soil 14 C for 6 weeks maize) illustrating the water soluble character of the freshly added material. The complexation potential of the soil extracts was evaluated by adding 57Co, 65Zn and 54Mn to the extract and determining their distribution among the organic fractions by a gel filtration technique. The results ~how that within four weeks a micro-environment is created around a plant root, characterized by an accumulation of root-derived organic materials. In parallel with this time dependent accumulation, a gradual shift from ionic metal to higher molecular weight forms occurred. The three metals were increasingly complexed throughout the growth period. Extracts from a fallow soil complexed minor amounts of the added tracer (6.4%; 1.9% and 0.2% for 57Co, 65Zn and 54Mn respectively) while cropped soil extracts after 6 weeks complexed 61%; 16% and 6% of respectively Co, Zn and Mn in the case of maize, and 31%, 15% and 1% in the case of wheat. Although the effects are most pronounced in the case of Co and maize, evidence is given for similar effects with wheat and the other metals. The results indicate that the physicochemical status of transition metals in the rhizosphere is entirely different from that in the bulk soil. * Contribution No. 2250 of the Radiation Protection Programme of the Commission of the European Communities.

Key words 14CO 2 atmosphere Maize Microbial biomass Rhizosphere Turnover root-derived material Wheat Summary The release of organic materials by roots of maize and wheat was studied using a growth chamber with a ~4CO2 atmosphere at constant total CO 2 concentration and constant specific activity. The distribution of t4C within shoots, roots and soil was determined for both plants after 4 and 6 weeks. After 6 weeks, 1.5% of the total amount ofl4C fixed by maize was found as a residue in the soil, while for wheat this figure was 2.0%.Rhizosphere ~4CO2 production was measured in a second experiment and plants were harvested after 3, 4, 5 and 6 weeks growth. The rhizosphere t4CO2 evolution by wheat accounted for some 20% of the total amount of fixed ~4C and was a constant value throughout the growth period. Rootderived products were slowly incorporated by the soil microbial biomass to a maximum of 20% of the residual soil 14C content after 6 weeks growth.

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