A greenhouse rhizobox experiment was carried out to investigate the fate and turnover of 13 Cand 15 N-labeled rhizodeposits within a rhizosphere gradient from 0-8 mm distance to the roots of wheat. Rhizosphere soil layers from 0-1, 1-2, 2-3, 3-4, 4-6, and 6-8 mm distance to separated roots were investigated in an incubation experiment (42 d, 15°C) for changes in total C and N and that derived from rhizodeposition in total soil, in soil microbial biomass, and in the 0.05 M K 2 SO 4 -extractable soil fraction. CO 2 -C respiration in total and that derived from rhizodeposition were measured from the incubated rhizosphere soil samples. Rhizodeposition C was detected in rhizosphere soil up to 4-6 mm distance from the separated roots. Rhizodeposition N was only detected in the rhizosphere soils up to 3-4 mm distance from the roots. Microbial biomass C and N was increased with increasing proximity to the separated roots. Beside 13 C and 15 N derived from rhizodeposits, unlabeled soil C and N (native SOM) were incorporated into the growing microbial biomass towards the roots, indicating a distinct acceleration of soil organic matter (SOM) decomposition and N immobilization into the growing microbial biomass, even under the competition of plant growth. During the soil incubation, microbial biomass C and N decreased in all samples. Any decrease in microbial biomass C and N in the incubated rhizosphere soil layers is attributed mainly to a decrease of unlabeled (native) C and N, whereas the main portion of previously incorporated rhizodeposition C and N during the plant growth period remained immobilized in the microbial biomass during the incubation. Mineralization of native SOM C and N was enhanced within the entire investigated rhizosphere gradient. The results indicate complex interactions between substrate input derived from rhizodeposition, microbial growth, and accelerated C and N turnover, including the decomposition of native SOM (i.e., rhizosphere priming effects) at a high spatial resolution from the roots.

The organic matter of five low-moor peat soils and one eutrophic raised-bog peat soil was chemically characterized by C fractionation and ion-exchange chromatography of amino acids and carbohydrates. C fractions were related to potential denitrification, D,,,, as a

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