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A pot experiment was carried out (1) to compare C and N yield of different plant parts, nutrient concentrations, and root colonization between the non-mycorrhizal mutant P2 (myc − ) and the symbiotic isoline Frisson (myc + ), (2) to investigate the effects of arbuscular mycorrhizal fungi and growing pea plants on microbial decomposition of 15 Nlabeled maize residues, and (3) to follow the distribution of the added substrate over different soil fractions, such as particulate organic matter, soil microbial biomass, and microbial residues. Yields of C in straw, grain, and roots of myc + peas were significantly higher by 27%, 11%, and 92%, respectively, compared with those of myc − peas. The δ 13 C values in the different plant parts were significantly higher in myc + than in myc − tissue with and without maize. Application of labeled maize residues generally resulted in 15 N enrichment of pea plants. At the end of the experiment, the ergosterol concentration in roots of mature peas did not differ between the two isolines, indicating similar colonization by saprotrophic fungi. The decomposition of added maize residues was significantly reduced by the myc − peas, but especially by myc + peas. The formation of microbial residue C was increased and that of microbial residue N was reduced in the presence of plants. The insufficient N supply to soil microorganisms reduced decomposition of maize residues in the presence of peas, especially myc + peas.

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