Influence of the rhizosphere on microbial biomass and recently formed organic matter

We have aimed to quantify the effect of roots on the size of the soil microbial biomass, and their influence on the turnover of soil organic matter and on the extent of the rhizosphere. We sampled a sandy clay loam topsoil from two subplots with different treatment histories. One had a normal arable fertilization record, the other had received only inorganic nitrogen fertilizer but no phosphorus and potassium for 30 years. Glucose labelled with C-14 was added to both samples which were then incubated for 4 weeks before the soil was packed in cylinders and planted with ryegrass. In both soils, microbial biomass at the root surface doubled during the first 8 days. At day 15, the microbial biomass had further increased in the fertile soil, and the rhizosphere effect had extended 2.5 mm into the fertile soil, but to only I mm. in the infertile soil. The microbial C-14 increased threefold near the roots in the fertile soil as a result of assimilation of previously formed microbial residues, but in the infertile soil there was no increase. There was a close relation between the increase in microbial C-14 and a decrease in C-14 soluble in 2 M KCl, indicating that the microbial residues were more weakly adsorbed in the fertile soil. We conclude that the increased microbial population living near the root surfaces re-assimilated part of the C-14-labelled microbial residues in the fertile soil. In the infertile soil, microbial residues resisted decomposition because they were more strongly sorbed on to soil surfaces.