It is noted that plant roots make a significant contribution to the formation of technozems during self-overgrowing of the technogenic substrate. The rate of decomposition of a mixture of thin roots (<0.5 mm thick) of meadow arbuscular mycorrhiza species (AM) in comparison with forest ecto mycorrhiza species (EM) in the power station ash substrate was studied in the laboratory 150-day experiment. The differences in the rates of decomposition of root mixtures, as well as in carbon (C) and nitrogen (N) content before and after decomposition in different variations of the experiment have been revealed. AM species have higher losses of biomass, nitrogen and carbon in contradistinction to ЕM species.