The aim of the current research was to assess the impact of root exudates of widely distributed natural plant species, a monocot (couch grass) and a dicot (dandelion), on the total amount of carbon (C) and nitrogen (N) in the rhizosphere soils of these plants when they are grown separately or in close proximity to each other. A field trial was carried out to study the short-term variations in the concentrations of C and N in the soil resulting from the activity of exudates produced by the plant roots and soil contamination by RbCl. During daytime, the total amount of C and N in the rhizosphere soil of both plant species was found to be constantly decreasing, from 7.0% to 4.5% (C) and from 0.3% to 0.2% (N). Probably, plant growth in the soil contaminated by RbCl and possible additional effect of organic compounds produced by roots of couch grass and dandelion caused the decrease in C concentration in the rhizosphere soil, while changes in the soil N concentrations were insignificant.