The influence of a small addition of argon (2-5%) on the parameters of a strongly non-uniform microwave discharge (with the electrode microwave discharge as an example) in nitrogen at reduced pressures was studied. Experiments showed that the small addition of Ar strongly affected the discharge: it increased in size, and the power absorbed in the plasma and the emission intensities of nitrogen bands reduced. A self-consistent 2D modeling of the discharge was carried out. The model included the Maxwell, Poisson and Boltzmann equations and a set of balance equations for neutral excited and charged plasma species. The processes involving vibrationally excited ground state molecular nitrogen were taken into account by the well-known analytic expression for the vibrational distribution of molecules in the diffusion approximation. The results of modeling and experiment were in qualitative agreement. Additional information about the discharge allowed us to explain the experimental results. It was shown that this effect was influenced by a strong spatial non-uniformity of direct electron impact ionization rate, difference in diffusion and mobility coefficients, and difference in diffusion and volume processes of the loss of the main ions. This effect can be observed in all types of discharges if these conditions are satisfied.