A field plant functional groups (PFGs) removal experiment was conducted in 2005-2007 to study the effects of plant diversity in a Leymus chinensis community of typical steppe in Inner Mongolia on the topsoil (0-10 cm) carbon (C) and nitrogen (N) pools. In the three successive years, the topsoil total C and N contents had a slight change (< 15%). They changed little in 2005, but were significantly higher in 2006 than in 2007 (P < 0.05). The topsoil NH4(+)-N content had a greater inter-annual change, being 80% higher in 2006 than in 2007, while the NO3(-)-N content had less significant change (P > 0.05). There was a positive linear relationship (P < 0.05) between the number of removed PFGs and the content of soil NO3(-)-N, which meant that the decrease of plant diversity increased soil NO3(-)-N content, and consequently, led to a possible soil nitrogen loss through leaching.