Exploring the mechanisms through which plant species coexist and a few species are dominant while many others are rare has long been an interest of ecologists.Much effort has been devoted to niche differentiation or dispersal abilities,but few studies among common and rare plant species in the uptake of ammonium,nitrate and glycine across three Inner Mongolia grasslands 2000 km apart.We find that common species used mainly(> 70%)the most available soil nitrogen form,while rare species relied more on less abundant form.In addition,we find that common species were able to switch to use the dominant nitrogen source when soil dominant nitrogen form had changed,but rare species were not.These results suggest that common and rare species partition nitrogen uptake,and particularly,common species possess an important plasticity in nitrogen uptake.The contrasting nitrogen use strategies provide an explanation for why some plant species are common and others are not in the studied temperate grasslands.