Abstract: Taking three succession plant communities in Hulunbeier Stipa krylovii steppe under different grazing intensities as test objects, this paper studied the underground biomass and root morphological characteristics of the communities and their constructive species. With increasing grazing intensity, both the community species composition and the root functional group types tended to be simplified. The underground biomass of the three communities presented a T-shape spatial distribution. The constructive species of the communities under different grazing intensities had an obvious substitution, with S. krylovii, Cleistogenes squarrosa and Allium polyrhizum as the constructive species under light, moderate and heavy grazing, and the root/shoot ratios of the constructive species decreased with increasing grazing intensity, being 0.47, 1.0, and 4.1 under light, moderate and heavy grazing, respectively. The root number, root volume, root biomass, specific root length, and root length density also varied with different grazing intensities. Moreover, the root/shoot ratio and the specific root length showed a significant correlation with the content of soil available nitrogen (P<0.05).

Key words: Mixed cropping, Gramineous plants, Root exudates, Apoplast Fe, Root Fe(Ⅲ) reducing capacity