Abstract: To reveal the difference of fine root biomass among different artificial forests and the driving factors in hilly loess region, we collected fine root samples from plantations with different tree species and stand ages. Fine root biomass and its vertical distribution, community diversity, soil physicochemical properties as well as their relationships were investigated in eight artificial forests, including 40-, 30- and 15-year-old Caragana korshinskii; 45- and 15-year-old Robinia pseudoacacia; 15-year-old Platycladus orientalis; 40-year-old mixed shrubs (Caragana korshinskiiand Prunus armeniaca) and 15-year-old mixed Robinia pseudoacaciaand Platycladus orientalis forest. The results showed that fine root biomass in the 0-30 cm soil layer in artificial forests with same age followed the order of 15-year-old Platycladus orientalis > 15-year-old Robinia pseudoacacia> 15-year-old Caragana korshinskii.The fine root biomass of Caragana korshinskii and Robinia pseudoacacia forests increased significantly with the increases of stand age (P<0.05). The fine root biomass of 40-year-old Caragana korshinskii and 45-year-old Robinia pseudoacacia increased by 57.8% and 41.0% compared with that of 15-year-old Caragana korshinskiiand 15-year-old Robinia pseudoacacia, respectively. There was no significant difference of the fine root biomass between the mixed forests and pure forests of similar stand age. The fine root biomass significantly decreased with the increases of soil depth. The results of RELATE (a nonparametric form of Mantel test) and “Best” model selection procedure showed that the relationships between fine root biomass and soil physicochemical properties were significant (r=0.303,P<0.01), with soil TN and SOC contents showing the greater impacts on fine root biomass. There was no significant relationship between fine root biomass and community diversity in different artificial forests. Our results suggested that fine root biomass of artificial forests can increase with the increases of stand age and thus soil carbon and nitrogen contents.

Key words: aggregate organic carbon, desert steppe, rotation method., soil aggregate