Identifying drivers of root community compositional changes in semiarid grassland on the Loess plateau after long-term grazing exclusion

Grazing exclusion has been widely studied to have impacts on grassland aboveground community characteristics, however little is known about responses of grassland root community to long-term grazing exclusion. We selected five grasslands with different grazing exclusion time (0, 5 years, 9 years, 22 years, and 30 years) to determine root traits (biomass, length, surface area) in plant functional group and plant community level, soil properties of 0-30 cm depth and relationship between root traits and soil properties. Our results showed that grazing exclusion increased grassland root biomass, root length density and root surface area, accompanying declined plant species richness. After grazing exclusion, with perennial bunchgrasses (PB) being predominant in root community all the time, proportion of perennial rhizome grasses (PR) increased and proportion of perennial forbs (PF) declined. Grasses had higher specific root length and specific root surface area than that of forbs. Grazing exclusion significantly increased soil water content, soil organic carbon, total soil nitrogen and available soil phosphorus. Grasses were mainly correlated with soil organic carbon and nitrogen, while forbs were mainly influenced by soil phosphorus. Our results highlight the grasses' enhancing effect on grassland root productivity, and the critical role of soil nitrogen and phosphorus in regulating root community compositional changes in semiarid grassland after long-term grazing exclusion.