Grazing intensity modifies alpine grassland fine root traits on the Qinghai-Tibetan Plateau

Newly-growing fine roots are essential absorption organs, but their responses to grazing at varying soil depths in alpine grassland remains unclear. Soil samples were taken from 0-10 cm and 10-20 cm depths in alpine grassland within the Lhasa River Basin, Tibet, China. We examined how grazing intensity affected root characteristics (biomass, architecture, morphology, and chemical properties) and root-economic-spectrum by analyzing roots in ingrowth bags. The results revealed that grazing intensity significantly affected root morphology at different soil depths. In the bottom layer, moderate grazing increased root average diameter by 21% but decreased specific root length (SRL) and specific root area (SRA) by 30% and 20%, respectively, indicating that bottom-layer roots became more selective in nutrient acquisition. However, heavy grazing decreased the top layer of root starch content, root length, root surface area, root volume, and root biomass by 12%, 37%, 40%, 44%, and 56%, respectively, but increased SRL and SRA by 71% and 69%, indicating that top-layer roots became morphologically less efficient for nutrient acquisition. Conversely, heavy grazing significantly reduced the bottom layer of root biomass by 68% but increased root length, root surface area, root volume, and root tissue density by 63%, 68%, 73%, and 137%, respectively, and slightly raised root nitrogen content, suggesting bottom-layer roots had greater longevity and resilience. Overall, grazing intensity modified the root characteristics and their root-economic-spectrum across different soil depths. These findings demonstrate grazing intensity affects root functional traits and nutrient acquisitions, highlighting the importance of adjusting grazing intensity for sustainable alpine grasslands management.