Soil gravel content and plant species configuration control vegetation restoration in Qinghai-Tibet Plateau

Soil gravel content strongly affects ecological restoration; however, the response and mechanism of plant traits to soil gravel content under the sensitive and fragile natural environment of Qinghai-Tibet Plateau remains unclear. Herein, soils with three gravel content (10%, 30%, 50%) in the southeastern Tibetan Plateau were selected, and three plant species (one native plants of Elymus dahuricus (Ed), and two introduced ones of Festuca elata (Fe) and Medicago sativa (Ms)) were used in seven planting patterns with different proportions (Fe, Ed, Ms, Fe + Ed (1:1), Fe + Ms (2:1), Ed + Ms (2:1), Fe + Ed + Ms (2:2:1)). Plant traits, phytochemical properties and soil stoichiometric characteristics were measured to explore the interactive effects of soil gravels and plant species on vegetation restoration. Average plant height, coverage, shoot biomass, and total biomass were most affected by plant species (F = 277-611, p < 0.01), followed by gravel content (F = 90-195, p < 0.01) and their interaction (F = 5-51, p < 0.05); root biomass was most affected by gravel content (F = 130, p < 0.01). Among plant species, shoot and root biomass, total biomass overall decreased in the order of Fe + Ed + Ms>Fe>Fe + Ms>Fe + Ed>Ms>Ms + Ed>Ed. Plant total biomass, shoot biomass, root biomass and shoot/root ratio among different soils overall decreased in the order of low> high> medium gravel contents. All plant species were restricted by soil nitrogen except for Ed and Ed + Ms (N:P > 14). In addition, increasing the gravel content in the soil will increase the soil bulk density and reduce the total soil porosity. The total soil porosity is significantly positively correlated with the average plant height, coverage, aboveground biomass, and total biomass of plants (r = 0.78-0.91, p < 0.05). The total nitrogen content, total phosphorus (TP) content, and N: P in rhizosphere soil were significantly positively correlated with the average plant height, coverage, aboveground biomass, root biomass, and total biomass of plants (r = 0.70-0.97, p < 0.05), but soil N was significantly positively correlated with aboveground biomass and total biomass (r = 0.69, 0.71, p < 0.05), and there was no significant difference in soil TP content between them. The gravel content directly affects plant growth by changing bulk density and total porosity, but the combined effect of soil nutrients and plants affects plant growth. Therefore, optimizing the configuration of soil properties (mainly nitrogen and compactness) and plant species (isecologic niche plants) is an effective strategy for ecological restoration in the Qinghai-Tibet Plateau.