Soil available phosphorus drives above- and below-ground biomass trade-offs under different grazing regimes in alpine meadows

Biomass allocation reflects plant adaptation strategies to the environment and governs ecosystem responses to environmental changes. In alpine meadows, where grazing is a common disturbance, different grazing methods can significantly affect biomass allocation. However, it is unclear how different grazing patterns affect biomass trade-offs through specific pathways. In the Qinghai-Tibet Plateau, alpine meadows included five grazing methods: continued grazing (CG), grazing during the growing season (GG), grazing during the dormant season (GD), traditional grazing (TG), and grazing exclusion (GE). We measured soil enzyme activities, plant biomass, and soil physicochemical properties. Additionally, plant biomass trade-off strategies were investigated using a biomass trade-off model, and key influencing factors were identified using structural equation modeling. Our results indicated that: 1) Plant biomass distribution under different grazing patterns was consistent with the optimal partitioning theory. 2) Under grazing during the growing season, shoots allocated greater biomass, with a trade-off value of 0.016, whereas under traditional grazing, continued grazing, grazing exclusion, and grazing during the dormant season, roots allocated more biomass, with trade-off values of 0.001, 0.032, 0.043, and 0.009, respectively. 3)Biomass trade-offs were directly controlled by soil available phosphorus and soil cellulase activity, and indirectly by soil alkaline phosphatase, available nitrogen, and organic matter via regulation of these direct factors. 4) Soil available phosphorus is a key factor driving the biomass trade-offs of plant communities, with the total effect value of -0.863. Our findings on plant biomass trade-off strategies provide a scientific basis for optimizing grazing management to maintain grassland health and productivity.