Grazing alters plant biomass allocation by regulating soil available nutrients and root biomass distribution in alpine meadows

• Belowground biomass responded more sensitively than aboveground to different stocking rates and grazing seasons. • Alpine meadow plant biomass allocation supported the optimal allocation hypothesis. • Plant biomass trade-offs were driven by soil available nutrients under different stocking rates. • Belowground biomass of the 0–10 cm soil depth contributed most to the changes in plant biomass trade-offs. • The warm-season grazing with the low stocking rate was a feasible grazing practice in alpine meadows. Plant biomass allocation is important for understanding the material cycle and turnover in grassland ecosystems under environmental changes. However, biomass allocation involves two main hypotheses, optimal and isometric allocation, that remain controversial, and how grazing management influences plant biomass trade-offs is poorly understood. Here, a five-year rotational grazing experiment was implemented in alpine meadows, involving three stocking rates (SR) and two grazing seasons, to test these hypotheses, examine how biomass allocation pattern responds to grazing, and identify key biotic and abiotic factors underlying changes in plant biomass trade-offs. The findings showed that the 8 sheep ha −1 SR in the warm-season grazing induced the allocation of more biomass to roots in 2014, while other treatments presented the opposite pattern. Furthermore, complex reactive dynamics of plant biomass allocation in relation to varying stocking rates and grazing seasons emerged across five years, lending support to the optimal allocation hypothesis. Plant biomass trade-offs were primarily determined by stocking rate and year, which modulated soil available nutrients. Additionally, belowground biomass within the 0–10 cm soil depth contributed most to these trade-offs, regulated by grazing season. Regarding biomass trade-offs, warm-season grazing with the low SR may be beneficial to better plant biomass allocation and to be a feasible grazing practice for alpine meadow ecosystems.