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Sedge replacement by grasses accelerates litter decomposition and decreases organic matter formation in alpine meadow soils

The dominance of sedges in Tibetan meadows is decreasing with the increasing abundance of grasses, mainly due to anthropogenic activities, nitrogen deposition, and climate warming. We investigated the effects of this vegetation shift on litter decomposition (in the field), mineralization to CO2, and litter carbon (C) incorporation into microorganisms (under laboratory conditions) under sedges and grasses at community and species levels. Litter C incorporation into soil organic carbon (SOC) was estimated using 13C labelling of the vegetation community. After one year of field incubation, a smaller proportion of shoot litter C was lost in sedges than in grasses at the species level and smaller proportions of root C were lost under sedges at community and species levels. Roots decomposed markedly slower than shoots across communities or species under sedges and grasses. Smaller proportions of the shoot and root C from sedges were incorporated into SOC than from grasses, and root C was less incorporated into the SOC than shoot C across communities. Over the 38‐day laboratory incubation, total microbial respiration and assimilation of litter C changed similarly to the C loss between litter types under the field condition. Across community and species levels, litter C loss in the field and incorporation of litter C into microorganisms under laboratory conditions all increased with initial phosphorus content in the litter and decreased with lignin content. Considering greater allocation of photosynthates belowground by sedges than grasses, we demonstrated that the ongoing replacement of sedges by grasses in the vast Tibetan alpine meadows is unfavorable for the maintenance of SOC storage.