Tibetan sedges sequester more carbon belowground than grasses: a 13 C labeling study

Background and Aims The Kobresia meadows widely distributed on the Tibetan Plateau are very tolerant to intensive grazing. The plant communities in these meadows are currently undergoing a shift in dominance from Cyperaceae (sedges) to Gramineae (grasses) species. We investigated the effects of the replacement of sedges by grasses in an alpine meadow on the allocation of photosynthetic carbon (C) in various plant and soil pools. Methods We selected patches (50 cm × 50 cm) of two vegetation types dominated by sedges ( Kobresia capillifolia , K. humilis and Carex spp., accounting for 88% of the total shoot biomass) and grasses ( Elymus nutans , Poa crymophila and Leymus secalinus , accounting for 86% of the total shoot biomass), respectively, in an alpine pasture grazed by yaks ( Bos grunniens ) and sheep ( Ovis aries ) in the winter season (November–June). Photoassimilate partitioning in various plant and soil pools was compared between sedge- and grass-dominated patches in the peak growth season (August) after labeling of plants in a 13 CO 2 atmosphere. Results The sedge-dominated patches had 85% smaller aboveground biomass but a much larger ratio of root (in the 0–50 cm soil depth) to shoot biomass (23.0 under sedges vs. 9.5 under grasses) than the grass-dominated patches. However, similar 13 C amounts were assimilated by sedges (1651 mg C m −2 ) and grasses (1575 mg C m −2 ) during 2 h of labeling, showing a greater capacity to assimilate C per unit of aboveground biomass in sedges. Over the period of 32 days after labeling, 13 C allocation to roots and soil was consistently greater under sedges than under grasses. By day 32 after labeling, the total 13 C amount remaining in roots and soil down to 50 cm depth was greater under sedges (78.5% of the total 13 C assimilated) than under grasses (65.7%). Conclusions Although similar C amounts were assimilated by two plant functional groups, sedges transferred more C to roots and soil (through rhizodeposition) than the grasses. This reflects the greater root-to-shoot ratio of sedges than grasses. Consequently, the ongoing replacement of sedges by grasses in plant communities will decrease soil C sequestration in the Kobresia meadows of the Tibetan Plateau.