Plant functional type composition drives summer carbon cycling in an ombrotrophic peatland

Abstract Peatlands are globally important carbon sinks because plant CO 2 uptake outweighs microbial decomposition. These ecosystems are characterised by a distinct microtopography of hummocks and lawns, supporting vegetation communities composed of various plant functional types (PFTs). In this study we assess the direct effects of PFT removal and microhabitats, and the indirect effects mediated through microbial community composition on CO 2 and CH 4 emissions. We utilised a long-term factorial vascular plant removal experiment in a Sphagnum -dominated peatland in southern Sweden. We used structural equation modelling (SEM) to analyse gas flux measurements and microbial community composition, and to assess both direct and mediated effects. Plots with intact vegetation exhibited the lowest CO 2 emissions, which increased progressively with ericoid, graminoid, and both ericoid and graminoid removal. Microhabitat type influenced microbial community structure, and only the removal of all vascular plants affected bacterial composition. However, neither CO 2 nor CH 4 fluxes were shown to be mediated by microbial community composition; instead, CH 4 emissions were directly shaped by microhabitat, with higher fluxes in lawns than hummocks. These results likely reflect the timing of sampling (August), when plant-driven CO 2 uptake dominates carbon cycling dynamics, and the limited sampling depth (~ 5 cm), which likely excluded deeper-residing microbial groups that are relevant to carbon cycling. Nevertheless, these results show that summer CO 2 emissions are primarily driven by vegetation composition, while microhabitat, i.e. water table and oxygen, differences are more important for CH 4 emissions.