Abiotic versus biotic changes: What are the main drivers of peat carbon transformations under vascular plants?

Peatland carbon (C) sink function is threatened by the concurrent changes in abiotic (temperature and moisture) and biotic (vegetation and soil biota community shifts) factors. We assessed the relative importance of microclimate and substrate quality gradients on the activity and vertical distribution of two crucial peat decomposers (enchytraeids and microorganisms) and their potential implications for C exports (CO₂ and dissolved organic carbon (DOC)) at three peatland habitats dominated by different vascular plant communities (sedge, grass, shrub). We studied this by turning intact peat blocks upside down and thus, reversing the substrate quality gradient but leaving the microclimate gradient along the peat profile intact. Our results revealed that the abiotic/biotic regulation on C dynamics in peatlands is habitat-dependent: (i) at the wettest (sedge-dominated) habitat, temperature was the main factor driving decomposition and only during the summer period CO₂ production was stimulated; (ii) at the grass habitat, the higher substrate quality prompted mesofaunal activities and caused the greatest C losses as DOC; (iii) at the driest (shrub-dominated) habitat, enhanced aerobic conditions led to a greater release of CO₂ to the atmosphere. These findings indicate that the two pathways of C losses are not linked and while warmer temperatures and oxygen availability have a significant stimulant effect on CO₂ emissions, DOC production was mainly driven by substrate quality, with plant-derived labile sources triggering bottom-up effects on decomposition. We suggest that the biotic control on the peatland C sink function must be central in model parameterisation for unravelling the fate of peatlands at global scale.