Above- to belowground carbon allocation in peatlands shifts with plant functional type and temperature

Background: Northern peatlands have accumulated vast amounts of carbon (C) as peat. Warming temperatures may affect peatland C stores by increasing microbial decomposition of ancient peat through enhanced input of labile root exudates by expansion of vascular plants, thereby accelerating atmospheric warming. Aims: We set out to explore how much freshly assimilated C is allocated belowground by vascular plants, and if the above- to belowground allocation is affected by temperature and plant functional types. Methods: We traced the C allocation pathways of two dominant plant functional types (i.e. sedges and shrubs) in two peatlands under different temperature regimes by combining selective plant removal in mixed sedge-shrub vegetation and in situ ¹³C pulse-labelling. Aboveground to belowground C allocation as well as the C turnover were assessed by quantifying ¹³C in plant leaves and soil respiration and by measuring δ¹³C in dissolved organic C. A depth-resolved quantification of ¹³C in the peat soil gave additional insight into belowground C allocation patterns. Results: Temperature did not affect the rate at which ¹³C was assimilated into shoots, but higher temperature decreased the fraction of assimilated C that was allocated belowground by vascular plants. Sedges assimilated CO₂ faster into their shoot biomass (faster depletion in ¹³C in shoots) and allocated more of the assimilated ¹³C belowground than shrubs. Conversely, sedges retained this belowground allocated C better than shrubs, leading to lower ¹³C in soil respiration measured under sedges. Conclusions: Climate induced vascular plant expansion will increase input of fresh assimilates into the peat substantially, even though part of this effect will be offset by reduced above- to belowground allocation rates. If shrub density increases relative to sedges, fresh assimilates are more likely to be respired than translocated to roots where they could reach and, potentially mobilize, ancient C stored in deeper peat layers.