Water Table and Vegetation Shape Peatland CO2 and CH4 Fluxes With Water Table Modulating Temperature Sensitivity.

Peatland rewetting can reduce CO2 loss but increase CH4 emissions. How vegetation shapes this trade-off and how the water table modifies temperature sensitivity remain unclear. We analysed 14,643 observations from 285 peatland sites. CH4 emissions increased more steeply with rising water-table level (WTL) under graminoid dominance, while the pattern of declining ecosystem respiration differed between vegetation classes. Combined CO2-equivalent emissions (GHGE; respiration + CH4 at GWP100 = 28) declined by 73.9% as WTL rose from 60 cm below to 15 cm above the peat surface and were still declining at the wet end of the analysed range. The decline persisted under GWP20, although the magnitude of the decline above the peat surface depended strongly on the CH4 weighting. The within-site analysis further showed that apparent CH4 temperature sensitivity (Q10) increased by 14.9% per 10 cm rise in WTL (95% CI 6.9%-23.5%), whereas respiration Q10 showed no detectable WTL dependence. A supplementary comparison within non-graminoid-dominated bogs showed that fitted CH4 Q10 increased with WTL where graminoids were present and decreased where graminoids were absent or negligible. Under +3°C warming, fixing CH4 Q10 at its value at WTL = -10 cm gave a predicted warming multiplier that was 24.7% larger at WTL = -63 cm and 11.8% smaller at +20 cm than the WTL-dependent formulation. These results suggest that assessments of peatland rewetting under warming should consider both vegetation differences in emission responses and the WTL dependence of CH4 temperature sensitivity.