The timing of warming matters as much as its intensity for the annual carbon balance of a degraded raised bog

Abstract

Pristine peatlands function as natural carbon dioxide (CO2) sinks, but anthropogenic drainage turns them into sources of CO2, responsible for 2 %–5 % of global annual greenhouse gas (GHG) emissions. Complex interactions between vegetation, soil, climate, and hydrology produce highly variable CO2 budgets on different types of peatlands and between years. Abandoned drained peatlands are considered low-hanging fruits for rewetting due to expected high GHG emissions and low resistance to repurposing yet remain underrepresented in research. To close this gap in the literature, we measured 3 years (2023–2025) of CO2 and methane (CH4) fluxes alongside meteorological and hydrological conditions in a drained shrub-dominated ombrotrophic raised bog in northwest Germany, investigating carbon flux budgets and the main seasonal drivers of fluxes. Methane fluxes were negligible throughout, likely due to water tables consistently deeper than 15 cm. Annual CO2 budgets were highly variable: the site was a considerable source of CO2 in 2023 and 2025 (112.6 ± 14 and 47.6 ± 27.8 gCm-2 a-1) but a weak sink (-24.8 ± 15.1 gCm-2) in 2024. An anomalously warm spring in 2024 triggered an earlier onset of CO2 uptake and increased maximum CO2 uptake capacity from April to June. In contrast, warming later in the growing season increased CO2 emissions due to a stronger reaction of respiration than of photosynthesis to warming – highlighting how the timing of climate anomalies matters. Partitioning the effects of high air temperature (TA) and vapor pressure deficit (VPD) revealed that high VPD suppressed carbon fluxes in the first half of the growing season but not the second, while extreme TA did not limit gross primary production (GPP) or ecosystem respiration the way extreme VPD did. TA and solar radiation were the dominant daily flux drivers; water tables had marginal effects on daily or interannual carbon flux variability. Together, our results demonstrate that the timing of TA and VPD anomalies – mediated through vegetation responses – decisively shapes their impact on the carbon balance. These results will become increasingly relevant as climate extremes intensify with ongoing global warming.