Temporal phases of GHG emissions in rewetted fen sites using multiyear ecosystem carbon flux measurements

Rewetting drained peatlands is recognized as a leading and effective natural climate solution to curbing greenhouse gas (GHG) emissions. However, CO₂ source-to-sink transition is not necessarily detected in years immediately following rewetting and temporal dynamics in carbon budgets can be seen in rewetted systems. Here, we investigate long-term (2008 & 2013 - present) ecosystem flux measurements using the eddy covariance technique, revealing the temporal patterns of annual carbon dioxide (CO₂) and methane (CH₄) fluxes, in a rewetted peatland site in northeastern Germany. We show that site-level annual emissions presented in this study are only approaching the IPCC default Tier 1 emission factors (EFs) and those suggested for the German national inventory report after 13-16 years of rewetting when the optimum range of water level (close to the soil surface) is reached during the vegetation period. Overall, our results indicate that in addition to annual variation in soil temperature, vegetation development (post-rewetting successional vegetation dynamics) along with water table depth have the greatest effect on the carbon sink function. We further observe a transitional change into a new phase in the ecosystem carbon status throughout the study period that includes a source-to-sink transition of annual CO₂ fluxes in 2020. The decreasing trend for CO₂ fluxes is estimated at -0.37 t CO₂-C ha^-1 yr^-1 and -44 kg CH₄ ha^-1 yr^-1 for CH₄ emissions for the period until 2021.  While we found a strong reduction in CH₄ emissions in 2019 (following a severe drought), the negative trend in CH₄ emissions in the years before the drought event is still statistically significant (-17 kg CH₄ ha^-1 yr^-1). Accordingly, a considerable reduction of the 100-year annual global warming potential (GWP) and sustained-flux global warming potential (SGWP) was observed during the course of the study and potentially approaching a new steady-state phase within the last few years. In Germany, the published long-term datasets from rewetted peatlands only cover a period of ≤10 years after rewetting, thus, the duration of potential phases and development of future emissions are still unclear. This outlines the need for more long-term datasets to cover the source-to-sink transition in rewetted peatlands that also capture the impacts of likely future climate extremes. Here, we aim to establish a baseline to contribute a better understanding of the transitioning, complexity, and climate sensitivity of rewetted systems by analyzing the dynamics within the site and the inter-annual variability via their respective drivers. The introduction, monitoring, and targeted management of an ensemble of site characteristics (coverage/type of dominant vegetation, average water level along with microclimate and microtopographic conditions) is necessary in understanding the transient nature of such systems and further refine the existing static default EFs.