Impacts of Active Versus Passive Re‐Wetting on the Carbon Balance of a Previously Drained Bog.

Peatland drainage depletes large carbon stocks by increasing carbon dioxide (CO2) emissions from the soil. Restoration via re‐wetting could play an important role in climate change mitigation, reducing CO2 emissions and increasing C storage within peatlands. However, re‐wetting leads to a biogeochemical compromise between increased CO2 uptake, and enhanced methane (CH4) release. The extent of this compromise in re‐wetted ecosystems with differing environmental conditions is uncertain. To assess re‐wetting effects, we analyzed eddy‐covariance flux measurements from a temperate bog near Vancouver, Canada, from two sites that have undergone different restoration techniques. By the end of the 1‐year study period, the actively re‐wetted, wetter site, was a weak CO2 sink (−26.1 ± 6.1 g C‐CO2 m−2), and the passively re‐wetted, drier site, was near CO2 neutral (3.8 ± 3.1 g C‐CO2 m−2). Higher CH4 emissions at the wetter site led to a larger radiative balance on 20‐ and 100‐year time horizons, implying that the strong radiative effect of CH4 can offset CO2 sink strength on shorter to medium timeframes. However, long‐term radiative forcing (RF) modeling suggests sustained CO2 uptake by the wetter site will eventually lead to a cooling effect on the climate. Furthermore, modeling results emphasize that despite both re‐wetted peatland sites having a positive RF over century timescales, the lack of restoration would have resulted in a significantly larger RF beyond the first few decades following restoration. Results highlight the importance of actively re‐wetting disturbed peatlands to mitigate climate warming and can be used to inform management decisions. Plain Language Summary: Peatlands are important ecosystems in the global carbon cycle as their soils can store vast quantities of carbon. However, when soils in peatlands are disturbed via drainage, they dry and release carbon dioxide (CO2) into the atmosphere, contributing to global warming. Scientists have proposed re‐wetting drained peatlands to reverse this effect and promote CO uptake. This can be an issue, because wet conditions often lead to increased emissions of methane (CH4), a much more potent greenhouse gas (GHG). This study presents results comparing GHG and environmental measurements from two peatlands undergoing restoration near Vancouver, Canada—one wetter, and one drier site. Restoration of both sites has a climate benefit compared to not re‐wetting, however, results reveal that although the wetter site had higher CH4 emissions, it was a larger CO2 sink than the drier site, and therefore will eventually have a cooling effect on the climate on long timescales. These results are useful for identifying which environmental conditions in re‐wetted peatlands are most beneficial for the climate. Key Points: Of two rewetted peatland sites, wetter conditions at one site corresponded with it being a stronger carbon dioxide sinkThe wetter site was a larger methane source than the drier one, resulting in a greater radiative balance on 20‐ and 100‐year timeframesOver century timescales, the wetter site had a lower (eventually negative) radiative forcing, and overall rewetting had climatic benefits [ABSTRACT FROM AUTHOR]