Carbon and climate implications of rewetting a raised bog in Ireland

Abstract

Peatland rewetting has been proposed as a vital climate change mitigation tool to reduce greenhouse gas emissions and to generate suitable conditions for the return of carbon (C) sequestration. In this study, we present annual C balances for a 5-year period at a rewetted peatland in Ireland (rewetted at the start of the study) and compare the results with an adjacent drained area (represents business-as-usual). Hydrological modelling of the 230-hectare site was carried out to determine the likely ecotopes (vegetation communities) that will develop post-rewetting and was used to inform a radiative forcing modelling exercise to determine the climate impacts of rewetting this peatland under five high-priority scenarios (SSP1-1.9, SS1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5). The drained area (marginal ecotope) was a net C source throughout the study and emitted 157 ± 25.5 g C m −2 year −1 . In contrast, the rewetted area (sub-central ecotope) was a net C sink of 78.0 ± 37.6 g C m −2 year −1 , despite relatively large annual methane emissions post-rewetting (average 19.3 ± 5.2 g C m −2 year −1 ). Hydrological modelling predicted the development of three key ecotopes at the site, with the subcentral ecotope predicted to cover 24% of the site, the sub-marginal predicted to cover 59% and the marginal predicted to cover 16%. Using these areal estimates, our radiative forcing modelling projects that under the SSP1-1.9 scenario, the site will have a warming effect on the climate until 2085 but will then have a strong cooling impact. In contrast, our modelling exercise shows that the site will never have a cooling impact under the SSP5-8.5 scenario. Our results confirm the importance of rapid rewetting of drained peatland sites to (a) achieve strong C emissions reductions, (b) establish optimal conditions for C sequestration and (c) set the site on a climate cooling trajectory.

Long-term flux measurements suggest dynamic emission factors are needed for rewetted peatlands

“…Seventeen years after rewetting, our site is still in a transitional phase in which a CO2 sourceto-sink shift took place. While the CO2 sink strength is increasing and annual CH4 emission is declining, this confirms that the time needed for a restored ecosystem to return to its long-term CO2 sink function may vary from years to several decades 41,42 . Though the ecosystem at our site has shifted to only a weak annual CO2 sink during the last two years of measurements (2020: -1.25 ± 0.09 and 2021: -0.46 ± 0.07 t CO2-C ha -1 yr -1 ), the avoided CO2 emission (over time) is already substantial when comparing to the net CO2 emissions from drained temperate nutrient-rich sites (3.6 t CO2-C ha -1 yr -1 : 1.8 -5.4, 95% confidence interval, n = 13) 4 .…”

Peatland restoration increases water storage and attenuates downstream stormflow but does not guarantee an immediate reversal of long-term ecohydrological degradation

“…Peatland restoration has been found to promote carbon dioxide sequestration 16 , 18 , but restoration effects are most positive where restoration increases vegetation cover which increases photosynthesis. The capacity for this change is greatest where the site is bare or sparsely vegetated prior to restoration 17 .…”

“…We hypothesised that a disruption in hydrological connectivity by peat blocks would reduce flow from the site (peak discharge and lag times) 9 ; this would raise and stabilise water table depths 9 , 14 , 15 , in turn this would decrease gaseous carbon losses 14 , 16 – 18 —with any increase in methane offset by a reduction in the net ecosystem exchange of carbon dioxide 16 , 18 . Raised water tables would also reduce the production of dissolved and gaseous carbon by peat soil decomposition (as measured by heterotrophic respiration) 19 reducing aquatic carbon losses 20 thereby improving water quality.…”

Active afforestation of drained peatlands is not a viable option under the EU Nature Restoration Law

“…Rewetting is always a choice between continued emissions of long-lived GHGs (CO 2 and N 2 O) versus emissions of a short-lived GHG (CH 4 ). In the short-term, rewetting will not result in net cooling because of increased CH 4 emissions, but will in the long-term (decades to centuries) (Wilson et al 2016 , 2022 ; Günther et al 2020 ; Ojanen and Minkkinen 2020 ). However in most cases, rewetting peatlands will pay off immediately, or after a very short time period, i.e.…”