Effect of re-wetting on greenhouse gas emissions from different microtopes in a cut-over bog in Northern Germany

Abstract

Under natural conditions, peatland ecosystems play an important role as a long-term net sink of carbon and nitrogen, storing 445-550 Gt of global carbon. Human activities, such as forestry or peat extraction, are known to affect peatland ecology and hydrology. These practices often lead to drainage, resulting in peat oxidization and enhanced greenhouse gas emissions (GHG) as long as the peatland area remains drained. Currently, the rewetting of drained peatlands is a common practice with the aim of re-establishing near-natural conditions and reducing peatland GHG emissions, especially of CO2 and N2O, which are enhanced under peatland degradation and the aeration of peat. At the cut-over drained bog, Himmelmoor (Quickborn, Germany), peat extraction started in 1780, and since the 1930s extraction has being occurring at a rate of about 38,000 m3 yr-1. Rewetting started stepwise here in 2004 by blocking drainage ditches with peat as well as creating polders surrounded by peat dams, and currently, more than 56 % of the former extracted central area has been rewetted. In this research project, five study sites in the Himmelmoor with different microtopes, land-use history, vegetation cover, and water table level were examined: a peat extraction area (E), a ditch refilled with peat (D), a peat dam (PD), and areas rewetted in 2004 (RV) and 2009 (R). In order to compare GHG emissions from rewetted and extraction areas, chamber measurements of CO2, CH4, and N2O fluxes were performed in the field over a two year period (2014-2015) in weekly to biweekly intervals. To complement these measurements, peat samples from rewetted and extraction areas were incubated under oxic and anoxic conditions in order to investigate their CO2 and CH4 production potentials. The results show that all study sites act as GHG sources, although large differences were identified between sites. For example, the total mean greenhouse gas emissions* over the two-year measurement period varied from 4.7 ± 1.8 t CO2-eq. ha-1 yr-1 at the rewetted site (R) to 38.1 ± 6.8 t CO2-eq. ha-1 yr-1 at the peat dam site (PD). However, in the longer term, it was found that the strong warming impact of GHG emissions from the extraction area had been reduced by more than 35 % only five years after rewetting. At all study sites, annual GHG emissions were dominated by CO2, which contributed between 50% and 74% of the mean annual emissions across the various sites. For a 1-hectare area, the annual emissions for rewetted sites were estimated at 123 ± 43 kg CH4, 3.01 ± 1.1 kg N2O, and 2.4 ± 0.8 t CO2; for ditches at 130 ± 16 kg CH4, 7.8 ± 3.3 kg N2O, and 6.6 ± 1.4 t CO2; and for drained bare sites (including peat dams) at 17.7 ± 10.8 kg CH4, 20.6 ± 4.3 kg N2O and 16.0 ± 2.9 t CO2. CO2 and N2O emissions from the dry bare peat dam areas were larger than previously observed for other harvested and drained bare peat sites in Europe. The estimated area-weighted mean annual GHG emission for the former extraction area and for the 74 ha investigation area as a whole amounted to 10.5 and 13.9 t CO2-eq ha-1 yr-1, respectively. Findings from the incubation experiments are in agreement with those from the chamber studies, with the mean CO2 production potential (for the top 1 m soil) of peat from the vegetated rewetted area incubated under anoxic conditions being considerably smaller (2.3 t CO2 ha-1 yr-1) than that observed in incubated peat material from the drained extraction area incubated under oxic conditions (8 t CO2 ha-1 yr-1). In terms of the success of rewetting practices, soil processes at sites that were rewetted 5- or even 25 years ago remain affected by previous management, and have not been fully restored to natural conditions. This practice was, however, found to have a significant effect on microbial activity and microbial biomass, as indicated by the microbial C and N concentrations in the upper horizons, which were 25-70 % higher in the rewetted bare and vegetated plots than in the drained plots. The findings of this study suggest that the full restoration of soil conditions as well as mire-typical microbial activity after ditch closing and rewetting will take at least decades to occur. The results of this study have been used to provide suggestions to improve restoration practices and enhance ecosystem services, such as reducing the height of dams.