Peatland degradation indicated by stable isotope depth profiles and soil carbon loss

Peatlands play a significant role in the global carbon cycle. Since the last glacial maximum, peatlands in the northern hemisphere have accumulated organic matter as peat with about 550 Pg carbon in their soils. The degradation of peatlands either by anthropogenic activities or by changing climatic conditions results in changes of the biogeochemical cycles. Increasing temperatures, especially in the high northern latitudes, lead to the accelerated permafrost thaw and the degradation of palsa peatlands and may lead to a positive carbon-climate feedback. Peatland drainage induces oxic conditions and causes increasing carbon dioxide emissions resulting in a decline of soil organic carbon. In Europe more than 50 % of the former peatland area has been drained for agricultural or forestry use contributing a significant proportion to the national greenhouse gas emissions.
The main objectives of this study were to use depth patterns of stable isotopes, both carbon and nitrogen, as indicators of peatland degradation, and to calculate the carbon balance of degraded peatlands by different profile-based methods. Various depth profiles from peatlands in Northern Sweden, Central Finland and Northern Germany were sampled to investigate these research questions. These peatlands present different causes of peatland degradation, including changes in climatic conditions in the subarctic region, drainage for forestry in the boreal region and drainage for grassland management in the temperate region.
The natural abundance of stable isotopes, particularly stable carbon and nitrogen isotopes, is a commonly used indicator in soil sciences to investigate biogeochemical processes in soils and soil degradation. Depth profiles of stable carbon isotopes generally reflect organic matter dynamics in soils with an increase of δ13C with depth during aerobic decomposition and stable or decreasing δ13C values with depth during anaerobic decomposition of organic matter. In addition, the δ15N values are assumed to increase with depth in degraded peatlands due to aerobic decomposition and show uniform depth patterns under anaerobic decomposition in natural peatlands.
In the palsa peatlands in Northern Sweden, stable carbon isotope depth profiles indicated changes in the decomposition processes over time. Recent degradation due to accelerated permafrost thaw as well as historical changes in decomposition processes from anaerobic to aerobic are displayed in the δ13C depth profiles. The historical changes indicated the uplifting of the palsa peatlands by permafrost. Furthermore, the time of the permafrost uplifting was determined by peat accumulation rates between 100 and 800 years ago. In addition, stable nitrogen isotope depth profiles indicated the change in decomposition processes and showed perturbation of the soil when relating to C/N ratios. The mean ages of permafrost uplifting of the two palsa peatlands identified by the stable nitrogen isotope depth profiles fall in the period of the Little Ice Age.
A land use gradient was investigated in Northern Germany including a near-natural wetland, an extensively managed and an intensively managed grassland site, which have all formed in the same peatland complex. Vertical depth profiles of δ13C, δ15N, ash content, C/N ratio, bulk density and radiocarbon ages were studied to identify peat degradation and to calculate carbon loss. The δ13C depth profiles indicated aerobic decomposition in the upper horizons at all sites. Moreover, depth profiles of δ15N differed significantly between the sites with increasing δ15N values of the upper horizons concurrent to increasing land use intensity due to differences in peat decomposition and fertilizer application.
There are different methods and approaches to determine the carbon balance of degraded peatlands. The profile-based methods compare the degraded soil with a reference soil. Differences in biogeochemical soil parameters, such as ash content, bulk density and radiocarbon age, are used by the profile-based methods to estimate the soil carbon balance of degraded peatland sites.
Peat and carbon loss could be quantified by the combination of ash content and bulk density and is supported by the radiocarbon ages. Increasing carbon loss with increasing land use intensity was calculated by two different profile-methods with 11.5, 18.8-38.2 and 42.9 52.8 kg C m-2 at the near-natural site, the extensively used grassland site and the intensively used grassland site, respectively. However, current greenhouse gas fluxes measured by the chamber technique indicated a carbon gain at the near-natural site, a neutral carbon balance at the extensive used grassland site and a carbon source at the intensive used grassland site. The historical carbon balance was assessed by the profile-based methods whereas the present changes in the carbon balance are captured by the flux measurements. Moreover, the combination of both approaches pointed out that the carbon balance of the peatland has changed over time. All biogeochemical soil parameters indicated peat degradation at all investigated sites along the land use gradient, however, at varying degrees.
In Finland more than half of the peatland area was drained during the 20th century for forestry use. The Lakkasuo peatland, Central Finland, includes a minerotrophic and an ombrotrophic part, both of which were partially drained for forestry. In addition to the δ13C depth profiles, four different profile-based methods were applied, using differences in ash content or radiocarbon dated peat samples to calculate the carbon balance of the soil. The δ13C depth profiles indicated that both undrained sites are in a natural state and that both drained sites are enriched in 13C in the topsoil indicating aerobic decomposition. At the minerotrophic drained site all four profile-based methods indicated a carbon loss but of different magnitude (0.058 to 0.272 kg C m-2 yr-1). However, at the ombrotrophic drained site both radiocarbon methods suggested a carbon gain (0.139 to 0.182 kg C m-2 yr-1) whereas the two other methods indicated a carbon loss (0.061 to 0.270 kg C m-2 yr-1). The results confirm that in boreal peatlands drainage for forestry leads to a higher risk of losing carbon when the peatland is minerotrophic.
This thesis demonstrates that examining stable carbon isotopes is a useful way of identifying peatland degradation by various causes, and this approach can be used as indicator of the natural state of a peatland. The carbon balance calculations by different profile-based methods help determining the long-term carbon changes of degraded peatlands since the beginning of peatland drainage.