Thawing Effects on Soil Organic Carbon in Permafrost Peatlands (TESOCPe)

• Applicants: Professor Dr. Werner Borken; Dr. Cristian Estop Aragonés
• Subject Area: Soil Sciences ; Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 537682730

Permafrost peatlands are hotspots of soil organic carbon (SOC) stocks and their accelerated thaw promotes SOC mineralization potentially contributing to higher CO2 release to the atmosphere. However, there is great uncertainty regarding the magnitude of SOC loss and the accumulation of SOC from increased plant production following thaw in permafrost peatlands. So far, the SOC balance of thawing peatlands has only been studied in very few permafrost regions. It is known from previous studies that the SOC balance in different peatland types and thawing regimes is very variable and forecasts on SOC stocks dynamics for thawing permafrost are uncertain. Permafrost peatlands in Finnmark, Norway's northernmost county, are currently thawing rapidly, but SOC losses and recent SOC accumulation have not been quantified in this region. We will investigate permafrost peatlands in this region that have been deeply thawed by thermokarst or superficially thawed by active layer deepening. These thawing regimes reflect different mode of thaw and soil conditions. When the active layer deepens near the surface due to partial thaw and drainage, oxidative conditions arise for microorganisms that promote SOC mineralization. Thermokarst forms when the entire soil profile thaws, subsides and becomes water-logged resulting in anoxic conditions. Different approaches are independently pursued to quantify the SOC losses due to thaw. We will quantify in situ mineralization rates of previously-frozen SOC by measuring soil CO2 emissions and their 14C-CO2 signatures. These results provide evidence for SOC mobilization of both thaw regimes in permafrost peatlands. We expect peatlands that have thawed near the surface to show higher SOC losses than thermokarst. In addition, soil cores will be collected from undisturbed and thermokarst peatlands i) to characterize the development of peat accumulation by macrofossil analysis and 14C dating of plant litter, ii) to quantify SOC stocks, iii) to characterize the degree of humification of organic matter and iv) to characterize the mineralization potential for SOC and dissolved organic carbon (DOC). We expect i) lower SOC stocks, ii) higher degrees of humification, iii) lower SOC and DOC mineralization potentials and lower microbial carbon use efficiency in thermokarst peatlands due to enhanced SOC mineralization following thaw compared to intact permafrost peatlands. Finally, recent SOC accumulation rates are determined by 210Pb dating. We expect thermokarst peatlands to have recently accumulated SOC that partially compensates for previously-frozen SOC loss by thaw. We will study different thermokarst peatlands to verify whether the post-thaw SOC accumulation rate is related to the degree of peat subsidence. The balance of SOC loss and SOC accumulation as well as the mineralization potentials in the different thaw regimes can make a valuable contribution to improve forecasts of the future development of SOC stocks in permafrost regions.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr. Klaus-Holger Knorr