Professor Ülo Mander, Institute of Ecology and Earth Sciences, University of Tartu
Senior Research Fellow Dr. Martin Maddison, Institute of Ecology and Earth Sciences, University of Tartu
Professor Dr. Jos T.A. Verhoeven, Ecology and Biodiversity, Utrecht University, The Netherlands
Natural peatlands are an important global carbon (C) sink. Within the past century, however, many peatlands have been drained and exploited for various purposes, including peat extraction for fuel and horticultural use. After cessation of peat extraction activities, vast areas of degraded peat soils remain acting as persistent net sources of C and greenhouse gases (GHGs) due to enhanced peat decomposition and absence of plant CO2 uptake. Thus, there is a need for after-use strategies that mitigate the GHG emission from these areas; currently, however, knowledge about the impact of different after-use options and associated management practices on annual C and GHG balances of abandoned peat extraction areas is limited. The goal of this dissertation was to investigate the impact of i) fertilized and nonfertilized reed canary grass (RCG; Phalaris arundinacea) cultivation and ii) peatland restoration with high and low water table levels (WTLs) on the GHG exchanges (incl. CO2, CH4 and N2O) in abandoned peat extraction areas. The results showed that RCG cultivation reduced the net GHG emissions compared to bare peat soil. Moreover, fertilization increased the climate benefit potential of RCG cultivation since the additional nitrogen supply resulted in enhanced biomass production and associated CO2 uptake which largely exceeded the concurrent increase in N2O emissions. However, the annual C and GHG sink-source strength of the RCG cultivations varied between a sink in a wet and a source in a dry year highlighting its sensitivity to climatic conditions. Restoration of the peat extraction area reduced the net GHG emissions by half relative to bare peat soil. This was mainly due to a large reduction in heterotrophic respiration which advocates raising the WTL as an effective method to reduce the aerobic peat decomposition. On an annual scale, however, both restored treatments acted as net C and GHG sources indicating that CO2 uptake by the re-established vegetation was not yet able to compensate for the GHG emissions. Furthermore, the effect of contrasting WTLs on the C and GHG balances of the restored treatments was limited. Overall, this dissertation concludes that both RCG cultivation as well as peatland restoration may serve as effective methods to mitigate the negative climate impact of abandoned peat extractions areas.