Doctoral defence: Reti Ranniku "Impact of environmental conditions and soil microbiome on greenhouse gas fluxes from soil and tree stems in hemiboreal drained peatland forest"

On 28 August at 10:15, Reti Ranniku will defend her doctoral thesis "Impact of environmental conditions and soil microbiome on greenhouse gas fluxes from soil and tree stems in hemiboreal drained peatland forest" for obtaining the degree of Doctor of Philosophy in geography (physical geography). 

Supervisors: 
Associate Professor Kaido Soosaar (University of Tartu)
Professor Ülo Mander (University of Tartu)

Opponent: 
Professor Vincent Gauci (University of Birmingham)

Summary:
In this doctoral thesis, seasonal patterns of soil and tree stem greenhouse gas (GHG) fluxes were studied in a hemiboreal drained peatland forest, focusing more closely on the previously understudied winter period and spring freeze-thaw cycles. The study found that the soil was a net annual sink for CH4 but a source of N2O and CO2. Soil CH4 fluxes remained near-zero during the dormant season but showed significant CH4 uptake in the drier period. Seasonal measurements highlighted that soil hydrology had a long-term impact on CH4 dynamics, while temperature played a more short-term role. Rapid changes in soil hydrology influenced soil N2O fluxes, particularly during freeze-thaw events. Soil thawing increased N2O emissions, primarily due to incomplete denitrification under prolonged anaerobic conditions. Tree stems emitted all measured gases, with birch stems playing a more significant role than spruce stems. Stem CH4 and N2O fluxes exhibited isolated emission peaks, driven by prolonged wetter periods for CH4 and rapid hydrological changes for N2O. Stem CO2 release followed a seasonal trend, influenced by temperature and photosynthesis. Stem-emitted GHGs likely had a primarily belowground origin. Stem CH4 emissions offset nearly a third of the soil sink annually, rising to almost half during wetter periods, highlighting the strong impact of soil hydrological conditions on CH4 dynamics. Stem N2O fluxes responded more to short-term hydrological changes, and their contribution to total N2O emissions remained low. Stem fluxes significantly contributed to total CO2 release, relating to the growing season. Neglecting stem fluxes can lead to inaccurate estimations of forest GHG budgets, underscoring the necessity of integrating these measurements for a comprehensive understanding of GHG dynamics in forest ecosystems.