PhD Defence: Johan Emil Kjær
Thesis title: Carbon dynamics in wetlands - patterns, drivers, and effects of restoration
Supervisor: Lars Båstrup-Spohr
Co-supervisor: Kaj Sand-Jensen
Assessment committee:
Ole Pedersen (chair), Department of Biology, Univerviity of Copenhagen
Anke Günther, Universität Rostock
Dominik Henrik Zak, Aarhus Universitet
Abstract:
Wetland soils contain enormous organic carbon pools because of steady carbon supply, but slow decomposition under anoxic conditions. Human activities have led to the degradation of organic carbon across vast wetland areas through drainage, resulting in intense greenhouse gas emissions. Restoring wetlands has the potential to reestablish them into carbon-sequestering natural areas, with the added benefits of increased biodiversity, nutrient retention and transformation, and flood control. However, restoration might not result in wetlands returning to their pristine state, and restored wetlands may become hotspots for methane emission; a powerful greenhouse gas (GHG). Although the main drivers of greenhouse gas emissions are firmly established, much remains to be learned about the effects of different hydrological regimes, the links between plant functional traits and carbon cycling, and the effects of restoration on the carbon cycling in riparian wetlands. I explore these topics in the three chapters of this thesis:
Chapter I investigates the spatiotemporal variations of methane (CH4) and carbon dioxide (CO2) emissions in fen areas experiencing different inundation regimes. It reveals that continuous water cover enhances CH4 emissions and reduces CO2 emissions, while periodic inundation has the opposite effect. We also find a substantial spatiotemporal variation in bubble flux (ebullition) of CH4. In this study, we used automated floating chambers and emphasize the importance of high-frequency measurements to estimate the bubble flux of CH4.
In Chapter II the focus is shifted to the interplay between environmental variables, plant functional traits, and carbon cycling in restored wetlands. The findings indicate that environmental conditions primarily dictate carbon cycling, with certain root traits showing significant differences between natural and restored sites, but with no significant impact on decomposition or primary production.
Chapter III assesses whether riparian wetlands can reestablish carbon cycling processes akin to those in natural wetlands 12-17 years after rewetting. Annual budgets of GHGs reveal that grazed sites, both near-natural and restored, generally act as carbon sinks with a net cooling effect on the climate. Conversely, ungrazed sites tend to be carbon sources, highlighting the influence of grazing on carbon sequestration in restored wetlands.
