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Urban runoff.

Doctoral defence of Taija Saarela, MSc, 5.5.2023: Aquatic ecosystems could benefit from biochar-based water purification

The doctoral dissertation in the field of Environmental Science will be examined at the Faculty of Science, Forestry and Technology, Kuopio Campus.

What is the topic of your doctoral research? Why is it important to study the topic?

This thesis investigates the effects of catchment characteristics on dissolved organic matter (DOM) microbial degradability and greenhouse gas dynamics in northern freshwater ecosystems. Under the changing climate, the processes regulating the production and consumption of global greenhouse gases carbon dioxide (CO2) and methane (CH4) require better understanding to provide more comprehensive estimates of future aquatic greenhouse gas emissions.

What are the key findings or observations of your doctoral research?

The results of this thesis highlight the important role of DOM characteristics and microbial degradability in northern freshwaters, and further shed light on factors controlling the production-oxidation processes of CO2 and CH4 in freshwater rivers and lakes. This thesis also provides evidence on potential solutions to reduce increasing DOM and nutrient leaching from forested peatlands by demonstrating effective adsorption of dissolved nutrient compounds onto biochar surfaces.

How can the results of your doctoral research be utilised in practice?

This thesis provides new information on potential methods for mitigating the impacts of increasing DOM and nutrient load on aquatic systems caused by forest management practices. This thesis tested the adsorption-based purification of peatland forest runoff water using biochar and found that biochar effectively adsorbs both organic nitrogen and inorganic nitrate-nitrogen from runoff water. The results also show that the adsorption of nitrogen onto biochar surfaces increases with increasing initial concentrations. Therefore, aquatic ecosystems exposed to high nitrogen loads from fertile peatlands could particularly benefit from biochar-based water purification.

What are the key research methods and materials used in your doctoral research?

The data for this thesis has been collected from study sites located in the boreal and subarctic regions of Finland. To investigate the effect of DOM source and composition on its microbial degradability and CO2 production, the combination of potential CO2 production measurements and molecular-level characterization of DOM with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was conducted in two subarctic river ecosystems that represent contrasting types of catchment characteristics. In addition, this thesis applied stable isotope techniques and 16S rRNA gene sequencing to reveal the controlling factors for CH4 production and oxidation in a typical, seasonally oxygen-stratified boreal lake. This thesis also assessed the potential of biochar for capturing the nutrients and DOM released from drained boreal peatlands to runoff water. The studies concerning the use of biochar were conducted as a part of a research project "Biochar, circular economy and the reduction of environmental impacts of forestry" (2018-2021, PI Marjo Palviainen, Helsingin yliopisto).

The doctoral dissertation of Taija Saarela, MSc, entitled The effect of catchment characteristics on the dynamics of dissolved organic matter (DOM) and greenhouse gases in northern freshwater ecosystems will be examined at the Faculty of Science, Forestry and Technology, Kuopio Campus. The opponent will be Professor Jan Karlsson, Umeå University, Sweden, and the custos will be Professor Jukka Pumpanen, University of Eastern Finland. Language of the public defence is English.

Public examination

Dissertation (PDF)