Disputation: Xiang Lu

Trial lecture

The candidate will first give a trial lecture on the following topic: 'Is the color of boreal lakes still increasing? An updated status on levels, causes, implications and future development of lake color in boreal lakes.'

Time and place for the trial lecture: 28 May, 2026, 10:15 AM, Nucleus, Bikuben, Kristine Bonnevies hus.

Main research findings

Boreal lakes receive and process substantial inputs of terrestrial derived dissolved organic matter (DOM), yet few studies systematically investigate their carbon dynamics and greenhouse gas (GHG) emission. This thesis integrated long-term empirical datasets, in‑situ high‑frequency monitoring, intensive field campaigns, dark incubations, and stable carbon isotope analyses to study the carbon mass balance and dynamics in lake Langtjern and assess its source-sink function in the catchment carbon budget.

We found terrestrial DOM input drives in-lake DOM levels whereas water residence time governs DOM removal rates. The lake is a substantial source of GHGs, and its carbon evasion is dominated by CO₂ (>99%) with CH₄ < 1%. In‑lake CO₂ production is the primary source of emitted CO₂, and sediment respiration is the largest contributor. The microbial mineralization rate of lake DOM is driven by temperature, and regulated by solar radiation and C:N of DOM. We anticipate the influence of wetting (negative) may outweigh that of warming (positive) in regulating lake DOM removal rate. The increasing hydrological flushing could shift sediments from carbon sink to carbon source. Extreme rainfall events can substantially increase lake GHG emissions by promoting lake mixing. Under a wetter climate, catchment soil can be a net carbon source.

Adjudication committee

• Deputy Director Blaize Denfeld, Swedish University of Agricultural Sciences (SLU), Sweden
• Professor Gunnhild Riise, Norwegian University of Life Sciences (NMBU), Norway
• Professor emeritus Stein Kaartvedt, Department of Biosciences, University of Oslo (UiO), Norway

Chair of the defense

• Professor Ketil Hylland, Department of Biosciences, University of Oslo (UiO), Norway

Supervisors

• Senior Research Scientist Heleen A. de Wit, The Norwegian Institute for Water Research (NIVA), Norway & Associate Professor at Centre for Biogeochemistry in Anthropocene (CBA), Department of Biosciences, University of Oslo (UiO), Norway
• Professor Dag O. Hessen, Centre for Biogeochemistry in Anthropocene (CBA), Department of Biosciences, University of Oslo (UiO), Norway
• Senior Environmental Advisor François Clayer, Norwegian Geotechnical Institute (NGI), Norway