PhD defence: Wenyi Xu
Wenyi Xu defends his thesis,
Nitrogen cycling and greenhouse gas fluxes in Arctic tundra soils. - Effects of experimentally deepened snow and tundra fire
Supervisor:
Professor Per L. Ambus, IGN
Assessment committee:
Assistant Professor Laura Kristiina Karhu, University of Helsinki - Finland
Senior Scientist Michael Dannenmann, Karlsruhe Institute of Technology - Germany
Professor Anders Michelsen (chair), Department of Biology, UCPH
Summary:
The Arctic is currently experiencing rapid increases in air temperature and changes in the precipitation patterns. Consequently, an increase in winter snowfall may be expected in the Arctic. Due to enhanced thermal insulation of deepened snow, soil temperature during winter becomes warmer, which could accelerate soil nitrogen (N) cycling rates and increase the availability of N during the subsequent growing season. The climate change in the Arctic also increases the frequency and severity of fire in many tundra regions. Fires can immediately remove vegetation, release carbon (C) and N into the atmosphere, and disrupt ecosystem N and other nutrient cycling.
This thesis addresses the effects of increased winter snow precipitation and tundra fires on soil N-cycle processes and GHG emissions in Arctic tundra ecosystems. Soil gross N cycling rates in response to experimentally deepened snow were investigated and compared among contrasting tundra sites across the Arctic. The fate of post-fire mineral N was assessed using stable isotopic labelling (15N) in a dry heath tundra (Disko, Greenland) and post-fire soil greenhouse gas emissions were also investigated.
The deepened snow enhanced soil gross N cycling rates in both winter and summer. In addition, soil N cycling rates were stimulated more strongly in wet tundra ecosystems. Tracing the fate of applied 15N following the fire suggests an increase in post-fire soil N retention, which was probably due to the increased incorporation of N into microbial biomass. The responses of plant N uptake to the fire differed among dominant shrub species. Moreover, fire increased soil NO3−-N, NH4+-N and phosphate (PO43−-P) concentrations as well as nitrous oxide (N2O) and carbon dioxide (CO2) production after two years. This work is an important contribution to improve our understanding of C and N biogeochemistry of tundra ecosystems in a changing Arctic and helps to better predict both permafrost carbon-climate and permafrost nitrogen-climate feedbacks.
A digital version of the PhD thesis can be obtained from the PhD secretary Mikala Heckscher at mikala@ign.ku.dk