Doctoral defence: Biplabi Bhattarai „Soil warming effect on belowground dynamics in subarctic grasslands: responses of roots, rhizomes, and rhizobiomes“

On the 25th of August at 14:15, Biplabi Bhattarai will defend her doctoral thesis "Soil warming effect on belowground dynamics in subarctic grasslands: responses of roots, rhizomes, and rhizobiomes", to obtain the degree of Doctor Philosophiae in Landscape Ecology and Environment Protection.

Supervisors:
Prof. Ivika Ostonen, University of Tartu

Prof. Boris Rewald, Mendel University in Brno, Brno (Czech Republic)

Prof. Marika Truu, University of Tartu

Opponent:
Dr. Lorna Street, The University of Edinburgh (Scotland)

Summary

Subarctic regions are warming faster than most other ecosystems globally as a result of climate change. Subarctic grasslands store significant amounts of carbon and nutrients in plant roots and soil microbes, which help maintain the overall ecosystem balance. Climate warming alters these belowground systems, accelerating decomposition and potentially amplifying the impacts of climate change. Previous scientific studies have primarily focused on broad community-level responses of plants and soil microbes to rising temperatures. However, the specific responses of different underground plant structures (for example, fine roots and rhizomes) and distinct microbial groups to warming have remained poorly understood. Moreover, the implications of these changes for carbon and nutrient storage and the timescale over which the ecosystems adapt to warming were unclear. This doctoral thesis revealed how fine roots, rhizomes, and soil microbes respond to soil warming in geothermally warmed subarctic grasslands. Measurements included biomass, carbon, nitrogen, and sugar content of fine roots and rhizomes, microbial growth rates both in the presence and absence of plant roots, and the dynamics of root growth over time. Belowground changes were linked to shifts in the overall plant community composition observed above ground. Novel outcomes from this study include: 1) Fine roots and rhizomes responded differently to warming: fine root biomass declined with warming, while rhizome biomass remained unchanged; 2) Plant roots stimulated microbial activity in ambient conditions, but such plant-root-microbe interaction was lost with long-term warming; 3) Sugar storage in belowground organs was dependent on both species and organ type, and of the three studied species, only Anthoxanthum odoratum increased sugar content in fine roots and rhizomes under warming; 4) The effect of warming on plant root growth was dependent on soil depth and season; 5) Changes in biomass, sugars and carbon and nitrogen content in belowground plant organs were closely associated with shifts in the plant community and soil microbial structure; 6) After about 60 years of soil warming, the amount of carbon and nitrogen stored in belowground plant organs returned to ambient levels, as the plant community adapted to long-term warming.