PhD defence: Relena Ribbons

Relena Ribbons defends her thesis,

Seeing the forest for the trees: Tree species effects on soil microbial communities and nutrient cycling dynamics

Assessment Commitee:
Professor Richard Bardgett, University of Manchester, UK
Associate professor Mona N. Högberg, Swedish University of Agricultural Sciences, Sweden
Associate professor Klaus Steenberg Larsen, IGN, University of Copenhagen, DK (observer)
Senior Lecturer Dr. Paula Roberts (chair), SENRGY, University of Bangor, UK

Supervisors:
Professor Morag McDonald, School of Environment, Natural Resources and Geography, University of Bangor, Bangor, UK
Professor Lars Vesterdal, IGN

Summary:
Tree species influence soils above and belowground communities through leaf litter and root inputs. Soil microbial communities can directly influence tree growth and development through processes such as decomposition of leaves, and indirectly through chemical transformation of nutrients in soils as an influence of altered C:N ratios due to leaf litter inputs. This thesis aims to document some of the mechanisms by which trees influence soil microbial communities and nitrogen cycling processes like gross and net ammonification and nitrification. This thesis also aims to determine the role of site nitrogen status on modulating those tree species effects. The effects of tree species on ammonification and nitrification rates in forest floors and mineral soils were explored, and related to functional genetic markers for ammonia-oxidation by archaea and bacteria (amoA AOA and AOB), bacterial denitrification (nirS and nirK), and the general markers for bacteria (16S) and fungi (ITS). Two paired high-resolution laboratory methods were used to investigate the relationships between trees, soils, and the microbial communities, including molecular techniques such as quantitative polymerase chain reaction (qPCR) to target gene abundances in soils, and 15N pooldilution experiments to understand how ammonium and nitrate are produced and consumed in soils. Soil samples were collected from two common garden experiments, EP571 in Canada (Ribbons et al. 2016) and Denmark, and both 15N and qPCR-based techniques were used to determine tree species effects and attribute N cycling processes to the abundances of functional genes. At EP571, western red cedar forest floor nitrogen transformation rates differed from Douglas-fir (Pseudotsuga menziesii), Sitka spruce (Picea sitchensis), and western hemlock (Tsuga heterophylla), which corresponded with western red cedar (Thuja plicata) had highest abundances of bacterial 16S and amoA AOA genes. A manipulative mesocosm (the Rhizotron) in Wales was used to determine how seedlings species mixtures and monocultures influenced tree growth, soil physical properties and soil microbial community structure and function within the first three years of growth. Within the Rhizotron experiment both alder (Alnus rubra) and Douglasfir grew taller when grown in mixtures compared with monocultures of each species. Sycamore maple (Acer pseudoplatanus) acquired slightly greater aboveground biomass when planted in mixtures with common oak (Quercus robur), which attained less biomass in mixtures than monocultures. C:N ratios of leaves, stems, roots, and rhizospheric soils were determined to see if mixtures influenced C:N storage in trees. Rhizospheric soil microbial communities (including bacterial and fungal markers and the 4 genes tied to N cycling) were compared among the 4 tree species in the Rhizotron. Soil samples for microbial analyses were collected before seedlings were planted, and just before the experiment was harvested. These data show differences in height, biomass and C:N ratios between species can be observed at a seedling growth stage, but microbial communities may require longer exposure to develop. Lastly, the Bangor Diverse experiment was used to further explore diversity and mixture effects on soil microbial communities and N transformations. We found few mixture or monoculture tree species effects on mineral soil microbial communities or net nitrification or ammonification rates. Collectively, these research papers shed light on the important functional role of soil microbes in forest soil N cycling. This thesis also highlights the use of isotope and microbial techniques for parsing out relationships between site, tree species identity and ecosystem functions, with the largest links observed between gross ammonification and microbial communities.

The thesis is available for inspection at the PhD administration office 03.1.353, Øster Voldgade 10, 1350 CPH K