PhD Defence: Lina Avila Clasen

Thesis title: Controls of nitrogen fixation associated with bryophytes in terrestrial ecosystems

Supervisors: Kathrin Rousk and Anders Priemé

Assessment Committee:
Per Ambus (Chair), Geoscience and Natural Resource Management
Steven Perakis, USGS Forest and Rangeland Ecosystem Science Center, Corvallis, OR
Annika Nordin, SLU Umeå and Stora Enso

Abstract
Nitrogen (N) fixation is a key process contributing for global biogeochemical cycles. This complex process is performed by N-fixing bacteria that converts atmospheric N into a bioavailable form that other organisms can assimilate. These bacteria often associate with bryophytes, non-vascular plants that are key ecological components of many ecosystems. Here, the role of nutrient availability affecting N fixation rates, a process that requires substantial energy, is emphasized in tropical ecosystems. The findings revealed that N additions reduced moss-associated N fixation rates. Phosphorus, an energy source, showed variable effects and did not exhibit a direct positive relationship with N fixation rates. No molybdenum limitation was observed. Climate was found to be a stronger driver of N fixation rates. A steep mountain gradient with a natural decrease in temperature was used to investigate N fixation rates which were higher in lower elevations with dense cloud forests. The higher elevations, which constitute the páramo ecosystem, encountered greater fluctuations in climate, which seemed to drive distinct microbial communities adapted to those conditions. The short-term fate of N was also investigated. Mosses retained most of the N within their tissues, with different precipitation regimes not greatly affecting the short fate of N. To further investigate other forest compartments and forests in different successional stages, N fixation in soil, litter, and bryophytes living epiphytically in pumpwood trees were sampled from the two forests. Significant higher rates were measured in association with bryophytes compared to soil or litter and in younger forests. Higher light intensity was a strong driver of N fixation rates, possibly contributing to the recovery of the regenerating forests. This PhD thesis explores the crucial role of microbial processes and N fixation to tropical ecosystems, which are under significant threat from human-induced climate change impacting N pools and global biogeochemical cycles.