Terrestrial Si dynamics in the Arctic: a study on biotic and abiotic controls

Sammanfattning: Silicon is the next most abundant element in the Earth’s crust and its biogeochemical cycle is linked with that of carbon. Further, silicon is a beneficial nutrient for plants in terrestrial ecosystems and a key nutrient for diatoms in aquatic ecosystems. During the last decade the important role of terrestrial vegetation in controlling Si fluxes downstream aquatic environments, via incorporation of Si into biomass (as amorphous Si) and subsequent storage in soil, has been realized. Due to the high prevalence of high Si-accumulating plants, cold temperatures and perenially frozen soil conditions, Arctic terrestrial ecosystems is hypothesized to store a significant fraction of the global soil ASi stock. The Arctic environment is highly sensitive to climate change, with unknown effects for terrestrial Si cycling. Hence, in this thesis we utilized archived soil samples collected from different geographical regions of the Arctic tundra and continuous permafrost region. By combining results obtained through soil chemical analysis with literature review this thesis provide a conceptual framework for how climate change may alter the biological component of terrestrial Si cycling in Arctic regions underlain by permafrost. Further, permafrost thaw can mobilize previously frozen soil material initiating biogeochemical processing of the newly thawed material, such as dissolution of plant derived amorphous silica stored in soil. Hence, an additional aspect of this thesis is to shed light on the potential biotic control (i.e. microbial influence) on plant derived ASi dissolution rates during litter degradation. This question was explored by utilization of microcosm laboratory experiments. Dependent on land cover type, we found total ASi storage to range between 1,030 - 94,300 kg SiO2 ha-1 in Arctic shrub/graminoid tundra and peatland ecosystems. Further, the first estimate of total ASi storage (0 - 1 m) in the northern circumpolar tundra regions is presented in this thesis. Our estimates, based on upscaling by vegetation and soil classes provide an estimated storage of 219 to 510 Tmol Si, which represents 2 - 6 % of the estimated global soil ASi storage. The results also show that the majority of the total ASi storage is allocated to the mineral subsoil, indicating that pedogenic rather than biogenically derived Si fractions dominate the ASi pool in the Arctic. Furthermore, the results suggest that at least 30 % of the total ASi pool is allocated to the permafrost layer, thus potentially representing an additional pool of Si that will become available for biogeochemical processing in a future warmer Arctic. Regarding the influence of microbes (bacteria and fungi) on amorphous silica dissolution during plant litter decomposition, we find that microbes can reduce the apparent release of Si and that the reduction in Si release increases with greater microbial colonization and decomposition of litter. This result is contrary to predicted results and common beliefs (i.e. that microbes can enhance Si release rates during litter decomposition). While the work carried out herein do not allow for the exact mechanism behind this pattern to be resolved, the results indicate that microbes may influence the availability of released Si. Overall, the work carried out in this thesis fills some of the existing knowledge gaps regarding the size and geographical/landscape distribution of the Arctic ASi pool, its significance in a global context as well as how microbes can influence Si release during plant litter decomposition, which previously were understudied.