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

Publishing year: 2015
Language: English
Pages:
Publication/Series: Lundqua thesis
Document type: Dissertation
Publisher: Department of Geology, Lund University

Summary

Popular Abstract in Swedish

Inom forskningsområdet biogeokemi studerar man hur naturens olika grundämnen, såsom kol, kväve, fosfor och kisel ändrar form och transporteras genom landskapet för att via bäckar och vattendrag till slut nå havet. Tillsammans beskriver dessa olika processer, som sker på olika tidsskalor, från sekunder till flera miljoner år, ett grundämnes kretslopp (eller biogeokemiska cykel).

Grundämnet kisel (Si) är det näst vanligaste i jordskorpan, efter syre, och därför ingår det också i de flesta mineraler och bergarter som exempelvis granit. När mineraler och bergarter vittrar sönder frigörs kisel och blir löst kisel som sedan transporteras via vattendrag och grundvatten genom landskapet. På vägen mot kust och hav tas en del av detta lösta kisel upp av vegetation och av små mikroskopiska organismer, t.ex. kiselalger, som lever i jorden, för att bilda så kallat amorft kisel. När växter och kiselalger dör, ackumuleras sedan detta amorfa kisel till olika grad i jorden där det kan bevaras i tusentals år. Sammanfattningsvis innebär detta att biologiska processer på land kan reglera flödet av löst kisel till akvatiska ekosystem nedströms. Hur mycket löst kisel som når akvatiska ekosystem har stor betydelse för produktionen av kiselalger som utgör en av de vanligaste grupperna av växtplankton i sjöar och hav. Förändringar i flödet av löst kisel till akvatiska ekosystem kan därmed ha inverkan på näringsväven i sjöar och hav. Många av människan orsakade processer, som t.ex. byggande av dammar för vattenkraft, kan minska flödet av löst kisel som når havet. Förändringar i landskapet, såsom vegetationsskifte och förändringar i hur vattnet rinner till följd av klimatförändringar, kan också påverka flödet av löst kisel till akvatiska miljöer. I denna publikation ges en sammanfattande bakgrund till dess frågor.
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.

Disputation

2015-10-23
13:15
Pangea, Geocentrum II, Lunds Universitet
  • Sophie Opfergelt

Keywords

  • Earth and Related Environmental Sciences

Other

Published
  • Daniel Conley
  • Johanna Stadmark
  • Wim Clymans
  • ISSN: 0281-3033
  • ISBN: 978-91-87847-11-0 (pdf)
  • ISBN: 978-91-87847-10-3 (print)