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Silica fluxes and wetlands: assessment of a missing processor in the global Si cycle

Final Activity Report Summary - SWAMP (Silica fluxes and wetlands: assessment of a missing processor in the global Si cycle)

Wetlands can modify and control nutrient fluxes between terrestrial and aquatic ecosystems, yet little is known of their potential as biological buffers and sinks in the biogeochemical silica cycle. This limited knowledge of the biological of Si cycling in wetlands is a critical knowledge gap in the rapidly evolving concept of the terrestrial Si cycle and the direct link to carbon cycling and ocean productivity. The overall objective of the proposed research was to increase our understanding of the critical role of wetland ecosystems in the biogeochemical cycling of Si. Through investigation of biogeochemical processes in a variety of wetland ecosystems, the research provided an integrated view of the role of wetlands in the biogeochemical Si cycle.

We investigated the storage and recyling of amorphous silica (ASi) in a central-European riparian wetland (Bierbza National Park, Poland) and in boreal wetlands in Northern Sweden. The variation in storage of ASi in the soil of a floodplain wetland in Bierbza National Park was significantly controlled by two factors: dominance of sedges and grasses and distance to the river. Highest ASi storage was found near the river and in sites with a dominance of grasses and sedges, plants which are well known to accumulate ASi. The management practice of mowing reduced the amount of variation attributed to both factors. ASi controlled the availability of dissolved silica (DSi) in the porewater, and thus potentially the exchange of DSi with the nearby river system through both diffusive and advective fluxes.

In boreal wetlands in Northern Sweden, we found enormous stocks of ASi in the upper soil layers (up to 11 % of dry weight), in the form of diatom frustules and plant ASi. A consistent exponential decrease in ASi concentrations was observed with increasing depth in the soil profile. An inverse modelling approach showed that 30 % of the DSi export from the Muddus watershed is directly related to ASi dissolution. Concurrent analysis of N and C indicates a faster turnover in and a higher leakage from the ASi pool. The magnitude of the biological buffering we observed is unprecedented and supports the emerging paradigm of the importance of biological uptake of DSi governing the export of DSi from terrestrial ecosystems.

Interestingly, detailed study of diatom assemblages showed diatoms flourish in boreal wetlands when higher vegetation is experiencing environmental stress, thus creating isolated hotspots for nutrients, even in peat characteristic of poor fen and bog vegetations. During such transitions, diatom diversity declines but ASi content increases. With climate change impacting most heavily on high-latitude regions, substantial environmental stress can be expected. Our results imply that a transition in diatom communities and a much higher productivity could be expected under such conditions.

Our results show that storage and recycling of ASi in wetland ecosystems can differ significantly on small spatial scales. Human management interferes with the natural control mechanisms. Our study demonstrates that wetlands have a large potential to modify the fluxes of both DSi and ASi along the land-ocean continuum and supports the hypothesis that wetlands are important ecosystems in the biogeochemical cycling of silica.