Continental export of si to the coastal zone is closely linked to the ocean carbon sink and to the dynamics of phytoplankton blooms in coastal ecosystems. Presently, however, the impact of human cultivation of the landscape on terrestrial si fluxes remains unquantified and is not incorporated in models for terrestrial si mobilization. In this paper, we show that land use is the most important controlling factor of si mobilization in temperate European watersheds, with sustained cultivation ( > 250 years) of formerly forested areas leading to a twofold to threefold decrease in baseflow delivery of si. This is a breakthrough in our understanding of the biogeochemical si cycle: it shows that human cultivation of the landscape should be recognized as an important controlling factor of terrestrial si fluxes.
*Although silica (Si) is not an essential element for plant growth in the classical sense, evidence points towards its functionality for a better resistance against (a)biotic stress. Recently, it was shown that wetland vegetation has a considerable impact on silica biogeochemistry. However, detailed information on Si uptake in aquatic macrophytes is lacking. *We investigated the biogenic silica (BSi), cellulose and lignin content of 16 aquatic/wetland species along the Biebrza river (Poland) in June 2006 and 2007. The BSi data were correlated with cellulose and lignin concentrations. *Our results show that macrophytes contain significant amounts of BSi: between 2 and 28 mg BSi g(-1). This is in the same order of magnitude as wetland species (especially grasses). Significant antagonistic correlations were found between lignin, cellulose and BSi content. Interestingly, observed patterns were opposite for wetland macrophytes and true aquatic macrophytes. *We conclude that macrophytes have an overlooked but potentially vast storage capacity for Si. Study of their role as temporal silica sinks along the land-ocean continuum is needed. This will further understanding of the role of ecosystems on land ocean transport of this essential nutrient.
Silica is an essential element for marine life and plays a key role in the biogeochemistry of the ocean. Glacial activity stimulates rock weathering, generating dissolved silica that is exported to coastal areas along with meltwater. The magnitude of the dissolved silica export from large glacial areas such as the Greenland Ice Sheet is presently poorly quantified and not accounted for in global budgets. Here we present data from two fjord systems adjacent to the Greenland Ice Sheet which reveal a large export of dissolved silica by glacial meltwater relative to other macronutrients. Upscaled to the entire Greenland Ice Sheet, the export of dissolved silica equals 22 ± 10 Gmol Si yr−1. When the silicate‐rich meltwater mixes with upwelled deep water, either inside or outside Greenland's fjords, primary production takes place at increased silicate to nitrate ratios. This likely stimulates the growth of diatoms relative to other phytoplankton groups.
Estuaries are naturally highly dynamic and rapidly changing systems, forming a complex mixture of many different habitat types. They are very productive biomes and support many important ecosystem functions: biogeochemical cycling and movement of nutrients, mitigation of floods, maintenance of biodiversity and biological production. Human pressure on estuaries is very high. On the other hand, it is recognized that estuaries have a unique functional and structural biodiversity. Therefore, these ecosystems are particularly important for integrating sound ecological management with sustainable economics. These opportunities are explored for the Scheldt estuary, a well-documented system with an exceptional tidal freshwater area. In this article a description of the Scheldt estuary is presented, illustrating that human influence is intertwined with natural dynamics. Hydrology, geomorphology, trophic status and diversity are discussed, and possible future trends in both natural evolution and management are argued.
Recent research has emphasized the importance of terrestrial ecosystems in the global biogeochemical cycle of silica (Si). The production, retention, and dissolution of amorphous silica of biological origin in soils and vegetation effectively control terrestrial Si fluxes. However, surprisingly little is known about the role of wetlands in these processes. Wetlands are known hotspots for both nitrogen and phosphorus cycling, and there have been countless studies and numerous reviews on these nutrients worldwide. By bringing together previously scattered results, we show that wetland ecosystems may be as important for Si transport and processing as they are for other important biogeochemical cycles. Yet, the range of studied systems is small and incomplete. This constitutes a serious gap in our understanding of both coastal eutrophication and climate change, issues that are strongly linked to Si biogeochemistry. Ecosystem scientists and wetland biogeochemists around the world need to begin addressing these issues.
The annual fixation of dissolved Si (DSi) into terrestrial vegetation has been estimated to range from 60 to 200 Tmole, or 10-40 times more than the yearly export of DSi and biogenic Si (BSi) from the terrestrial geobiosphere to the coastal zone. Ecosystems form a large filter between primary mobilization of DSi from silicate weathering and its eventual export to the oceans, and a large reservoir of BSi accumulates in aquatic and terrestrial ecosystems. Although a number of synthesis activities within the last decade have discussed biological transformations in the terrestrial Si cycle, the timescales at which BSi is stored and recycled within ecosystems, BSi persistence and reactivity throughout soil profiles, the dependence of the BSi storage and recycling on ecological processes, the feedbacks to hydrology, the interaction with man's activities and ultimately the global relevance in Si budgets are poorly constrained. Here we discuss 5 key controls on the ability of ecosystems to filter and control the export of DSi: ecosystem biodiversity, BSi dissolution rates and reactivity, hydrology, interaction with the geosphere and anthropogenic impacts. These controls need to be further studied to better quantify the global and local importance of the terrestrial biogeochemical Si cycle and specifically the BSi reservoir in ecosystems.
Abstract. Human land use changes perturb biogeochemical silica (Si) cycling in terrestrial ecosystems. This directly affects Si mobilisation and Si storage and influences Si export from the continents, although the magnitude of the impact is unknown. A major reason for our lack of understanding is that very little information exists on how land use affects amorphous silica (ASi) storage in soils. We have quantified and compared total alkali-extracted (PSi a ) and easily soluble (PSi e ) Si pools at four sites along a gradient of anthropogenic disturbance in southern Sweden. Land use clearly affects ASi pools and their distribution. Total PSi a and PSi e for a continuous forested site at Siggaboda Nature Reserve (66 900 ± 22 800 kg SiO 2 ha −1 and 952 ± 16 kg SiO 2 ha −1 ) are significantly higher than disturbed land use types from the Råshult Culture Reserve including arable land (28 800 ± 7200 kg SiO 2 ha −1 and 239 ± 91 kg SiO 2 ha −1 ), pasture sites (27 300 ± 5980 kg SiO 2 ha −1 and 370 ± 129 kg SiO 2 ha −1 ) and grazed forest (23 600 ± 6370 kg SiO 2 ha −1 and 346 ± 123 kg SiO 2 ha −1 ). Vertical PSi a and PSi e profiles show significant (p < 0.05) variation among the sites. These differences in size and distribution are interpreted as the long-term effect of reduced ASi replenishment, as well as changes in ecosystem specific pedogenic processes and increased mobilisation of the PSi a in disturbed soils. We have also made a first, though rough, estimate of the magnitude of change in temperate continental ASi pools due to human disturbance. Assuming that our data are representative, we estimate that total ASi storage in soils has declined by ca. 10 % since the onset of agricultural development (3000 BCE). Recent agricultural expansion (after 1700 CE) may have resulted in an average additional export of 1.1 ± 0.8 Tmol Si yr −1 from the soil reservoir to aquatic ecosystems. This is ca. 20 % to the global land-ocean SiCorrespondence to: W. Clymans (wim.clymans@ees.kuleuven.be) flux carried by rivers. It is necessary to update this estimate in future studies, incorporating differences in pedology, geology and climatology over temperate regions, but data are currently not sufficient. Yet, our results emphasize the importance of human activities for Si cycling in soils and for the land-ocean Si flux.
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