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Hot-spots in biological transformation of silica

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Role of silica in biodiversity hot-spot

The Okavango Delta is Africa's largest inland wetland and home to an exceptional biodiversity that is largely a result of its unique island–floodplain pattern. Although silicon (Si) plays a major role in island formation, its biogeochemistry in tropical wetlands remained a mystery until an EU-funded initiative investigated the phenomenon.

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Movement of Si from terrestrial habitats to the aquatic environment is biologically controlled. Vegetation takes up dissolved silica from soil and groundwater and stores it in biomass. The EU-funded 'Hot-spots in biological transformation of silica' (HOBITS) project was set up to study biological buffering of the Si cycle. HOBITS partners investigated biological Si cycling in Botswana's Okavango Delta, in southern Africa. A sampling expedition collected soil water, vegetation, soil and sediments, and surface water from three transects along typical island–floodplain gradients. A second expedition conducted a detailed study of biogeochemical processing across the Okavango Delta. An investigation into the impact of aquatic plants on the processing of Si and organic matter was also carried out. The full trans-Okavango transect carried out by the project enabled the detailed analysis of Delta’s water quality for the first time ever. First sampling campaign revealed large amounts of biogenic Si storage in the upper 30 cm of sediments and soil, particularly in grass-dominated sites. This corresponds to about 4 kg of Si per square metre in the biologically active top layers, indicating the wetlands’ capacity to retain biogenic Si. The data showed the importance of the recycling of nutrients in maintaining the Delta's huge biodiversity. The most abundant primary producers – diatoms and grasses, require Si for their production. Findings from the second sampling campaign revealed large variations in concentrations of Si, magnesium (Mg), potassium (K), sodium (Na) and calcium (Ca) unexplained by evaporation alone. Scientists sampled soil cores up to 6 m deep to track the fate of Si and other elements along an island–floodplain gradient. The results revealed that the role of surface biological processing of elements in the formation of the unique island–floodplain structure had been previously underestimated. Work conducted by the HOBITS project showed that vegetation plays a previously unknown role in Si retention in the Okavango Delta. This was a significant finding as retention of Si by vegetation is essential for supporting the basic food web. In addition, the biogeochemical patterns observed by HOBITS provided insight on the impact of hydrological and nutrient management on the Delta's productivity and biodiversity.

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