Occurrence and chemistry of selenium in Finnish soils
Hartikainen, Helinä (2005)
Hartikainen, Helinä
Julkaisusarja
Agrifood Research ReportsMaa- ja elintarviketalous
Numero
69
Sivut
s. 18-24
MTT
2005
Tiivistelmä
Soil selenium is ultimately derived from the parent material in the bedrock, wherefore its content is markedly dependent on the origin and geological history of soil and dictated by the mineralogical characteristics of the parent material, weathering degree of mineral constituents and soil formation processes. In Finland, the bedrock is characterized by an abundance of plutonic and metamorphic rocks that are typically low in selenium. However, mineral soils of contrasting texture can be expected to differ in their element concentrations, because the various particle-size classes differ in the mineralogy. Generally, the clay fraction consists of minerals higher in selenium than minerals in the coarse fractions. Furthermore, the clay fraction is also rich in aluminium and iron oxides that decrease the mobility of selenium through their high sorption tendency. In organogenic soils, the native selenium content varies depending on the origin of the soil. The chemical characteristics explain the low bioavailability of selenium in Finnish soils irrespective of their total selenium content. Selenide in rocks is oxidized during weathering predominantly to selenite. Only in arid regions under very oxic condition it can be oxidized up to selenate. These two species decisively differ in their reaction mechanisms and mobility. Selenate is retained very weakly and is easily leached. Selenite, in turn, is specifically retained on oxide surfaces by ligand exchange, which reduces its mobility and bioavailability. Furthermore, our soils are high in organic matter, an efficient electron source, wherefore selenate added in fertilizers will be easily reduced to selenite and retained. This reaction patter diminishes the plantavailability but, on the other hand, the leaching losses. Thus, we can conclude that, similarly as with phosphorus, the main selenium losses from fields take place with eroded soil particles transferred with surface runoff water. The chemical retention in peaty soils is limited by the low amount of binding metals and minerals. Selenium losses from our soils as volatile compounds have been reported to be small.
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