Selenium Distribution and Speciation in Dutch Agricultural Soils.

Selenium (Se) is an essential micronutrient for livestock and human. Application of Se fertilizer on grassland soils with relatively low available Se, such as Dutch soils, is one way to increase Se intake by livestock (through grass). Understanding Se distribution and speciation in soil phases (i.e. soil solution and solid phases) and the key factors is an approach to understand solid – solution distribution of Se, which would be useful to estimate Se fertilizer recommendation. The research aimed to (1) develop methodology to determine and quantify Se distribution and speciation in soils, i.e. single extractions, not sequential extractions; (2) understand Se distribution and speciation in Dutch agricultural soils (i.e. grassland and arable land soils) and the key factors. Single extractions using 0.01M CaCl₂; ammonium oxalate – oxalic acid; and aqua regia were performed on 42 grassland soils and 41 arable land soils, except for 1M NaOCl pH 8 extraction that performed on selected soils. Se speciation analyses in CaCl₂ and ammonium oxalate extractions were performed on selected soils using Donnan membrane technique (DMT) and HPLC coupled with ICP-MS, respectively. Se distribution in Dutch agricultural soils showed that on average 1.5% of total Se (aqua regia) is in soil solution and around 98.5% is in soil solid phases. Se in soil solution is mainly present as colloidal sized organic Se (>90% of total soluble Se) and some small Se (i.e. inorganic selenite, selenate, and/or small organic Se). In soil solid phase, <5% of total Se (aqua regia) is present as inorganic selenite mainly adsorbed onto Fe/Al-oxides; > 85% is present as Se associated with organic matter; and the rest is in the form of refractory Se. The results imply that organic Se is the major pool in Dutch agricultural soils, which is most likely due to high organic matter content.

Keywords: selenium distribution, selenium speciation, soil solution, solid phase