Selenium speciation and bioavailability in Dutch agricultural soils: the role of soil organic matter

Selenium (Se) is an essential micronutrient for animals and humans. In the food chain, the intake of Se by animals and humans depends largely on Se content in plants, whereas the major source of Se in plants lies in the soil. Therefore, understanding Se bioavailability in soils for plant uptake and its controlling factors and mechanisms is important. The objective of this thesis is to study the amount, speciation, bioavailability, plant uptake and fertilization of Se in agricultural soils in the Netherlands and underlying controlling factors and mechanisms, to provide guidance for soil testing and fertilization recommendation for efficient Se management in agriculture. The majority of agricultural soils (grassland and arable land) in the Netherlands contains low total Se (i.e. in the range of Se deficient), which is predominantly present as organic Se. Only a small fraction of total Se is present as inorganic Se (mainly as selenite) and residual Se. In this thesis, the evidences of association between Se and soil organic matter in these low Se soils have been shown. The associations include: (1) the total Se content is positively correlated to soil organic matter content; (2) the solubility and extractability of Se in soils follow the solubility and extractability of soil organic C; (3) the majority of Se present in soils is in organic form, both in the soil solution and solid phase; (4) the distributions of Se and organic C in the different fractions of solid organic matter (i.e. humic acids, hydrophobic organic neutral, hydrophilic acids) and dissolved organic matter (i.e. hydrophilic acids and fulvic acids) are comparable; and (5) the Se richness in solid and dissolved organic matter are related to properties of soil organic matter from different land uses. The relatively high soil organic matter content in these low Se soils is likely responsible for these associations. In general, Se content in crops (e.g. grass and wheat) grown on grassland soils and arable land soils, respectively in the Netherlands is low due to low amount of bioavailable Se in the soils. Different soil parameters determine Se plant uptake in these low Se soils with predominantly organic Se, depending on the properties of Se-containing soil organic matter. The intensity parameter of Se-rich dissolved organic matter (DOM) in soil solution (i.e. Se to DOC ratio in 0.01 M CaCl2 extraction) determines Se plant uptake in soils containing Se-rich organic matter (e.g. potato arable land soils), whereas the buffer capacity of labile organic Se to supply Se-rich DOM in soil solution limits Se plant uptake in soils containing Se-poor organic matter (e.g. grassland soils). Further research is needed to confirm the generality of the conclusion above, because the two experiments were carried out under different conditions (pot experiment and field experiment), using different plant species (wheat and grass) and covering different soil types from different land uses (potato fields and grassland). Site-specific properties in the field in addition to soil parameters included in the current study may largely (> 50%) determine Se content in grass under field conditions, which is in contrast with the results of the pot experiment in which the soil parameter explains 88% of Se content in wheat shoots. In general, the content of Se-rich DOM in soils increases with the increase of soil pH (with the decrease of soil C:N ratio), and the amount of labile organic matter in soils that can resupply Se-rich DOM is determined by the amount of clay (and Fe-(hydr)oxide). NPK fertilization, as one of the external factors, can reduce Se plant uptake, especially in organic-rich soils. Selenium (as selenate) fertilization on grassland with N plus cattle slurry or NPK application shows a positive effect to increase Se content in grass grown on different soil types with a large range of total Se, pH, clay content and organic matter content. Selenium content in grass grown on different soil types upon Se fertilization becomes more similar than before the fertilization. The results indicate that the effectiveness of Se fertilization is only weakly modified by soil properties, probably due to the high solubility of selenate in the soils. Nevertheless, the Se fertilization tends to be slightly more effective on sandy soils than on clay and organic rich soils. This thesis has shown that the content and quality of soil organic matter play an important role in determining the amount, speciation and bioavailability of Se in low Se soils with predominantly organic Se. The results in this thesis can be used as guidance to develop soil testing and fertilization recommendation for efficient Se management, especially in low Se soils with predominantly organic Se, such as in Dutch agricultural soils.