Speciation of Se and DOC in Soil Solution and Their Relation to Se Bioavailability

Abstract

A 0.01 M CaCl(2) extraction is often used to asses the bioavailability of plant nutrients in soils. However, almost no correlation was found between selenium (Se) in the soil extraction and Se content in grass. The recently developed anion Donnan membrane technique was used to analyze chemical speciation of Se in the 0.01 M CaCl(2) extractions of grassland soils and fractionation of DOC (dissolved organic carbon). The results show that most of Se (67-86%) in the extractions (15 samples) are colloidal-sized Se. Only 13-34% of extractable Se are selenate, selenite and small organic Se (<1 nm). Colloidal Se is, most likely, Se bound to or incorporated in colloidal-sized organic matter. The dominant form of small Se compounds (selenate, selenite/small organic compounds) depends on soil. A total of 47-85% of DOC is colloidal-sized and 15-53% are small organic molecules (<1 nm). In combination with soluble S (sulfur) and/or P (phosphor), concentration of small DOC can explain most of the variability of Se content in grass. The results indicate that mineralization of organic Se is the most important factor that controls Se availability in soils. Competition with sulfate and phosphate needs to be taken into account. Further research is needed to verify if concentration of small DOC is a good indicator of mineralization of soil organic matter.

Drying effects on selenium and copper in 0.01 M calcium chloride soil extractions

“…4A), indicating that there is an association between Se and DOC upon drying. Similar relationship between DOC and soluble Se has been observed using other dried soil samples from grassland (Weng et al, 2011). The absence of a correlation between DOC and soluble Se in the dried samples from potato field can be due to very low level and small range of DOC and soluble Se in the samples.…”

“…In the dried clay-peat soil around 3% of sub-nanometer Se measured in the acceptor is probably present as subnanometer organic Se associated with sub-nanometer Hy DOC. Previous DMT analysis using other dried soil samples from grassland has also shown that colloidal sized organic Se accounts for 67 to 86% of total soluble Se in CaCl 2 soil extracts, whereas selenate, selenite and small organic Se molecules account for the remaining 15 to 33% (Weng et al, 2011). The results of DMT analysis indicate that the increase of total soluble Se upon drying is mainly due to the increase of colloidal sized organic Se.…”

“…Simple organic acids, e.g. citrate, can pass the anion exchange membrane in the DMT and be detected in the acceptor solution (Van der Stelt, 2007), whereas colloidal-sized organic molecules, such as HA and FA cannot (Van der Stelt, 2007;Weng et al, 2011). The results indicate that most of the DOC fractions in the dried samples, including the Hy fraction, are colloidal in size (1 nm to 0.45 μm).…”

Selenium-rich dissolved organic matter determines selenium uptake in wheat grown on Low-selenium arable land soils

“…Among different Se forms present in soil, soluble Se is the main source available for plant uptake. However, in the soil solution, Se can be present as both inorganic selenate, selenite and as organic Se (Weng et al 2011). Probably not all soluble Se species can be taken up by plants directly.…”

Selenium speciation and extractability in Dutch agricultural soils

“…In soil solution, Se is present as selenate and selenite anions as well as organic Se (Stroud et al, 2012;Tolu et al, 2011;Weng et al, 2011). Soluble selenate and selenite are the major bioavailable chemical forms for plants, although some research showed that selenoamino acids such as selenomethionine and selenocysteine can also be taken up (Abrams et al, 1990b;Kikkert and Berkelaar, 2013;Williams and Mayland, 1992).…”