European Journal of Soil Science,2013doi:10.1111/ejss.12051 Microbial selenium transformations in seleniferous soils J.W.F e l l o w e s a,R.A.D.P a t t r i c k a,C.B o o t h m a n a,W.M.M.A l L a w a t i a,b,B.E.v a n D o n g e n a, J.M.C h a r n o c k a,J.R.L l o y d a&C.I.P e a r c e c
a School of Earth,Atmospheric and Environmental Sciences and Williamson Research Centre for Molecular Environmental Science, University of Manchester,Manchester,M139PL,UK,
b Higher College of Technology,Ministry of Manpower,Muscat113,Sultanate of Oman,and
c Pacific Northwest National Laboratory,Richland,Washington99352,USA
Summary
Selenium(Se)is an essential trace element for animals and displays a narrow range between dietary deficiency
and toxicity.The toxicity of Se depends on its bioavailability,which is directly related to its oxidation states,
of which four occur in the environment(Se VI,Se IV,Se0and Se II−).Microbial communities drive the cycling
of Se between these oxidation states.In order to investigate the effect of microbial activity on Se cycling in
the environment,afield site in County Meath,Ireland,was identified with anomalously large concentrations
of Se as a result of weathering of black shales within the Lucan formation,leading to cases of Se toxicity
in farm animals.Soil cores were extracted from the site for Se speciation and microbial community analysis
prior to microcosm experiments to assess Se stability and microbial Se transformations.Selenium was present
as a recalcitrant,reduced organic phase that was strongly coordinated with carbon,concordant with suggested
hypotheses of Se phyto-concentration within a clay-lined,postglacial marshland.Selenium was not mobilized in
microcosm experiments,and supplementation with Se VI resulted in rapid reduction and removal from solution
as Se0.Additional electron donors did not affect Se stability or removal from solution,although nitrate did
hinder Se VI reduction.Terminal restriction fragment length polymorphism analysis indicated a significant shift
in microbial community after amendment with Se VI.This work extends the current knowledge of Se cycling
in the environment,and provides information on the bioavailability of Se in the soil,which determines Se
content of foodstuffs.
Introduction
Selenium(Se)is an essential trace nutrient,present in a range of seleniferous proteins and enzymes but the small range between human dietary defficiency(<40μg day−1)and toxicity(>400μg day−1)requires careful control,especially when monitoring human diets and supplementing feedstuffs for livestock (Fordyce,2007).The bioavailability to plants and,thus,toxicity of Se in crops is largely dependent on local geochemical conditions. In aqueous solution,such as groundwater and soil-pore waters, Se is commonly found as the Se IV and Se VI oxyanions,with the latter being predominant and mobile under more oxic,alkaline conditions(Oremland et al.,2004;Fordyce,2007;Lenz&Lens, 2009).Under reducing conditions,insoluble Se0and Se II−mineral phases are expected(Fordyce,2007;Lenz&Lens,2009).V olatile organic species,such as dimethylselenide,are also found in soils and contaminated ground and formed as a result of biotic
Correspondence:J.W.Fellowes.E-mail:Jonathan.Fellowes@
Received15August2012;revised version accepted25February2013methylation(Lenz&Lens,2009).Biological transformations are a major driving force in the change of Se from one species to another,and a variety of biochemical mechanisms are responsible for the large range of seleniferous compounds found in the environment(Li et al.,2008;Lenz&Lens,2009).
Global soil Se concentrations are small,typically between0.1 and2.0μg g−1(Girling,1984;Berrow&Allan,1989),resulting in the need to amend Se-deficient agricultural soils to increase dietary uptake(Broadley et al.,2006;Li et al.,2008).The underlying lithology largely determines the Se content of overlying soils (Fordyce,2007),and soils formed above Se-rich rocks such as black shales and coal measures,when coupled with evapo-or phyto-concentration mechanisms,can contain elevated Se concentrations in the mg g−1range(Fordyce,2007;Lenz&Lens, 2009).Accordingly,reports of Se toxicity in humans and livestock have been recorded in such areas(Crinion,1980;Rogers et al., 1990;Dhillon&Dhillon,1991;Fordyce et al.,2000).
Cases of Se toxicity in animals in Ireland date back to the late 19th century(Parle&Fleming,1983),and Rogers et al.(1990)list seven counties affected by large soil Se concentrations.Fleming (1962)identifies a site near Trim,County Meath,with a soil Se
©2013The Authors
Journal compilation©2013British Society of Soil Science1
