SELENIUM GEOCHEMICAL RELATIONSHIPS OF SOME NORTHERN NEVADA SOILS

7 Pages·1994·3.6 MB·English

by  Stephen Poole

GreatBasinNaturalist54(4),© 1994,pp.335-341
SELENIUM GEOCHEMICAL RELATIONSHIPS OF
SOME NORTHERN NEVADA SOILS
Stephen Pooled Glenn Gross^, and RobertPotts'
—
Abstract. Soilsamples,onefionieachof10locationsinnorthernNevada,wereevaluatedforredoxpotential,total
and e.xtractable seleniimi, phosphate, free iron oxide, total and ferrous iron. Mole fractions for extractable selenium
specieswerecalculated from redox potentials. Datawere usedtoextrapolategeneral geochemical relationships forsoil
selenium at thesamplesites. Resultsobtainedfrom one sample perlocation allowedonlythemostgeneralconclusions
to be drawn. Soil phosphate levels, which affect the adsorption ofselenite species on iron oxide by competing for
adsoiption sites,werenotcorrelatedwithlevelsofextractableseleniuminthis study. Thiswouldsuggestthatselenium
wouldexistinsolution,havingbeendisplacedfromadsorptionsitesbyphosphorus. Ferrousiron, ironoxides,andredox
potential had a combined effect on the level ofextractable selenium at all sites. Soils in this study support selenite
speciesthatarenotreadilyavailabletoplantsandthereforecouldnotsupportvegetationadequateinSe.
Keywords:selenium,soil, redoxpotential,geocheinisfnj,plantbioavailability.
Selenium (Se) is a significant micionutrient dominant mobile forms in a soil solution and
in production agriculture; because of this, are available forplant uptake.
knowledge of the Se status of rangelands is Redox potentials are important in soils, and
important. Distribution oftotal and extractable theoretical relationships can be used to pre-
Se can vary widely over short geographic dis- dict and interpret metal solubilities (Lindsay
tances (Fisheretal. 1990). Because the geology and Sadiq 1983). Redox potentials have been
of Nevada is complex, relationships between used in Nevada to interpret observed
critical plant Se levels and geological forma- sequences ofminerals in an alteration zone in
tions are difficult to define. Recently, a review Ely, Nevada (Raymahashay and Hollard 1969),
ofthe Se status ofsoils, plants, and animals in interpret hydrogeochemistry ofthe Red Rock,
Nevada reported deficiency problems in west- Nevada, area (Fricke 1983), and evaluate trace-
em Nevada, variable amounts in northern and element content ofsediment and water in west
central portions ofthe state, and adequate lev- central Nevada (Rowe et al. 1991). Soil redox
els in the southern portion of the state. potential dataare lackingforthe state.
Selenium accumulatorplants grow throughout Thepuipose ofthis studywas to investigate
Nevada on limited seleniferous geological for- soil Se geochemical relationships for 10
mations (Poole et al. 1989; Fig. 1). The narrow Nevada sites using redox potential (pe -I- pH)
gap between essential and toxic concentra- and extractable and total Se levels. Phosphate
tions ofSe makes it imperative that processes (P), iron (Fe), and iron oxide (Fe203) levels
controlling the distribution ofthis element be were also investigated to determine their
understood (McNeal and Balistrieri 1989). effect on Se bioavailabilit\' for plants growing
Uptake ofSebyplants is governedby many on the soils.
soil and plant factors including type ofplant,
soil pH, clay content, and mineralogy. Most Experimental Procedure
important factors determininguptake are form
and concentration in the soil. Chemical form A soil sample was taken from each of 10
is controlledby redox potential parameters (pe sites: Battle Mountain and Gund Ranch in
-f pH; Elrashidi et al. 1989, Mikkelsen et al. central Nevada (Eureka and Lander counties);
1989). Although Se may exist in four oxidation Minden (Douglas County); Reno, Red Rock
states, selenate (VI) and selenite (IV) are pre- area north of Reno, Spanish Springs (Washoe
'SierraEnvironmentalMonitoring,Inc..1135FinancialBlvd.,Reno,Nevada89502.
335
Great Basin Naturalist [Volume54
Fig. 1. Selenium in Nevada forage. Very low = 81% of Fig. 2. Soil sample locations in northern Nevada:
samples with Se concentration of <0.01-0.05 ppm Se; 1-Battle Mountain, 2-Gund Ranch, 3-Spanish Springs,
variable — 74% ofsamples with Se concentration of 4-Reno, 5-Minden, 6-Fallon, 7-Salmon Falls Creek,
0.05-0.5 ppm Se; adequate — 78% ofsamples with Se 8-HuntingtonViilley,9-CloverValley, 10-RedRock.
concentrationof0.1-1.0ppm Se.
County); Fallon (Churchill County); Salmon calculated from millivolt readings using the
Falhs Creek near Contact and two locations relationship pe = Eh(millivolts)/59.2.
near the Ruby Mountains (Elko County) in To obtain total soil Se levels, we digested
Nevada (Fig. 2). Samples were taken appro.xi- samples in aliquots of 1:1 hydrochloric acid
mately 12-15 cm below the surface so as to in- and 5% potassium persulfate for 15 min fol-
clude the root zone. Air-dried samples <2 mm lowed by 3.5% o.xalic acid solution for 15 min.
(No. 10) were used foranalysis. The resulting solution was then treated with
Redox potentials were measured according concentrated hydrochloric acid for 42 min
to the procedure ofLindsay and Sadiq (1983). prior to diluting to 100 niL volume with
Soil suspensions were prepared in conical deionized water. Se concentrations of the
flasks to contain 50 g air-dried soil and 100 digests were determined using hydride gener-
niL deionized water. Each treatment was pre- ation atomic absorption spectroscopy (AAS;
pared in duplicate, degassed with argon (Ar), Varian SpectrAA 10with VGA accessoiy).
stoppered, and shaken. Millivolt readings Soluble Se was measured in a saturation
were taken on soil suspensions with a plat- paste extract from each soil (Jump and Sabey
inum (Ft) electrode and a glass Ag/AgCl refer- 1989) using hydride generation AAS. The
ence electrode using an Altex Selection 5000. extract Se concentration was used for calcula-
The platinum/reference electrode system was tion ofSe species. The mole fraction ofsoluble
standardized using a ferrous/ferric ion refer- Se species was calculated emploving methods
ence solution (ZoBells; ASTM 1978). Soil sus- ofElrashidietal. (1987).
pension pH was determined using a combina- Bicarbonate extractable P was determined
tion electrode that was calibrated with stan- using the method of Olsen (Council on Soil
dard buffers (ASTM 1978). Suspension pe was Testing and Plant Analysis 1980). To evaluate
1994] Selenium in Northern Nevada Soils 337
Fe203 levels, we extracted 4 g soil overnight Tablk 1. Total selenium, extractable selenium, redox
with 4 g sodium dithionite (Na2S204) and 75 potential(pe + pH),andpredominantseleniumspeciesin
mL deionized water. Suspensions were fil- soilsamples.
tered, brought to volume (Kilmer 1960), and
analyzed for Fe by AAS. Total soil Fe was
determined by flame AAS on nitric acid
digests ofsoil samples.
Ferrous iron (Fe II) in sample soils was
determined colorimetrically. Samples were
digested using concentrated sulfuric acid and
30% hydrofluoric acid, neutralized with 4%
boric acid, and made to volume with deion-
ized water (Walker and Sherman 1962). To an
aliquot ofthe digest we added 0.001 M batho-
phenanthroline in 50% ethanol and acetate
buffer. Isoamyl alcohol extracted the ferrous-
bathophenanthroline complex from the solu-
tion. The alcohol layer was drained into a 25-
niL volumetric flask, made to volume with
95% ethanol, and the absorption of the solu-
tion read on a spectrophotometer (Baush and
Lomb Spectronic 710) at a wavelength of538
nm. Standards and blanks were treated simi-
larly. Ferrous iron standards were derived
from a stock solution of ferrous ammonium
sulfate.
For the interpretation of data, we used
redox and adsorption relationships developed
by Howard (1977), Balistrieri and Chao (1987,
1990), and Schwab and Lindsay (1983) for the
behavior of Se, Fe, and FO4 and equilibria
described by Elrashidi et al. (1987) for Se in
soils.
Regression and multiple regression analy-
ses were performed following methods of
Damon and Hai-vey (1987). Regressions were
evaluated for significance at the 95% confi-
dence level.
Results
Total Se, extractable Se, redox parameters,
and general site descriptions are presented in
Table 1 for sample soils. Mole fi-actions ofthe
Se species for each sample are presented in
Table2.
Howard (1977) summarized Se geochem-
istiy on an Eh-pH diagram and found that Fe,
with which Se is closely associated inboth oxi-
dizingand reducingenvironments, controls Se
geochemistiy. In aerated soil suspensions the
Se (IV) oxyanions HSeO^" and Se03^~ are
strongly adsorbed by hydrated surfaces offer-
ric oxides over the pH range 2-8; above pH =
338 Great Basin Naturalist [Volume 54
Table2. Logmolefractionofseleniumspecies-'
1994] Selenium in Northern Nevada Soils 339
Table 3. Total iron, extractable iron oxides, andferrous Table 4. Extractable phosphate phosphorus, selenite
ronin soilsamples. selenium,andphosphate/selenitemolarratios.
340 Great Basin Naturalist [Volume54
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