Thesis (PhD (Botany and Zoology))—University of Stellenbosch, 2009. / ENGLISH ABSTRACT: Metal pollution is a problem of increasing global concern. It could arise from
industrial activities, as well as pesticide use in agriculture, among other sources. For
adequate protection of the soil ecosystem from metal toxicity, the bioavailability of
metals must be properly evaluated. A plethora of soil factors affect the bioavailability
of metals to soil organisms. These include pH, clay and organic matter contents,
salinity among others. While much is known about the influence of some of these
parameters, little is known on how clay content and salinity modify the bioavailability
of metals to soil organisms.
This study investigated the influence of clay content and salinity on
partitioning, uptake and toxicity of two essential metals (Cu and Zn) to the earthworm
Eisenia fetida in separate laboratory trials. Partitioning of the metals was evaluated
with 0.01 M CaCl2, DTPA (di-ethylene-triamine-penta acetic acid), and nitric acid
extractions. The metal content of worms was determined by acid digestion, while
growth, cocoon production, and mortality were used as endpoints showing toxicity to
metals and/or salinity. To test the validity of some of the laboratory results, a field
study was undertaken, using the earthworm Aporrectodea caliginosa. Further, the
study assessed the effect of salinity, using a battery of laboratory tests (acute, chronic
and avoidance tests) with natural and/or artificial soils on four species of organisms (a
collembolan Folsomia candida, a potworm Enchytraeus doerjesi and two earthworm
species E. fetida and A. caliginosa), representing different feeding patterns and
ecological roles in soil.
Results showed that with increased clay content, there was increased
availability of Cu in the substrate, and increased toxicity to E. fetida as shown by data
for mortality and growth. The situation with Zn was less significant at sub-lethal
concentrations but much so at lethal concentrations. DTPA and CaCl2 extracted
metals revealed changes in partitioning of Cu and Zn with changes in clay content,
but this trend was not always consistent. Both DTPA and CaCl2 revealed increased
availability of Zn in substrates with increased salinity. Salinity had an additive to
synergistic effect with Zn in toxicity to E. fetida. When combined with Cu, salinity
also increased the availability of Cu as shown by CaCl2 extracted fraction, and had
additive effect on toxicity of Cu to the earthworm. The field study did not succeed in in
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confirming the results of the laboratory study due to confounding role of flooding
after heavy rainfall and subsequent leaching of salts and Cu. The results of the
experiment on acute and chronic toxicity tests for NaCl on E. fetida showed LC50 of
5436 mg/kg NaCl and EC50 for growth and cocoon production of 4985 and 2020
mg/kg NaCl. These values showed that earthworms might be negatively affected in
many soils containing fairly moderate concentrations of salts. Similarly, A. caliginosa
could not survive in natural soil containing relatively low salt concentrations (EC =
1.62 dS/m) while reproduction was severely affected at lower EC value of 0.52 dS/m.
F. candida and E. doerjesi could survive in the highest salinity soil (EC = 1.62 dS/m)
used in this study but their reproduction was severely affected from 1.03 dS/m.
Overall, it appears that of all the taxa used, earthworm species were the most sensitive
to saline stress and could proof useful in determining ‘safe levels’ of salt in
contaminated soils. The results of the avoidance test showed that A. caliginosa
avoided both natural and artificial saline soil containing concentrations lower than
those avoided by E. fetida.
The conclusion is that the influence of clay content and salinity on the
bioavailability of Cu and Zn depends largely on the metal in question, but generally
speaking, bioavailability and toxicity of the metals were reduced with increased clay
content while the opposite was true for salinity. If the species used in this study can be
seen as fairly representative of a wide range of soil organisms, the conclusion is that
salinisation of soil will be detrimental to most soil organisms at relatively low saline
concentrations. Given the role of beneficial soil organisms in several soil processes
which in turn contribute to soil fertility and sustainable use of land, it is recommended
that any farming practices that may lead to an increase in salt content of agricultural
soils should be discouraged. / AFRIKAANSE OPSOMMING: Metaalbesoedeling is ‘n probleem wat toenemende globale kommer
veroorsaak. Dit kan ontstaan as gevolg van industriële aktiwiteite sowel as van
plaagmiddelgebruik in die landbou en ander bronne.Ten einde die grondekostelsel
genoegsaam te beskerm, moet die biobeskikbaarheid van metale ge-evalueer word. ‘n
Verskeidenheid van grondfaktore be-invloed die biobeskikbaarheid van metale vir
grondorganismes Hulle sluit onder andere in pH, klei, organiese inhoud en
soutgehalte. Hoewel heelwat bekend is oor die rol van sommige van hierdie
parameters, is min bekend oor hoe klei en soutgehalte die biobeskikbaarheid van
metale vir grondorganismes kan modifieer. Hierdie studie het die invloed van kleiinhoud
and soutgehalte op die verdeling/partisie, opname en toksisiteit van twee
essensiële metale (Cu en Zn) vir die erdwurm Eisenia fetida in afsonderlike
laboratoriumproewe ondersoek. Kompartementele verdeling van die metale is geevalueer
deur middel van ekstraksie-metodes met 0.01 M CaCl2, DTPA (di-etileentriamien-
penta asysnsuur), en salpertersuur ekstraksies. Die metaalinhoud van wurms
is bepaal deur suurverterings en spektrofotometriese analises te doen terwyl groei,
kokonproduksie en mortaliteit van organismes gebruik is as gevoeligheidseindpunte
om toksisiteit van metale en soutgehalte aan te toon. Om die geldigheid van somige
van die laboratoriumresultate te toets, is ‘n veldstudie ook onderneem met die
erdwurm Aporrectodea caliginosa. Die effek van soutgehalte is verder ondersoek
deur ‘n battery van laboratoriumtoetse met vier spesies (‘n kollembool Folsomia
candida,’n potwurm Enchytraeus doerjesi en twee erdwurmspesies E. fetida en A.
caliginosa), wat verskillende voedingspatrone verteenwoordig.
Die resultate het getoon dat met toenmende klei-inhoud was daar ‘n toename
in die beskikbaarheid van Cu vir opname vanuit die substraat, asook ‘n toename in
toksisiteit vir E. fetida soos deur die gegewens vir mortaliteit en groei uitgewys. Die
situasie met Zn was minder betekenisvol by subletale konsentrasies en selfs baie
minder so by letale konsentrasies. DTPA en CaCl2 ge-ekstraheerde metale het
veranderinge in die partisie/verdeling van Cu en Zn uitgewys met verandering in
klei-inhoud, maar die tendens was nie altyd konstant nie. Beide DTPA en CaCl2
ekstraksie het toenemende beskikbaarheid van Zn in substrate uitgewys met
toenemende soutinhoud. Soutinhoud het ‘n additiewe/toegevoegde tot sinergistiese
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toksisiteitseffek saam met Zn vir E. fetida. In kombinasie met Cu het soutgehalte ook
die geskatte biobeskikbaarheid van Cu verhoog soos uitgewys deur die CaCl2, geekstraheerde
fraksie, en het ‘n additiewe effek gehad op die toksisitiet van Cu vir die
erdwurm.
Die veldstudie kon nie die resultate van die laboratoriumstudie bevestig nie
weens die belemmerende rol van vloede na swaar reënneerslae en daaropvolgende
uitloging van soute en Cu. Die resultate van die eksperimentele ondersoek na die
akute en chroniese effekte van NaCl op E. fetida het ‘n LC50 van 5436 mg/kg NaCl
en EC50 vir groei en kokonproduksie van 4985 en 2020 mg/kg NaCl opgelewer.
Hierdie waardes het aangetoon dat erdwurms moontlik negatief beinvloed kan word
in baie gronde wat ‘n redelike gemiddelde konsentrasie van soute bevat. Soortgelyk
kon A. caliginosa nie oorleef in natuurllike grond wat relatief lae soutkonsentrasies
bevat het (EC=1.62 dS/m) nie terwyl voortplanting sterk ge-affekteer is by ‘n lae EC
waarde van 0.52 dS/m. F. candida en E. doerjesi kon oorleef in die grond met die
hoogste soutgehalte (EC= 1.62 dS/m) maar hulle voortplanting is ernstig geknou
vanaf 1.03 dS/m. In geheel blyk dit dat van allle taksa wat gebruik is, erdwurms die
sensitiefste was vir die stres wat deur soutgehalte veroorsaak is. Die kennis kan nuttig
wees in die bepaling van “veilige vlakke” van sout in gekontamineerde gronde. Die
resultate van die vermydingstoetse het getoon dat A. caliginosa beide natuurlike en
kunsmatig versoute gronde vermy het by konsentrasies wat heelwat laer was as dié
wat deur E. fetida vermy is
Die gevolgtrekking is dat die invloed van klei en soutgehalte op die
biobeskikbaarheid van Cu en Zn grootliks afhanklik is van die metale wat betrokke is
en dat biobeskikbaarheid en toksisiteit normaalweg verminder het met verhoogde
klei-inhoud, met die teenoorgestelde wat waar was in die geval van soutgehalte.
Indien die spesies wat in die studie gebruik is beskou kan word as redelik
verteenwoordigend van ‘n wye reeks van grondorganismes, is die gevolgtrekking dat
versouting van gronde nadelig sal wees vir meeste grondorganismes, selfs by relatief
lae soutkonsentrasies. In die lig van die rol wat nuttige grondorganismes speel in
verskeie grondprosesse wat bydraend is tot grondvrugbaarheid en volhoubare gebruik
van gronde, word dit aanbeveel dat enige boerderypraktyk wat mag lei tot verhoging
van die soutinhoud van landbougronde ontmoedig moet word.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/1484 |
| Date | 12 1900 |
| Creators | Owojori, Olugbenga J. (Olugbenga John) |
| Contributors | Reinecke, A. J., Rozanov, A., University of Stellenbosch. Faculty of Science. Dept. of Botany and Zoology. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | University of Stellenbosch |
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