Thesis (MScAgric (Soil Science))--University of Stellenbosch, 2009. / ENGLISH ABSTRACT: The sandy soils of the Riverlands Nature Reserve, near Malmesbury, and the
neighbouring farm were studied to determine the effects of long term invasion of the
legume Acacia saligna on the soil nutrient content of a soil previously vegetated with
fynbos. The effect of the removal of this alien legume on general soil properties and
groundwater quality were also studied. The changes in nitrates and nitrites (NOX) due to
the invasion and removal of the alien legume were investigated in more detail than
changes in other soil nutrients. In addition to that emphasis was placed on the effect of
vegetation clearing on groundwater quality, specifically relating to potential
contamination with nitrates.
This study was initiated after Conrad et al., (1999) found increased NOX concentrations
in ground water while studying the effects of pig farming on ground water nitrogen (N)
near a site cleared of Acacia saligna by Working for Water (WFW). Since many sites are
scheduled for removal of this alien vegetation it was deemed necessary to study the
effects that clearing alone had on groundwater quality. It was suspected that there would
be an increase in soil and groundwater NOX with vegetation removal due to the inputs
from the legume alien invader.
Soil sampling was done continuously throughout the rainy season of 2007 (From May to
December) on three adjacent sites separated by some 50 m of distance, consisting of a
natural fynbos site and two Acacia saligna sites. The sites were selected approximately
on the same contour line to prevent interaction through lateral water flow. One of the
Acacia sites was cleared by the Working for Water programme in the usual manner
leaving slash on the ground. Soil samples were collected at regular intervals throughout
the season from all three sites (fynbos, Acacia and cleared site) using a Jarrett soil auger.
They were airdried (to achieve full oxidation of mineral N) and sieved though a 2mm
sieve. Soils were analysed by atomic absorption spectroscopy for basic cations and by ion
chromatography for anions, including nitrates and nitrites. Total carbon and nitrogen was
determined by combustion, pH (1M KCl and H2O) and EC (1:5 H2O) were also measured. The present study was part of a wider investigation into the quality of
groundwater, modelling flow and contaminant transport (Jovanovic et al., 2008).
The largest changes in soil properties were observed in the top (0-5 cm) layer. The fynbos
site had a lower nutrient status by comparison to the Acacia site and the cleared site. The
sum of cations from the soils of the fynbos site at a soil depth of 0-5cm was 100mg/kg
and soils from the cleared and Acacia sites were about 190mg/kg. The Acacia site had a
higher NOX status and experienced a larger NOX fluctuation during the rainy season than
the fynbos site; minimum NOX values were similar (below 10mg/kg) but the Acacia site
had a maximum NOX value of near 60mg/kg and the fynbos site just below 30mg/kg.
There was little difference in general soil characteristics (exchangeable cations, pH and
EC, total soil C and N) during the first season after clearing, between the Acacia and the
cleared sites.
The effect of soluble nitrogen changes due to alien legume invasion and removal on
groundwater quality, relating to NOX, during the first season after clearing, was
determined. It was found that the Acacia site had higher NOX concentrations than the
fynbos site. At 0-5cm the fynbos site NOX was less than 30mg/kg and the Acacia site was
between 30 and 110mg/kg for most of the season, with values lower than 30mg/kg for the
last four sampling dates only. N concentrations on the cleared site behaved in a similar
manner to the uncleared Acacia site, but generally N values were lower on the cleared
site, there were only two sampling dates where the cleared site had higher NOX values
than the Acacia site at 0-5cm. The average groundwater N in NOX under the cleared site
was 4.34 mg/l, and under the Acacia site 3.78mg/l, these values are both below the level
determined for water contamination with nitrates. However, the increase in ground water
nitrate levels after A. saligna clearing was significant.
It was concluded that there is a change in the nutrient status of soil with Acacia invasion
and again with removal. NOX migrates to the groundwater to a larger degree once vegetation has been removed, although during this study the nitrate pollution threshold of
10 mg/l nitrate N was not reached. / AFRIKAANSE OPSOMMING: Die sanderige gronde van die Riverlands Natuur Reservaat, naby Malmesbury, en die
aangrensende plaas was bestudeer om die effek van die langtermyn indringing van die
peulgewas, Acacia saligna, op die voedingswaarde van ‘n voorheen fynbos begroeide
grond, sowel as die effek van die verwydering van die indringer op die algemene
samestelling van die grond en grondwater kwaliteit. Oplosbare stikstof veranderings
(NOX) wat plaasvind as gevolg van die indringing en verwydering van die indringer
peulgewas, was in meer diepte bestudeer as die ander elemente. Klem was ook geplaas op
die effek van die verwydering van plantegroei op grondwater kwaliteit, met spesifieke
verwysing na potensiële nitraat besoedeling.
Die studie was beplan na Conrad et al., (1999) ‘n toename van NOX konsentrasies in
grondwater ontdek het, tydens ‘n studie van die effek wat vark boerdery het op
grondwater N naby ‘n area waar Acacia Saligna verwyder was deur Working For Water
(WFW). As gevolg van die feit dat verskeie areas in die proses is om skoongemaak te
word van Acacia Saligna, is dit nodig geag om die effek daarvan of grondwater kwaliteit
te ondersoek. Die hipotesis was dat daar ‘n toename in NOX konsentrasies in grond en
grondwater sal wees as gevolg van die verwydering van plantegroei.
Grond monsters was deurlopend geneem gedurende die reën seisoen van 2007 (Vanaf
Mei tot Desember) uit nabygeleë areas wat omtrent 50 meter uitmekaar is. Die volgende
tipes grond was verteenwoordig: ‘n Fynbos begroeide grond sowel as ‘n grond begroei
met Acacia saligna. Die areas was geselekteer ongeveer op die selfde kontoer lyn om
interaksie tussen areas te voorkom as gevolg van laterale water vloei. ’n Gedeelte van die
A. saligna area was skoongemaak deur die Working for Water program op die gewone
manier deur die afgesnyde plant materiaal op die grond te los. Grond monster was
geneem met gereelde intervalle gedurende die seisoen op al drie areas (fynbos, A. saligna, en skoongemaakte area) deur die gebruik van ’n Jarret grondboor. Die monsters
was lugdroog gemaak (om volle oksidasie van die mineraal N te verseker) en toe gesif
deur ‘n 2mm sif. Die gronde was analiseer deur atoom absorpsie vir basiese katione en
deur ioon chromatografie vir anione, insluitende nitriete en nitrate. Totale koolstof en
stikstof was bepaal deur verbranding terwyl pH (1M KCl en H2O) en EC (1:5 H2O) ook
gemeet was. Hierdie studie was deel van ‘n wyer ondersoek na die kwaliteit van
grondwater, vloei modelering en vervoer van kontaminante (Jovanovic et al., 2008).
Die grootste veranderinge in die grond eienskappe was in die boonste grondlaag (0-5cm)
waargeneem. Die bevinding was dat die fynbos area ‘n laer voedingswaarde het as die
area begroei met die indringer sowel as die die skoongemaakte area. Die som van katione
onder fynbos grond by ‘n grond diepte van 0-5cm, was 100/mg/kg en die som van
katione by die ander twee areas was omtrent 190mg/kg. Die Acacia area het ‘n hoër NOX
inhoud en het ‘n groter NOX fluktuasie ervaar gedurende die reën seisoen as die fynbos
area. Die minimum NOX waardes was soortgelyk (minder as 10/mg/kg), maar die Acacia
area het ‘n maksimum NOX waarde van omtrent 60mg/kg terwyl die fynbos area se
maksimum net minder as 30mg/kg gehad het. Daar nie veel verskil tussen die algemene
grond eienskappe (uitruilbare katione, pH en EC, totale C en N) van die Acacia en
skoongemaakte areas gedurende die eerste seisoen na die indringer verwyder is nie.
Die effek van oplosbare stikstof veranderings as gevolg van die peulgewas se indringing
en verwydering op grondwater kwaliteit, met verwysing na NOX, gedurende die eerste
seisoen na verwydering van die indringer was bepaal. Dit was bevind dat die Acacia area
‘n hoër NOX konsentrasie as die fynbos area het. Op ‘n diepte van 0-5cm was die fynbos
NOX laer as 30mg/kg terwyl die Acacia area tussen 30 en 110mg/kg vir die grootste
gedeelte van die seisoen was met waardes laer as 30mg/kg vir die laaste vier datums
waarop monsters geneem was. Veranderings in N op die skoongemaakte area en die
Acacia area was soortgelyk, maar oor die algemeen was N waardes laer op die
skoongemaakte area. Daar was slegs op twee datums gevalle gevind waar die
skoongemaakte area ‘n hoër NOX inhoud as die Acacia area gehad het by ‘n diepte van
0-5cm. Die gemiddelde N in NOX in die grondwater onder die skoongemaakte area was 4.34mg/l en by die Acacia area was dit 3.78mg/l; hierdie waardes is onder die drempel
vir nitraatbesoedeling in grondwater. Die nitraat inhoud van die grondwater het wel
beduidend toegeneem as gevolg van verwydering van A. saligna.
Die gevolgtrekking was dat daar ‘n verandering plaasvind in die voeding status van grond
met Acacia saligna indringing en weer met verwydering. NOX migrasie na die
grondwater verhoog met verwydering van plantegroei. In hierdie studie was die nitraat
besoedeling drempel van 10mg/l nitraat N nie bereik nie.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/2489 |
| Date | 12 1900 |
| Creators | Van der Merwe, Nicolette |
| Contributors | Rozanov, A., University of Stellenbosch. Faculty of Agrisciences. Dept. of Soil Science. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | Unknown |
| Type | Thesis |
| Rights | University of Stellenbosch |
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