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In vitro polyploidization of selected indigenous plant species.Reddy, Viloshanie. January 2005 (has links)
Many plant species indigenous to South Africa have ornamental, medicinal and horticultural value. Polyploidization is one technique that has been used to artificially produce superior genotypes, particularly in horticultural species. In the current investigation two antimitotic substances, colchicine at concentrations of 0.1% and 0.01% and oryzalin at concentrations of 0.01% and 0.001%, were used in an attempt to polyploidize microshoots of Dorotheanthus bellidifonnis (Burm.f.) N.E.Br and Mondia whiteii (Hook.f.) in vitro. Microshoots of D. bellidifonnis and M. whiteii obtained from nodal cuttings of in vitro germinated seedlings were maintained for 48 hours in liquid medium containing the antimitotic substances and thereafter cultured on sucrose-supplemented MS medium. The treated microshoots were evaluated for elongation, necrosis, contamination and phenolic exudation. Best results were observed in M. whiteii microshoots treated with antimitotic substances and transferred on to solid sucrose-supplemented MS medium containing 0.2% activated charcoal for 4 weeks. Leaves from the surviving treated plants were excised and used for flow cytometric analyses to evaluate changes in chromosome number. Shoots of M. whiteii treated with 0.01 % colchicine showed no changes in chromosome number, while the higher concentration used produced polyploids and mixaploids. However, oryzalin at 0.01 % concentrations produced a comparatively higher number of microshoots that were polyploids and mixaploids. Shoots of M. whiteii that have altered chromosome number have been transferred onto multiplication medium, for future evaluation of changes in phenotypic characteristics. The germination response of seeds of D. bellidifonnis was evaluated in the presence of oryzalin (0.01% and 0.001%) and colchicine (0.1% and 0.01%). Poor germination was observed in seeds germinated in the presence of 0.01 % oryzalin. Upon transfer of the germinated seedlings treated with antimitotic substances onto sucrose-supplemented MS medium, subsequent growth and development was restricted. Shoot and root development was different for the seedlings germinated in the presence of the two antimitotic substances. Shoot elongation and root development was vigorous in seedlings geminated in the presence of 0.01 colchicine and stunted development was observed in seedlings germinated in 0.1 % colchicine. On the other hand restricted rootind was observed in seedlings germinated in the presence of oryzalin, and the shoots lacked pigmentation. Meristematic cells excised from the shoot tips of the treated seedlings showed several ultrastructural changes including abnormal mitochondrial development, endomembrane formation and vacuolation. It was concluded that oryzalin and colchicine influence ultrastructure in plant cells differently. Practical constraints associated with plant tissue culture also influence the rate of in vitro polyploidization. Since different plant species require different conditions for optimal growth, it was also noted that no unique polyploidization treatment can be used for a wide range of plant species, individual species require different growth conditions. / Thesis (M.Sc.)-University of KwaZulu-Natal, 2005.
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Investigations into aluminium toxicity and resistance in cynodon dactylon using invitro techniques.January 2004 (has links)
Aluminium toxicity is a significant limiting factor to agricultural crop production globally, promoting the need for plants that are resistant to low pH and high Al3 +, Current literature suggests that Al3 + inhibits plant growth by stopping root elongation. Although considerable research has been directed towards Al3+-inhibited root growth, the initial cellular targets and primary mechanisms of Al3+ toxicity still remain unclear. The present study, therefore, considered an alternate approach to investigating Al3 + toxicity and Al3 + resistance. Callus, a group of actively dividing meristematic cells, was exposed to Al3 + and the influence of Al3 + on callus growth was investigated. In South Africa, gold mining results in the production of wastes that require vegetation cover resistant to low pH and high Al3+, in order to promote stabilisation and prevent erosion. Cynodon dactylon was considered a key species for such a purpose since small populations of this grass were found growing on the acidic gold tailings. Different C. dactylon genotypes were exposed to Al3 + and the feasibility of using differences in callus growth to identify potential Al3 +-resistant individuals was assessed. An in vitro method for indirect somatic embryogenesis was concurrently established to regenerate whole plants from such calli. Embryogenic calli were initiated from young leaf segments, using 2,4-D. Somatic embryo maturation and plant regeneration were achieved on a hormone-free Murashige ~d Skoog (MS) nutrient medium. In addition to this protocol for micropropagation via indirect somatic embryogenesis, nodal cuttings, on a single hormone-free MS nutrient medium, were shown to be suitable explants for micropropagation via direct organogenesis, albeit resulting in low plantlet yields (1 plant/explant). In the investigation of Al3 + resistance, each of the three parameters tested (genotype, Al3+ concentration and exposure time) had a significant influence on callus growth rate. The nutrient medium supporting callus growth was modified in order to ensure known concentrations of free Al3 + ions (0.08-2.3 mM). This was achieved through the use of a chemical speclatlOn model (MINTEQA2). Fresh callus mass measurements for three genotypes were recorded at two-weekly intervals for a total of 8 weeks. Significant differences in callus growth rate were used to identify the genotypes as Al3+-sensitive (AlS), moderately Al3+-resistant and Al3+-resistant (Al-R), suggesting that it is feasible to use undifferentiated meristematic callus cells to screen for resistance to Al3+. In addition to callus growth rate, it was also possible to differentiate between the Al-S and the Al-R genotype using differences in cell numbers. Exposure to 0.8 mM AI3+ for 2 weeks resulted in an 88% reduction in the Al-S meristematic cell number whereas no Al3 + concentration tested had a significant inhibitory effect on the Al-R cell number. Aluminium was detected inside the callus cells, with the Al-S cells accumulating three times more Al in the nucleus than did the Al-R cells. It is suggested, therefore, that Al3 + inhibited meristematic cell number in the Al-S genotype by interfering with cell division. Two possible mechanisms by which the Al-R genotype was able to exclude Al3+ from its cells were investigated. The Al-R callus was able to maintain a higher extracellular pH (4.34 in Al-R and 4.08 in Al-S) and immobilise more Al in the cell wall (33% more in the Al-R) than the Al-S genotype. The present study has developed a valuable tool for investigating the physiological effects of Al3 + on actively dividing meristematic cells. In addition, the somatic embryogenesis route allows for the concurrent in vitro selection and plantlet regeneration of genotypes of interest. Future work is necessary to confirm that the properties of undifferentiated cells in culture are maintained by the ex vitro whole mature plants. / Thesis (Ph.D.)-University of KwaZulu-Natal, 2004.
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Studies on polymerised dispersions as soil conditioners : their effects and feasible applications.Bishop, Richard Timothy. 19 September 2014 (has links)
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1978.
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Investigation of 2:1 layer silicate clays in selected southern African soils.Buhmann, Christl. January 1986 (has links)
As very little detailed X-ray diffraction investigations have been carried out in South Africa on 2:1 phyllosi1icates in soils,
the aim of the present study was to contribute to the knowledge
of soil genesis, as well as K-fixation and swelling, by investigation
of the clay fraction of selected soils known to be rich
in these minerals. X-ray diffraction analysis has been used
almost exclusively as the investigative technique.
In Chapter 1 a literature review is presented on the reasons
for X-ray diffraction peak broadening and the problems encountered
in the identification of swelling clay minerals. For interstratifications,
the concept of an ABAB layer sequence, considered
as having
suggested
an abab
that the
inter1ayer space,
X-ray diffraction
is questioned.
data from which
It is
the ABAB
arrangement is inferred can as well be explained in terms of
an alternative AAAB layer sequence, having an aabb interlayer
arrangement.
Chapters 2, 3 and 4 deal with layer silicate formation/alteration
in the course of soil development in dolerite and shale-derived
profiles. Dolerite-derived pedons could· be characterized by
one of the following layer silicate suites :
suite i : discrete smectite (Fe-containing beidellite-montmorillonite)
with or without traces of kaolinite and
talc (Vertisol)
suite ii : smectite-kaolinite interstratification (Vertisol)
suite iii : 14 ft minerals (vermiculite, beidellite, montmorillonite,
chlorite) and 7 ft minerals (halloysite, kaolinite )
in about equal proportions (Vertisol and Mollisol)
suite iv : kaolinite with subordinate chlorite and traces of talc
(Oxisol, Ultisol). Eccashale-derived Vertisols are dominated by mica-smectite interstratifications.
The occurrence of an iron-rich pedogenic talc is discussed in
Chapter 4. X-ray and chemical data suggest 30 - 50 mole percent
substitutions of iron for magnesium.
The mineralogical basis for K-fixation has been established in
Chapter 5. Two K-fixing components could be identified :
dioctahedral high-charge vermiculite as a discrete mineral and
random mica-smectite interstratifications with 20 - 60% mica.
In Chapter 6, some of the most expansive soils in South Africa
have been investigated. They can be subdivided into two groups
denoted by the swelling component as follows :
(a) smectite-dominated (the smectite species involved being
most probably beidellite with a heterogeneous charge distribution);
(b) mica-smectite interstratification with random or ordered
stacking arrangement. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1986.
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Soil physical factors affecting root growth and maize yield in four Rhodesian soils.Rankin, James Malcolm. 23 September 2014 (has links)
The platinum microelectrode technique for measuring
oxygen flux in soils has been reviewed. Shortcomings in the
existing technique and instrumentation have been discussed.
The new instrumentation, electrode standardization and
measurement techniques developed enable the method to be used
with confidence in unsaturated soil systems. Measurements
of oxygen flux index in four soil samples showed a very highly
significant regression relationship between oxygen flux index
and air space within the range 3 - 15% air space on each soil.
There was no significant difference in the regression relation
between soils.
A field penetrometer, designed to measure the presence
and strength of subsurface pans in field soils has been
described. Measurements with the penetrometer on three depth
of ploughing treatments (100, 230 and 355 mm) on tillage trials
at four sites with different clay contents showed that hard
layers were present on all the treatments. Except on the
shallowest ploughing depth treatment on the fine-textured
soil, where the pan was 225 mm below the nominal ploughing
depth, the hard layers were present between a few mm and 150 mm
below the nominal ploughing depth, and had strengths of between
16 and 24 bars.
The theory and factors affecting measurement of soil
strength with needle penetrometers have been investigated.
The design and operation of a laboratory penetrometer used to
measure soil strength under closely controlled laboratory
conditions has been discussed.
Physical factors likely to affect root growth, viz. soil
texture, air space, bulk density, soil strength and available
moisture, have been measured in a comprehensive range of
undisturbed cores taken from the four tillage trials. High
soil strength is considered as being the soil physical factor
most likely to restrict root growth in these soils. Physical
factors affecting soil strength have been investigated. Soil
strength is shown to be highly dependent upon bulk density, matric potential and soil texture.
The hard pans shown to exist in all the tillage trial soils
exhibit many of the characteristics of tillage pans, but their
existence cannot be attributed exclusively to the ploughing
depth treatments imposed in the tillage trials. Rather, the
pans have resulted from a combination of interacting factors,
including the previous history of the soils, the imposed
tillage treatments, crop, and climatic factors.
A study of some of the data from the literature on root
growth and soil strength shows that root growth is severely
restricted by soil strengths of the order of 20 to 30 bars.
In order to determine whether root growth was being restricted
in the tillage trials, root profiles were extracted from one
of the trials. These showed that the pans severely restricted
root growth.
Analysis of maize yield data from the tillage trials
showed that on the three coarse-textured sites yield increased
with increased depth of ploughing, and that there was a marked
seasonal effect, ploughing depth having a relatively greater
effect on maize yield in dry seasons than in wet. On the
fine-textured site, however, where no pan existed near the
surface in the shallow ploughed treatment, the ploughing depth
effect was not significant, nor was there any marked seasonal
effect of ploughing depth on maize yield. Evidence presented
shows that the pans, by restricting root growth are reducing
the amount of water available to the plant. This effect is
greater in dry seasons, and in soils with low available water . / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1976.
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An evaluation of conventional and no-tillage systems on soil physical conditions.Nzeyimana, Innocent. January 2002 (has links)
The use of no-tillage (NT) system has increased in the past few years in South Africa, but
its effects on soil physical conditions have not been adequately documented. This study
was undertaken to ascertain these effects, as compared to Conventional tillage (CT)
system. Several sites were selected in the Bergville and Winterton areas of the midlands
of KwaZulu-Natal, and at the Cedara Agricultural Research Station.
NT generally increased bulk density in the topsoil and this altered total porosity and poresize
distribution. Water retention, organic C and aggregate stability were increased under
NT, partly due to the maintenance of the mulch cover on the surface soil. Organic C and
aggregate stability were positively correlated with each other. Differences in bulk density
between tillage systems with soil depth did not clearly indicate where soil compaction had
occurred. Significant differences in soil compaction between treatments were, however,
illustrated by changes in soil penetration resistance (SPR), especially at the. 150 mm
depth. In addition, depending on the soil type, SPR was greater in the topsoil under NT
than CT. It was suggested that conversion from CT to NT was carried out when the
topsoil of the CT-fields was structurally poor, due to a previous history of continuous CT.
Tractor traffic under CT and repeated tillage when the soil was wet had, in some cases,
resulted in the formation of a compacted layer at the depth of cultivation. In clay soils, this
has resulted in subsoil compaction. The formation of compacted layers caused major
changes to pore size distribution and continuity and this resulted in substantially reduced
hydraulic conductivity, infiltration rate,air-filled porosity and air permeability. It was
concluded that compacted subsoil layers need to be broken up prior to conversion from
CT to NT, and that compaction in the surface soil under NT has occurred and, in some
cases, this will be a limitation to crop production. The use of minimum tillage systems
should be considered and researched in these cases. / Thesis (M.Sc.)-University of Natal, Pietermaritzurg, 2003.
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The effects of surface-applied poultry manure on top- and subsoil acidity and selected soil fertility characteristics.Judge, Angus. January 2001 (has links)
The effects of surface applications of poultry manure on pH, exchangeable AI, exchangeable
Al saturation and levels of available macronutrients in the surface and subsurface layers were
investigated in some acid soils from the KwaZulu-Natal Midlands. Three field sites with a
history of long-term applications of poultry manure were compared with adjacent fields
where no manure had been applied. Results generally showed an increase in pHwater, pHKCI,
exchangeable Ca, Mg, and K and extractable P and a decrease in exchangeable Al and
exchangeable Al saturation in the surface soil to the depth to which the manure had been
incorporated. Results also provided evidence for substantial downward movement of Ca and
Mg into the subsoil layers (i.e. 30-60 cm) and their accumulation in exchangeable forms.
There was a concomitant general increase in pHKcl and pHwater and decrease in exchangeable
Al and exchangeable AI saturation in the subsoil layers. These results demonstrate that, under
field conditions, surface applications of poultry manure can cause the amelioration of subsoil
acidity. This is an extremely important finding since subsoil acidity is characteristically
extremely difficult and costly to ameliorate.
A leaching column study was subsequently conducted to investigate the mechanisms by
which surface applications of poultry manure ameliorate both top- and subsoil acidity. The
soil used, originating from one of the field sites, had not previously been treated with poultry
manure and had a subsoil pHKCI of 4.25 and an exchangeable acidity concentration of 1.79
cmolJkg. Three forms of poultry manure (layer, broiler and free range) were incorporated
into the surface 5 cm of soil columns at rates equivalent to 5, 10 and 30 Mg/ha. Columns
were maintained in glasshouse conditions for a period of 108 days and over that period they
were leached on four separate occasions (receiving a total of 825 mm of simulated rainfall).
At the conclusion of the experiment, the soil columns were cut into sections for chemical
analysis.
Applications of poultry manure to the surface soil markedly increased pHwater, pHKcl,
exchangeable Ca, Mg, K and Na concentrations and decreased exchangeable Al levels in the
surface 5 cm layer. They also increased the concentrations of soluble C, soluble salts, total Al
and organically-complexed Al in soil solution. These effects were most pronounced with
layer manure and greater at the higher rate of application. The manure-induced decrease in
exchangeable AI, but increase in total AI, in soil solution was attributed to soluble salts,
originating from the manure, displacing exchangeable Al into solution where it was
subsequently complexed by soluble organic matter.
Analysis of subsoil layers (5-15, 15-25, 25-35 and 35-45 cm) at the conclusion of the
experiment showed that surface applications of poultry manure decreased concentrations of
exchangeable Al in the subsoil but had no effect on pHKCl and depressed values for pHwater. It
was suggested that manure-derived urea leached into the subsoil and was then hydrolysed
causing an increase in pH and precipitation of exchangeable Al as insoluble hydroxy-AI
oxides. Towards the conclusion of the experiment nitrification began to proceed, causing
subsoil pHKCl values to decrease back to their original values. Analysis of the inorganic-N
content of leachates and soil layers provided circumstantial evidence for this mechanism.
High concentrations of soluble salts in the subsoil layers (caused by leaching from the
manure) resulted in displacement of exchangeable A13+ and W into soil solution so that the
electrical conductivity and concentrations of total and monomeric Al were elevated and
pHwater was depressed in the subsoil (15-45 cm) of poultry manure-treated columns.
It was concluded that the results underline the opposing effects that poultry manure
applications have on (i) raising soil pH and lowering exchangeable Al but at the same time
(ii) greatly increasing soluble salt concentrations and thus displacing At3+ and H+ back into
soil solution. They also have suggested the importance of the release of N during manure
decomposition in influencing soil pH (through the processes of ammonification, urea
hydrolysis and nitrification) and therefore other soil chemical properties. It is, however, clear
that the long-term effect of surface applications of poultry manure is generally to ameliorate
subsoil acidity by raising subsoil pH and lowering exchangeable Al concentrations. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2001.
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Some soil chemical and fertility aspects of the land disposal of a water treatment residue on selected soils of KwaZulu-Natal, South Africa.Buyeye, Sicelo Malizo. January 2005 (has links)
The environmental and agricultural viability of land disposal of a water treatment residue (WTR) from the Midmar Water Treatment Works of Umgeni Water was investigated by determining answers to four broad questions: 1. What effects would the application of the WTR have on plants growing on the treated soils? 2. What effects would application of the WTR have on soil chemical properties? 3. What effects would the WTR have on the soil solution composition (and by implication the quality of the groundwater)? 4. Could this material be used to reduce solubility of potential pollutants? To answer these questions, the following experiments were set up, and their respective results are reported. 1. Effects of the water treatment residue on plant growth This was investigated in a pot experiment and two field experiments. In the pot experiment five soils, two Huttons (Hu-M and Hu-T), an Inanda (la-C), a Namib (Nb-F) and a Shortlands (Sd) were used to grow perennial ryegrass ((Lolium perellne). All samples were fertilized with a basal dressing of N, P, K, Mg and S. Two lime levels were added to the Ia-C and Nb-F soils, the higher calculated to reduce acid saturation to 1%, and the lower being half of that. The WTR was applied at rates of 0, 40, 80 and 120 Mg ha-1. All treatments were in triplicate. Eight cuts in all were made of the perennial ryegrass. The dry matter (DM) yield of perennial ryegrass grown in the pot experiment increased with the WTR applied in all five soils although the highest increase was with the acidic Ia-C and Nb-F soils. The fact that the highest yields were on the strongly acid soils suggests that the liming effect of the WTR could have contributed, more so considering that lime also increased yields in these soils. It was, however, clear that no one factor was responsible for the increase in yield as the timing effect could not explain the results of the other three soils. At the two field experiments perennial ryegrass was grown at Brookdale Farm from 1998 to 2001, after which the site was re-seeded with tall fescue (Festuca arundinaceae). At Ukulinga Farm tall fescue was grown from the outset in 2000. In the two field experiments with both perennial ryegrass and tall fescue, no significant increase in yield was apparent. Importantly, however, from an environmental point of view there was no decrease in yield whether the WTR was incorporated or applied as a mulch. This was observed even at the highest rates of application, namely 1280 Mg ha-1. The growth on the mulched plots was often observed to be better than any of the other treatments, including the control. Analysis of the plant material from both pot and field experiments indicated that the WTR neither pollution of the groundwater by nitrates. However, analysis of saturated pastes from soils at both field experiments showed that the levels of nitrate were increased by application of the WTR in only the fallow plots. 4. The water treatment residue as a possible pollutant-reducing agent The effect of the water treatment residue on the sorption of P and heavy metals (Cd, Ni and Zn) was studied in the laboratory. Soils treated with WTR were equilibrated for 6 hours in 0.005 M calcium cWoride solution containing a known concentration of each element. For the coarse-textured soils, initial P concentrations ranged from 0 to 1000 mg kg-1 as opposed to 0 to 1800 mg kg-1 for the clay soils. Treatments of WTR used were 0, 80, 320 and 1280 Mg ha-1, both incubated and non-incubated. At high initial P solution concentrations, the WTR increased the extent of sorption in the coarser textured soils (Hu-T, Nb-A, Nb-F, Va and We), and decreased it in highly sorbing Av, Hu-M, la-C and la-W soils. In general though, the WTR greatly reduced soluble P. For Cd, Ni and Zn only one concentration, 50 mg kg-1, was studied using the incubated soil samples as affected by WTR rates from 0 to 1280 Mg ha-1. For all three metals, the amount sorbed increased with increase in amount of WTR for the nine soils studied, namely the Av, Hu-F, Hu-M, Hu-T, la-C, la-W, Nb-F, Va and We. In many cases the sorption was so high that more than 40 mg kg-1 of the initial concentration was removed from solution. Even for those soils with high sorption capacity e.g. the Va and We, the WTR still increased sorption by up to an average of more than 25% for Cd and more than 40% for Ni and Zn. Because for the Av and la-W soils liming also increased sorption, it could be assumed that the accompanying increase in pH as a result of the addition of WTR promoted precipitation of metals, and/or the resultant increase in negative charge increased their adsorption. These results show that where excess concentrations of soluble heavy metals may occur (especially in coarse-textured soils), and where there is concern about run-off with high P concentrations then this WTR could be considered to immobilize these elements and render them less harmful to the environment. General comments and management guidelines. Based on the results reported above, it is apparent that the WTR can be safely disposed of onto land. It has been demonstrated in the current investigation that rates of application can be as high as 1280 Mg ha-1. Rates of application to land higher than 1280 Mp; ha-1 could probably be acceptable - this was the highest rate tested in this investigation - where the residue is produced in large amounts at the plant, and land for disposal is somewhat limited. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2005.
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Identification of the principal mechanisms driving soil organic carbon erosion across different spatial scales.Müller-Nedebock, Daniel January 2013 (has links)
Soil water erosion is recognized as the principal mechanisms behind soil organic carbon
(SOC) losses from soils, a soil constituent essential for ecosystem functions. SOC erosion can
thus be far-reaching, affecting the future human welfare and the sustainability of ecosystems.
Little research has yet been done to investigate the main mechanisms involved in the lateral
translocation of SOC on the landscape. Understanding the effects of the different water
erosion mechanisms, which control SOC losses (SOCL) at the hillslope level, creates scope
for further scientific studies.
Empirical data from 357 plots, with a range in slope length from 1 (n=117) to 22.1m (n=240)
were analysed to estimate the global variations of particulate organic carbon content (POCC),
POC losses (POCL) and sediment POC enrichment ratio (ER). The global average POCL rate
was calculated to be 12.1 g C m-2 y-1. Tropical clayey soil environments revealed the highest
POCL (POCL=18.0 g C m-2 y-1), followed by semi-arid sandy (POCL=16.2 g C m-2 y-1) and
temperate clayey soil environments (POCL=2.9 g C m-2 y-1). The global net amount of SOC
displaced from its original bulk soil on an annual basis was calculated to be 0.59±0.09 Gt C,
making up an approximated 6.5% of the net annual fossil fuel induced C emissions (9 Gt C).
POCL data for different spatial scales revealed that up to 83% of the eroded POC re-deposits
near its origin in hillslopes, and is not exported out of the catchment. The low organic carbon
sediment ER obtained from the data of clayey soils (ER of 1.1) suggests that most of the
eroded POC remains protected within soil aggregates. Consequently, erosion-induced carbon
dioxide (CO2) emissions in tropical areas with clayey soils are likely to be limited (less than
10%), as the process of POC re-burial in hillslopes is likely to decrease the rate of organic
matter (OM) decomposition and thus serve as a potential carbon sink. Water erosion in sandy
and silty soils revealed organic carbon sediment ER as high as 3.0 and 5.0, suggesting that in
these soils the eroded POC is not re-buried, but is made vulnerable to micro-decomposers,
thus adding to the atmospheric CO2 influx. The results obtained in the review study only
reaffirm that large variations of POCL are evident across the different pedo-climatic regions
of the world, making it a scientific imperative to conduct further studies investigating the link
between SOC erosion by water and the global carbon cycle.
A field study was designed to quantify the POC exported in the eroded sediments from
1x1m2 and 2x5m2 erosion plots, installed at different hillslope aspects, and to further identify
the main erosion mechanisms involved in SOC erosion and the pertaining factors of control.
The erosion plots were installed on five topographic positions under different soil types,
varying vegetation cover, and geology in the foothills of the Drakensberg mountain range of
South Africa. Soil loss (SL), sediment concentration (SC), runoff water (R) and POCL data
were obtained for every rainfall event from November 2010 up to February 2013. Scale ratios
were calculated to determine which erosion mechanism, rain-impacted flow versus raindrop
erosion, dominates R, SL and POCL. Averaged out across the 32 rainfall events, there were
no significant differences in R and POCL between the two plot sizes but SL were markedly
higher on the 5m compared to the 1m erosion plots (174.5 vs 27g m-1). This demonstrates that
the sheet erosion mechanism has a greater efficiency on longer as opposed to shorter slopes.
Rain-impacted flow was least effective where soils displayed high vegetation coverage (P <
0.05) and most efficient on steep slopes with a high prevalence of soil surface crusting. By
investigating the role of scale in erosion, it was possible to single out the controlling in situ
(soil surface related conditions) and ex situ (rainfall characteristics) involved in the export of
SOC from soils. This information will in future contribute toward generating SOC specific
models and thus further inform erosion mitigation. / M. Sc. University of KwaZulu-Natal, Durban 2013.
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Estimation of hydrological properties of South African soils.Hutson, John Leslie. January 1983 (has links)
A computer simulation model of the soil water regime can be a
useful research, planning and management tool, providing that the data
required by the model are available. Finite difference solutions of the
general flow equation can be applied to complex field situations if soil
profile characteristics are reflected by appropriate retentivity (B( Ψ))
and hydraulic conductivity (K(Ψ)) functions.
The validity of a flow simulation model depends upon the degree
to which simulated flow corresponds to the flow pattern in real soils.
Macroscopic flow in apedal soils is likely to obey Darcy's law but in
structured or swe~ling soils, macro-pores and shrinkage voids lead to
non-Darcian flow. Physical composition and structural stability properties
of a wide range of South African soils were used to assess swelling
behaviour and depth-related textural changes. The applicability of a one-dimensional
Darcian flow model to various soil types was evaluated.
Core retentivity data for South African soils were used to
derive regression equations for predicting B (Ψ) from textural criteria
and bulk density. A sigmoidal, non-hysteretic two-part retentivity function
having only two constants in addition to porosity was developed for use
in water flow simulation models. Values of the constants, shapes of the
retentivity curves and soil textural properties were related by fitting
the retentivity function to retentivity data generated using regression
equations~ Hydraulically inhomogeneous soils may be modelled by varying
the values of the retentivity constants through the profile to reflect
changing soil properties. Equations for calculating K(B) or K(Ψ) from retentivity data
were derived by applying each of three capillary models to both exponential
and two-part retentivity functions. Comparison of these equations showed
that the definition and value of semi-empirical constants in the capillary
models were as important as the choice of model in determining K(B).
K(Ψ) was calculated using retentivity constants corresponding to a range
of bulk density, clay and silt content combinations. Three retentivity
constant-soil property systems were evaluated. These were derived from
retentivity data for South African soils between 1) -10 and -1500 kPa,
2) 0 and -50 kPa and 3) from published retentivity data for British
soils. Only that derived from retentivity data accurate in the 0 to -50 kPa
range led to K(Ψ) relationships in which saturated K and the slope sK/sΨ
decreased as bulk density, clay or silt content increased. Absolute values
of K were unreliable and measured values are essential for matching
purposes.
A method for evaluating the constants in a K(Ψ) or K(B) function
from the rate of outflow or inflow of water after a step change in
potential at the base of a soil core was described. Simple exponential
g (Ψ) and K(Ψ) functions were assumed to apply to each pressure potential
range. Retentivity parameters were obtained by fitting the 8(Ψ) function
to the measured retentivity curve. A value for K[s] , the remaining unknown
parameter in the K(Ψ) function, was obtained by matching measured outflow
and inflow data to a family of simulated curves. These were computed using
measured retentivity parameters, core dimensions and ceramic plate
conductivity, and a range of K[s] values. An advantage of this method is that there are no limitations on core length, plate impedance or pressure
potential range which cannot be ascertained by prior simulation.
Regression equations relating texture to retentivity, and a
conductivity model were applied in a simulation study of the water regime
in a weighing lysimeter in which gains and losses of water were measured
accurately. Active root distribution was assumed proportional to root
mass distribution. Relative K(Ψ) curves for each node were computed
using one of the conductivity equations derived earlier. Daily water
potentials for a month were simulated using three conductivity matching
factors. By matching simulated Ψ values to tensiometer potentials measured
at five depths an appropriate matching factor was chosen. The effects
of an over- or underestimate of K(Ψ) were demonstrated.
This work simplifies the prediction and use of retentivity and
conductivity relationships in soil water flow simulation models. These
models can be used for assessing the water regime in both irrigated and
dry-land crop production. Other applications include catchment modelling,
effluent disposal and nutrient and solute transport in soil. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1983.
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