Doctoral Degrees (Soil Science)

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Browsing Doctoral Degrees (Soil Science) by browse.metadata.advisor "Hardie-Pieters, Ailsa G."

Now showing 1 - 4 of 4
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    Determination of optimal soil conditions and foliar nutrient levels in commercial rooibos tea production
    (2023-03) Smith, Jacobus Francois Naude; Hardie-Pieters, Ailsa G.; Hoffman, J. E.; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
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    Effect of different biochars on inorganic nitrogen availability
    (Stellenbosch : Stellenbosch University, 2018-03) Aghoghovwia, Makhosazana Princess; Hardie-Pieters, Ailsa G.; Rozanov, Andrei Borisovich; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
    ENGLISH ABSTRACT: Biochar (a fine pyrolysed organic material) is an amendment used to increase and sustain productivity, reduce environmental pollution and sequester carbon (C) in soils. Successes were reported in improving acidic, sandy and highly weathered soil. However, the effects are strongly influenced by biochar physico-chemical characteristics, which vary widely depending on feedstock and pyrolysis conditions. The main objective of this study was to determine the effects of six biochars (commercially-produced in South Africa under various pyrolysis conditions from maize stover, grape pip, grape skin, pine wood, rubber tyre and sugarcane pith) on nitrogen (N) interactions in a sandy soil. The physico-chemical properties of the above biochars were characterised, three main experiments were conducted to study the effects of biochar addition to soil on (1) inorganic (ammonium and nitrate) N adsorption and desorption of added ammonium and nitrate in aqueous solution; (2) soil C and N mineralisation; and (3) leaching of inorganic N fertiliser. Maize stover and grape skin chars were suggested to be imperfect biochars due to low total C contents. Characterisation suggested that the sugarcane pith char was either not a suitable raw material for biochar or it was not actually a biochar due to its low stability and high chemical reactivity. However, its high ash content (66%) suggests good nutrient delivery as a soil amendment. Pine wood biochar was the most recommendable because of its low ash (3.5%), high total C (80%) and high surface area (344 m2 g-1), which all aid nutrient and water holding. However, the grape pip biochar had a low surface area (9.8 m2 g-1) and the highest fixed-C content (87%) which can be good for soil C storage. This work shows that despite many positive effects of biochar application to soil reported in literature, the negative effects of such applications on N availability are clear. All six biochars had a stronger nitrate removal affinity (82-89%) compared to ammonium (33-39%). It was also shown that adsorbed nitrate was not desorbable (0.01-0.23%) compared to adsorbed ammonium removal which was around 50% desorbable with KCl. Based on the shape of the adsorption isotherms, physisorption was the suggested mechanism for this behaviour. Competing reactions such as redox reactions in nitrate adsorption and volatilisation of ammonium were also suggested to have influenced the adsorption study results. Laboratory incubation studies showed that biochars enhanced N immobilisation along with increase in absolute and suppression of relative soil respiration. Pine wood and sugarcane pith biochars were found to reduce inorganic N availability the most due to net N immobilisation. The following biochar property may be linked to N immobilisation: inherent inorganic N in the soil-biochar system. Suppression of relative soil respiration may be due to biochar fixed-C content. Sugarcane pith char had the least effect on relative respiration because of its low fixed-C content (15.6%). However, the remaining biochars were substantially limiting the relative CO2 emissions. Rubber tyre char was the best performer in this regard with 75% lower cumulative relative CO2 emissions compared to the control. Among the plant-derived biochars, grape pip had the lowest CO2 released with 59% lower cumulative relative CO2 release. The leaching column experiment showed that application of biochars at 2.5% (w/w) to sandy soil reduced cumulative leaching of NH4+ and NO3- by 15-26% and 11-54%, respectively, compared to unamended soil. Using 15N labelled ammonium nitrate, it was found that 0.77-10.81% of applied fertiliser N remained in soil-biochar treatments after leaching. Only the pine wood and sugarcane pith biochar treatments significantly increased N fertiliser retention by 136 and 157% compared to the control soil. Whereas, the rubber tyre biochar treatment significantly reduced N fertiliser retention by 81%. The study concludes that all six biochars make inorganic N less available by mechanisms such as nitrate capture which is related to aromaticity and metal content of the chars and by enhancing biological immobilisation.
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    Lipid humification by soil clays
    (Stellenbosch : Stellenbosch University, 2019-04) Adams, Adrian Richard; Clarke, Catherine E.; Hardie-Pieters, Ailsa G.; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
    ENGLISH ABSTRACT: We studied three aspects of the natural polymerisation (humification) of lipids by soil clays – namely, the products formed, reaction mechanism and kinetics – at environmental temperatures (c. 20–50°C). Various clays were reacted with oleic acid (our chosen model lipid). The Mn-oxide birnessite was the most reactive toward oleic acid, polymerising it into quasi-solid polyesters. A probing of the birnessite-oleic acid reaction mechanism revealed that the formation of a surface exchange complex between oleic acid carboxyl groups and birnessite surface sites (>Mn(III)/>Mn(IV)) is a crucial first step of the reaction. Subsequent chelation and one-electron reduction of Mn(III) to Mn(II) forms radical oleic acid species which couple and thereby polymerise. Kinetic studies revealed that the birnessite-oleic acid reaction was near-linearly dependent on birnessite mass-loading (rate order ~ 0.75) but virtually independent of birnessite surface pH (rate order ~ 0.2). A determined activation energy for the reaction of 12.8 ± 4.2 kJ/mol revealed that it is energetically more spontaneous than the usual autoxidation pathways. These findings broaden our understanding of the role soil clays play in lipid humification in soils.
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    Mitigation of soil and ground water pollution caused by on-land disposal of olive mill wastewater
    (Stellenbosch : Stellenbosch University, 2016-03) Umeugochukwu, Obiageli; Rozanov, Andrei Borisovich; Sigge, G. O.; Hardie-Pieters, Ailsa G.; Stellenbosch University. Faculty of Agrisciences. Dept of Soil Science.
    ENGLISH ABSTRACT: Olive mill wastewater (OMW) is generated in large quantities, particularly in the regions with a Mediterranean climate where olive oil is produced on a commercial scale. Some producers collect the effluent and dispose of it as hazardous waste at significant expense, while others dispose of it directly on land, claiming the potential benefits to productivity from the plant nutrients present in the OMW. It was shown that the OMW also contains some phytotoxic phenols, which may have both immediate and cumulative negative effects on plant growth. The long-term effects on the soil and crop growth have been shown to be detrimental. Sandy soils are of particular concern due to the possibility of phenol penetration into deeper soil layers and potential ground water contamination. The study explores in-situ (soil amendment with biochar prior to the OMW disposal) and ex-situ (OMW filtration through a biochar bed) options to mitigate the negative effects of the OMW on-land disposal. A laboratory batch sorption experiment was set up using 0.2 g pinewood biochar to explore the possibilities of removing the phenols from 50 mL of the OMW or gallic acid (GA) solutions at different concentrations. The results showed that the sorption process was rapid and stabilized within one hour. The kinetic process followed a pseudo-second-order model and was described by the Freundlich multi-layer isotherm. The pinewood biochar had a sorption capacity of 30 mg·g-1 and 100 % removal was obtained with 300 g·l-1 of the OMW load. It was found that pinewood biochar could be used to remove the phenols contained in the effluent. A column experiment was set up to determine the effectiveness of biochar and biochar-soil mixtures in removal of phenol and Chemical Oxygen Demand (COD) from the OMW compared to sand filtration. The breakthrough curves for phenol and COD were determined, while the pH and EC of the filtrates were monitored. Ten PVC columns of 30 cm height and 5 cm diameter were filled with five different materials: sand, sand + biochar, Hutton clay loam soil, Hutton clay loam soil + biochar and biochar alone. Two different treatments were given to the columns; five of the columns were prewashed with 2 liters of deionized water and the other five were not washed before the OMW filtration. The performance of the columns was determined in respect of the phenol and COD removal capacities, hydraulic conductivities and porosity changes. The results showed that washing enhanced the phenol sorption but not the COD sorption. The addition of the biochar at 2%wt load significantly improved the effectiveness of the filtration. The best performance was achieved in terms of COD removal in pure biochar columns, but in terms of the phenol, the best performance was on a pre-washed Hutton clay loam soil with 2%wt biochar addition. Both the washing and biochar addition affected the porosity and reduced the hydraulic conductivity of the columns. The greenhouse experiments were conducted to confirm the above statement using pot trials laid out in a 4 x 4 factorial Randomized Complete Design (CRD) to determine the effect of effluent and biochar on wheat and green beans on alkaline sand. Results showed that the increasing effluent rate up to 200 m3·ha-1 gave significantly negative results on wheat growth, even with fertilizer application. But the effect was different for beans where low effluent loads gave positive results though not significant while with fertilizer (N and P) 50 m3·ha-1 performed better. With the addition of biochar there was no significant effect on wheat, but it significantly affected beans at the application rate of 2.5 and 5%wt. The interaction of biochar and effluent showed that the best performance was at 5% biochar application and effluent loads of 50 and 100 m3·ha-1, but increased effluent rate decreased production even with a 5% wt biochar application rate. It was suggested that a leguminous crop should tolerate OMW application better compared to wheat even in the adverse conditions of the alkaline sand. A second greenhouse experiment was conducted with another legume, an indigenous African crop, the bambara groundnut, on an acidic Hutton clay-loam soil (Oxisol) sourced locally. The experiment was laid out in a 2 x 6 CRD factorial design to determine the effect of the biochar and effluent combination on the yield and growth parameters of bambara as well as the effect on soil conditions and nutrient availability. The result showed that biochar addition improved seed germination, which was retarded by effluent loading. The effluent rate of 200 m3·ha-1 and biochar 2% gave the best yield performance. The biochar addition increased the pH and hence affected the release of P and N whereas Na and K availability were reduced. We conclude that biochar may be used for both ex-situ filtration to treat the OMW, and as a soil amendment to allow safe on-land disposal of the OMW. The estimations of safe disposal loads and the required application rates of the biochar should be made individually for a specific soil type. Pinewood biochar was proven to be a cheaper source of activated carbon for the treatment of olive mill wastewater organic contaminants in South Africa.