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Thesis (MSc. Agriculture(Soil Science) -- University of Limpopo, 2018 / Soil acidity is one of the abiotic stress factors that greatly limit the productivity of crops on farmers’ fields. A greenhouse study was carried out over two summer growing seasons to evaluate the effect of lime and phosphorus (P) application rates on the growth, yield and root attributes of a determinate cowpea variety on acid soil. The experiment was laid out as a 4x5 factorial arrangement with 4 replications. Treatment factors comprised of variable rates of Vaalburg dolomitic lime (0, 2, 4 and 6 t ha-1) and P (0, 15, 30, 45 and 60 kg ha-1) using single super phosphate, 10.5% P. The two treatment factors were combined resulting in a total of 20 treatment combinations. Data collected included cowpea growth parameters, crop phenology, yield attributes and root characteristics. While cowpea plants with no P application consistently gave the least plant height, stem diameter, number and length of trifoliate leaves, the 6 t ha1 lime rate appears to be completely disadvantaged for all measured parameters with generally lower values than in soil filled pots without lime application. Results showed that soil pH was increased with 6 t ha-1 lime application while soil electrical conductivity (EC), percent of organic matter (OM) and total organic carbon (TOC) were all increased with increasing P and lime rates. All measured cowpea growth attributes such as plant height, stem diameter, number of trifoliate leaves, and leaf area were significantly increased (p≤0.05) with increasing P and lime rates. During the two planting seasons, P and lime application resulted in reduced (p≤0.05) duration to flowering, pod formation and physiological maturity. The 6 t ha-1 lime application produced higher number of pods (2.50) compared to the other rates. Application rates of 45 kg P ha-1 and 6 t ha-1 of lime produced superior number of seeds per pod with high values of (13.71) and (12.85), respectively. However, cowpea root attributes namely number of nodules per plant, the third branching root diameter, angle of adventitious root, tap root diameter at 5 and 10 cm, shallow and deep score were significantly increased at moderate P rate of 30 kg P ha-1. Overall, findings of this study revealed that application of both P fertiliser and lime were able to ameliorate the negative effect of P deficiency from soil acidity on the evaluated cowpea variety and promoted increased yield. Keywords: Acid soil, grain cowpea, P fertiliser, lime, growth, root characteristics, yield / National Research Foundation (NRF) and Department of Agriculture, Forestry and Fisheries (DAFF)

Gegenstand dieser Arbeit ist die Untersuchung von zeitabhängigen Verfestigungseffekten in Sandböden. Dies geschieht im Rahmen des Leitthemas „Modifizierung und Optimierung von Baugrund zur Ressourcenschonung“ bei der G²-Gruppe Geotechnik. Die bereits in Feld- und Laborversuchen beobachtete und untersuchte Zunahme von Festigkeits- und Steifigkeitsparametern in grobkörnigen Böden über Zeit, die nicht auf die Setzung zurückzuführen ist, wird anhand von zwei Versuchsmethoden unter Laborbedingungen untersucht. Die Versuche werden unter Betrachtung verschiedener Einflussfaktoren durchgeführt. Die verwendeten Methoden sind eine Labor-Drucksondierung und eine Ultraschalluntersuchung, deren Prüfgeräte für die Untersuchung der zeitabhängigen Verfestigungseffekte entwickelt wurden. Die Kontrolle der Geeignetheit für den Nachweis der zu untersuchenden Effekte und die damit verbundenen Anpassungen der Prüfgeräte sollen die Erstellung und Optimierung eines Versuchsprogramms ermöglichen.:Einleitung 1 Überblick 1.1 Hinweis zum Ultraschallversuch 2 Ziele 3 Grundlagen und Stand der Wissenschaft 3.1 Beschreibung des Effekts 3.2 Abgrenzung zu zeitabhängigen Verfestigungseffekten in bindigen Böden 3.3 Sand in der Geotechnik 3.3.1 Makroskopische Betrachtung geotechnischer Eigenschaften 3.3.2 Mikroskopische Betrachtung geotechnischer Eigenschaften 3.4 Kenntnisstand der Wissenschaft zum Aging-Effekt in grobkörnigen Böden 3.5 Labor- und Feldversuche zur Untersuchung von Aging-Effekten in Sandböden 4 Versuchsmethodik 4.1 Einführung 4.2 Untersuchung des Versuchsmaterials 4.2.1 Bodenkenngrößen 4.2.2 Mikroskopische Bodenuntersuchung 4.3 Labor-Drucksondierung 4.3.1 Versuchsaufbau 4.3.2 Übersicht der Versuchsreihen 4.3.3 Versuchsauswertung 5 Ergebniszusammenstellung und Auswertung 6 Fazit und Ausblick 7 Literaturverzeichnis Erklärung / The subject is the investigation of time-dependent consolidation effects in sandy soils. This takes place within the framework of the main theme 'Modification and optimization of subsoil to conserve resources' at the G²-Gruppe Geotechnik. The increase in strength and stiffness parameters in granular soils over time, which has already been observed and investigated in field and laboratory tests, and which is not due to settlement, is investigated using two testing methods under laboratory conditions. The tests are carried out considering various parameters. The methods used are a Mini-CPT and an ultrasonic examination, the test equipment of which was developed for the examination of the Aging-effect of soils. The control of the suitability for the verification of the effects to be examined and the related adaptations of the test devices should enable the designing of a test routine.:Einleitung 1 Überblick 1.1 Hinweis zum Ultraschallversuch 2 Ziele 3 Grundlagen und Stand der Wissenschaft 3.1 Beschreibung des Effekts 3.2 Abgrenzung zu zeitabhängigen Verfestigungseffekten in bindigen Böden 3.3 Sand in der Geotechnik 3.3.1 Makroskopische Betrachtung geotechnischer Eigenschaften 3.3.2 Mikroskopische Betrachtung geotechnischer Eigenschaften 3.4 Kenntnisstand der Wissenschaft zum Aging-Effekt in grobkörnigen Böden 3.5 Labor- und Feldversuche zur Untersuchung von Aging-Effekten in Sandböden 4 Versuchsmethodik 4.1 Einführung 4.2 Untersuchung des Versuchsmaterials 4.2.1 Bodenkenngrößen 4.2.2 Mikroskopische Bodenuntersuchung 4.3 Labor-Drucksondierung 4.3.1 Versuchsaufbau 4.3.2 Übersicht der Versuchsreihen 4.3.3 Versuchsauswertung 5 Ergebniszusammenstellung und Auswertung 6 Fazit und Ausblick 7 Literaturverzeichnis Erklärung

Many environmental crises are threatening to collapse human societies, but also life on the Earth as we may know. Climate change due to anthropogenic global emissions is one of the main issues, but also soil degradation, and the management of the enormous amount of organic wastes that are harmfully released into the environment by human activities. On the one hand, direct spreading of biowastes onto the land is often limited due to eutrophication or pollution by toxic compounds. On the other hands, these biomasses may contain nutrients, such as phosphorus (listed as a critical raw material) and nitrogen, which could be recovered to sustain the high demand in expensive soil amendments and fertilizers. Thus, finding new solution to close the loop towards circular economy and sustainable processes is crucial, nowadays, to convert the global trends and restore the delicate equilibrium in the water-food-energy nexus, with soil ‘under special surveillance’. In this framework, many researchers are focusing their attention on hydrothermal carbonization (HTC) as a possible candidate, in particular to treat moist biowastes and obtain useful solid (hydrochar) and liquid (HTC liquor) products. Nutrients contained in biowastes are sensitive to HTC process parameters and initial feedstock properties, thus it is crucial to analyze their distribution and (im)mobilization among the HTC products for their reuse in soil. On the other hand, since some organic compounds are generated during the thermochemical process, which may be responsible for toxicity to plants and other organisms, analysis of their possible toxicity towards the biosphere becomes crucial before implementation at a large scale. This thesis collects our efforts to explore new processes and deepen the knowledge about the possibility to produce amendments suitable for soil application from hydrochar. The core is from a process engineering perspective, focusing on the production phase and the characterization of the products, never forgetting any possible limits or the implications on ecotoxicological issues. In the first part of the thesis, ‘How to make hydrochar a soil amendment?’ is the main question. Based on few pioneer studies about composting of hydrochar, we assessed a complete analysis of hydrochar co-compost, produced from the 25-day aerobic stabilization of digestate of organic fraction of municipal solid waste, together with a fraction of its hydrochar, and green waste. Under the umbrella of C2Land project (funded by The European Institute of Innovation & Technology -EIT), we produced this new amendment in specifically designed bioreactors, then we characterized the products from the point of view of physico-chemical properties, nutrients distribution, toxicity to plants and mammalian cells. We are confident that the crucial result of toxicity removal from hydrochar through composting will attract the interest of many stakeholders, since implementing HTC in anaerobic/aerobic plants is almost ready to be applied at a large scale. Moreover, an experimental campaign performed at the University of Leeds further explored the effect of some other post-treatments on hydrochar composition and phytotoxicity, depending on the initial feedstocks (manure, sewage digestate, water hyacinth, and grass). The second part of the thesis was dedicated to nutrients recovery, necessary when some harmful matrixes - such as the municipal sewage sludge digestate - are not allowed to be reused for agricultural purposes, due to law limitations or pollution issues. In this case, nutrients recovery (N, P) could be achieved chemically, via HTC and struvite precipitation, being struvite recognized as a good fertilizer. Firstly, we performed a techno-economic assessment of the implementation at a district level of HTC-centered sewage sludge management. The mass reduction of waste streams may have some important effects on transportation costs, while the recirculation of HTC liquor back to the anaerobic digestors can imply more biomethane production with increased revenues. A mass balance for the potential of N and P recovery through struvite precipitation in the centralized facility was also assessed. Secondly, a near-zero waste treatment process for municipal sewage sludge via HTC was designed according to the available literature and preliminarily analyzed at lab level. The fractionation of the different types of phosphorus was analyzed through the STM-P protocol before and after HTC, confirming the mineralization of organic phosphorus during the process. Citric acid was validated as a good green solvent for phosphorus extraction and high-quality struvite precipitation, even if the process was not still optimized in terms of yields. Future works may identify the best process parameters for final metal recovery from liquid byproducts in order to internally recirculate them back to water line, and to characterize the leached hydrochar as purified renewable fuel or soil amendment. Thus, the proof-of-concept chain would be confirmed and could be applied to the sustainable HTC-centered biorefineries of the future.