The Karoo-aged basins evolved from assembly to break-up of the supercontinent Gondwana and were filled by denudating major mountain ranges accompanied by vast sedimentary recycling processes. A succession of rift episodes caused the emergence of a great number of these intra-cratonic basins throughout the Gondwana interior, e.g. the Aranos, Karasburg and Huab basins, which are scattered across today’s Namibia. This evolution may be split into a Permian to early Triassic and a Jurassic phase. The Karoo I phase is confined by ‘passive’ continental rifting and a retro-arc extension at the SW margin of Gondwana. The early Jurassic Karoo II rifting phase of east Africa eventually disintegrated Gondwana and led to the opening of the West Indian Ocean. A terminal early Cretaceous rifting phase led towards the opening of the southern Atlantic Ocean and ended the Gondwana supercontinent sedimentary regime. In the course of this evolution, the Namibian late Paleozoic to Mesozoic sedimentary record yields evidence for changing climates from icehouse towards extreme hothouse conditions. As based on sporadic datings the timeline of this evolution remains mostly unclear. The lack of data is in great contrast to the importance of determining the speed of major climate changes. In addition, sediment fluxes within such a supercontinent regime are not well studied but are key in understanding sediment dynamics during severe ecological and environmental changes. Therefore, this thesis tries to establish a timeframe of the sedimentary deposits for the Namibian Karoo Supergroup sedimentary deposits and furthermore tries to explore the laws of sediment dispersal prevailing in southern Gondwana. In order to answer these research questions a comprehensive dataset comprised of 41 samples with more than 5.700 U-Th-Pb LA-ICP-MS age determinations and over 1.000 Lu-Hf isotopic measurements on single zircon grains of siliciclastic rock material of the vast majority of all Permo- Carboniferous to early Cretaceous Karoo-aged Namibian formations was compiled. All of the investigated zircon crystals were also studied with respect to their grain morphology, including length, width, surface, and roundness, providing valuable information concerning transport distances and energies. In combination with whole-rock geochemical data of a majority of the investigated samples, they help deciphering the sedimentary deposition history during the Gondwana supercontinent cycle. A compiled set of southern African U-Th-Pb zircon age data is of great help interpreting sediment fluxes and inferring provenance areas. The onset of Karoo-aged sedimentation is recorded within the Aranos and Karasburg Basin successions and is represented by glacially induced diamictites of the Dwyka Group partially resting directly on pre-Cambrian basement complexes. In places, two distinct E-W directed ice advances are present. The deposition of these glacial diamictites was prior to 296 Ma, as two ash beds incorporated within the overlying shale successions yield Asselian deposition ages. Further hints concerning ice-induced deposition disappear at the Sakmarian-Artinskian boundary, as the lowermost succession of the Ecca Group yields a maximum deposition age of ca. 290 Ma, documenting the end of the Dwyka ice age in the southern Namibian area. The lowermost Ecca Group deposits of the Huab Basin yield a maximum deposition age of ca. 295 Ma, suggesting an earlier termination of the Dwyka ice age in the north. The uppermost strata of the Aranos and Karasburg Basins were dated ca. 265 Ma and 255 Ma, respectively, revealing a disparate depositional history. Due to a lack of datable ash beds as well as no detrital zircon grain ages near the assumed sedimentation age it was not possible to determine a detailed sedimentation history for the Huab, Kunene River, and Waterberg Basin deposits. Detrital zircon U-Th-Pb ages are routinely used in order to trace siliciclastic sedimentary rocks to their bedrock sources, deriving transport directions. This classic ‘source-to-sink’ approach is most likely obscured by several cycles of sediment homogenization processes. A majority of all investigated samples yield high portions of detrital zircon fractions of late Mesoproterozoic (950-1150 Ma) and Neoproterozoic (440-650 Ma) age. In addition, all Jurassic and Cretaceous samples yield a prominent Permian age fraction of 250-280 Ma, suggesting a Gondwanides orogen provenance. Thus, the investigated siliciclastic rocks consist of already recycled sedimentary material. This observation is supported by a high degree of zircon grain roundness. As of zircon grain hardness long transport distances are necessary to achieve latter. This suggests that one sedimentary sink is source for the next sedimentary cycle. A comparison with the detrital zircon record of other southern Gondwanan Permo-Carboniferous successions shows similar results, strongly pointing towards a supercontinent-wide sedimentary recycling regime. Therefore, detrital zircon age patterns within supercontinent scenarios reflect large-scale sedimentary processes rather than primary provenance information.:1 Introduction 1 1.1 Evolution of the Namibian landscape from Carboniferous to Cretaceous times 2 1.2 Thesis format 5 1.3 References 5 2 Methods 10 2.1 Sample preparation and zircon morphometrics 10 2.2 U-Th-Pb age determination 10 2.3 Lu-Hf model age determination via LA-(MC)-ICP-MS 12 2.4 Geochemical analysis 12 2.5 Comparative statistics 12 2.6 References 13 3 Study I: The Permo-Carboniferous Dwyka Group of the Aranos Basin (Namibia) – How detrital zircons help understanding sedimentary recycling during a major glaciation 15 3.1 Introduction 17 3.2 Regional geological setting 17 3.2.1 Geology of the Permo-Carboniferous Dwyka Group (Aranos Basin) 19 3.2.2 Paleotectonic significance of the Dwyka formations 22 3.3 Methods 24 3.4 Results 26 3.5 Discussion 30 3.5.1 Significance of zircon morphologies for sediment fluxes 32 3.5.2 Potential sedimentation rates and source areas indicated by U-Pb age data 35 3.5.3 Implications for the evolution of the Dwyka Group 42 3.6 Conclusions 46 3.7 References 47 4 Study II: The evolution of the southern Namibian Karoo-aged basins: Implications from detrital zircon geochronologic and geochemistry data 63 4.1 Introduction 64 4.2 Geology of the Aranos and Karasburg basins 66 4.3 Tectonic and structural framework of the southern African Karoo aged basins 70 4.4 Methods 71 4.5 Results 75 4.6 Discussion 80 4.6.1 Timing of the Formation of the Aranos and Karasburg basins 80 4.6.2 Provenance and evolution of the upper Paleozoic Aranos and Karasburg basins 85 4.6.3 Implications for the Karoo-aged basin sedimentary record 93 4.7 Conclusions 95 4.8 References 98 5 Study III: Mesozoic deposits of SW Gondwana (Namibia): Unravelling Gondwanan sedimentary dispersion drivers by detrital zircon 109 5.1 Introduction 110 5.2 Geological background 113 5.2.1 Evolution of the southwestern Gondwanan Mesozoic successions 113 5.2.2 Namibian Mesozoic successions 117 5.3 Methods 120 5.4 Results 123 5.5 Discussion 130 5.5.1 Protosources of the sediments 130 5.5.2 Recycling dynamics of the Mesozoic sediments 133 5.6 Conclusions 139 5.7 References 140 6 Study IV: Tracing southern Gondwanan sedimentary paths: A case study of northern Namibian Karoo-aged sedimentary rocks 153 6.1 Introduction 154 6.2 Geological setting 156 6.2.1 SW Gondwanan rifting history and sediment dispersal 156 6.2.2 The northern Namibian Karoo-aged Huab Basin and Kunene section 157 6.3 Methods 161 6.4 Results 165 6.5 Discussion 175 6.5.1 Timing of deposition of the Huab Basin strata 175 6.5.2 Protosources of the sediments 176 6.5.3 Detrital zircon grain morphology and isotope analysis 178 6.5.4 The northern Namibian Karoo-aged basins within the southern Gondwanan framework 185 6.6 Conclusions 186 6.7 References 187 7 Conclusions and outlook 199 8 Supplements 201

African tropical soils play an essential role in global biogeochemical cycles due to carbon (C) they store and the ecosystems they support. Rapid population growth is accelerat- ing land-use changes, particularly the conversion of forest to cropland, which can profoundly affect soil geochemical and microbial properties. How microbial properties relate to geochemical properties in these soils is poorly understood, thus limiting our ability to predict C and nutrient cycling. Our knowledge of the relationships between microbial and geochemical properties comes primarily from temperate regions and tropical regions of America and Asia. However, because of differences in climate, landform, and soil development, this knowledge is not readily transferable to tropical Africa. The objective of this research was to investigate the relationships between microbial and geochemical properties in African tropical forest and cropland soils developed from varying parent material to gain better insight into microbial strategies of nutrient acquisition and investment. Tropical montane forest and cropland soils developed from geochemically different parent ma- terials (mafic, mixed sedimentary rocks, and felsic) were collected in tropical Africa. These samples were analyzed under standardized moisture and temperature conditions to determine microbial properties, including microbial biomass C, potential extracellular enzyme activity, mi- crobial communities, and extracellular polymeric substances (EPS). The standardization was designed to minimize site-specific differences in climatic conditions and thus better assess how geochemical soil properties may influence microbial properties. Despite their long history of chemical weathering, tropical soils developed from different parent materials exhibit geochemical differences that subsequently influence microbial properties to varying degrees. In this study, the chemical index of alteration (CIA) was identified as a useful indicator of a range of geochemical properties. This index, often used to indicate the degree of silicate weathering, was innovatively used in this study to correlate geochemical properties with microbial properties. The role of geochemical properties in controlling microbial properties is particularly pronounced in organic matter (OM)-depleted soils, where geochemical properties likely determine the availability of essential nutrients for microbial life. The influence of OM on patterns of microbial properties is not only a matter of quantity, but also of quality. It has also been found that land-use conversion of forest to cropland does not necessarily reduce microbial diversity as long as soil fertility is maintained at a higher level. Nevertheless, land-use change causes a shift in microbial communities towards microbes that have different resource demand and allocation strategies. Overall, microbial communities in deeply weathered tropical soils are highly adaptive, particu- larly concerning their nutrient acquisition strategies. These strategies provide valuable insights into microbial resource demand and allocation, and potential impacts on C and nutrient cycling in tropical soils. Observed microbial nutrient acquisition strategies suggest that managing mi- crobial communities could potentially be leveraged to maintain or even improve soil fertility and C storage.:Summary Zusammenfassung List of Figures List of Tables List of abbreviations Thesis at a glance 1 General introduction 1.1 Why tropical soils? 1.2 Organic matter cycling: why do controls of soil microbial properties matter? 1.3 Geochemical properties of tropical soils: to what extent do they affect microbial properties? 1.4 Land-use change and its influence on soil properties 1.5 Research objective, questions, and hypotheses 1.5.1 Objective 1.5.2 Research questions and hypotheses 1.6 Outline 2 Materials and methods 2.1 Study area 2.2 Soilsampling 2.3 Incubation experiment 3 Microbial properties in tropical montane forest soils developed from contrast- ing parent material – an incubation experiment 3.1 Abstract 3.2 Introduction 3.3 Laboratory analyses 3.3.1 Soil enzyme assays 3.3.2 Microbial biomass 3.3.3 Soil properties 3.3.4 Data analysis 3.3.5 Statistics 3.4 Results 3.4.1 Soil properties of the different geochemical regions 3.4.2 Patterns of microbial biomass and salt-extractable DOC 3.4.3 Patterns of potential extracellular enzyme activity 3.4.4 Patterns of microbial investment in C and nutrient acquisition 3.4.5 Rotated principal components and mechanistic interpretation 3.4.6 Controls on microbial biomass and nutrient acquisition 3.4.7 Controls on microbial properties after statistical removal of soil depth effects 3.5 Discussion 3.5.1 Patterns of microbial properties in relation to resource availability and geochemical soil properties 3.5.2 Microbial investment in C and nutrient acquisition 3.6 Conclusion and outlook 4 Relationships between geochemical properties and microbial nutrient acquisition in tropical forest and cropland soils. 4.1 Abstract 4.2 Introduction 4.3 Laboratory analyzes 4.3.1 Microbial biomass and enzyme activity 4.3.2 DNA extraction and Quantitative Real-Time PCR (qPCR) 4.3.3 Soil properties 4.4 Data analysis. 4.4.1 An index to describe variation in geochemical soil properties 4.4.2 Statistics 4.5 Results 4.5.1 Variation in geochemical soil properties 4.5.2 Relationship between geochemical soil properties and microbial properties 4.6 Discussion 4.6.1 The CIA is a useful proxy for geochemical variation in tropical soils 4.6.2 Land use-dependent effects of geochemical changes on microbial C and P acquisition 4.6.3 Shifts in bacteria versus fungi abundance affecting C dynamics along the gradients 4.7 Conclusion 5 Extracellular polymeric substances are closely related to land use, microbial communities, and enzyme activity in tropical soils 5.1 Abstract 5.2 Introduction 5.3 Laboratory analyzes 5.3.1 Microbial biomass C, enzyme activity, and nutrient acquisition 5.3.2 Extracellular polymeric substances 5.3.3 DNA extraction 5.3.4 Illumina sequencing and sequence processing 5.3.5 Geochemical soil properties 5.4 Data analysis and statistics 5.4.1 K-means clustering of geochemical soil properties 5.4.2 Variance analysis of nutrient acquisition and EPS 5.4.3 Microbial communities 5.5 Results 5.5.1 EPS concentration and production efficiency 5.5.2 Microbial investment in nutrient acquisition 5.5.3 Patterns and drivers of microbial communities 5.5.4 Differential abundance of microbial taxa between forest and cropland soils 5.5.5 Drivers of EPS concentration and production efficiency 5.6 Discussion 5.6.1 Patterns of EPS concentration and production efficiency depend more on land use than on geochemical soil properties 5.6.2 Land use shapes microbial community assemblages 5.6.3 EPS concentration and production efficiency are related to fungal and bacterial taxa 5.6.4 EPS concentration and production efficiency is related to microbial nutrient acquisition 5.6.5 EPS production efficiency is negatively related to C availability 5.7 Conclusion 6 General discussion and conclusions 6.1 Controls of microbial properties in deeply weathered tropical soil 6.1.1 Organic matter: it is not just a matter of quantity 6.1.2 Geochemical soil properties: a single proxy can be reliable 6.1.3 The unexpected influence of land use 6.2 Microbial nutrient acquisition strategies in deeply weathered tropical soils: implications for C and nutrient cycling 6.3 Final remarks–limitations and perspectives Acknowledgments Bibliography A Appendix to Microbial properties in tropical montane forest soils developed from contrasting parent material – an incubation experiment A.1 Effects of site elevation on microbial properties A.2 Use of vector analysis in assessing microbial resource acquisition B Appendix to Relationships between geochemical properties and microbial nutrient acquisition in tropical forest and cropland soils C Appendix to Extracellular polymeric substances are closely related to land use, microbial communities, and enzyme activity in tropical soils List of publications in peer-reviewed journals Curriculum Vitae Declarations

West Africa is subject to frequent yield losses due to erratic rainfall and degraded soils. At the same time, its population is expected to double by 2050. This situation is alarming in northern Togo, a West African dry savannah area, where rainfed maize is a staple food. Thus, it is necessary to improve agricultural productivity, e.g., by evaluating and introducing alternative irrigation management strategies, which may be implemented in this region. For this purpose, the present investigation focused on evaluating the potential of deficit and supplemental irrigation, as well as assessing the impact of climate and soil variability on maize yield under irrigated agriculture using irrigation optimisation strategies in northern Togo. The Optimal Climate Change Adaption Strategies in Irrigation (OCCASION) framework was adapted and employed to address the research objectives. It involves: (i) a weather generator for simulating long-term climate time series; (ii) the AquaCrop model, which was utilised to simulate the irrigation during the growing periods and the maize yield response to given irrigation management strategies; and (iii) a problem-specific algorithm for optimal irrigation scheduling with limited water supply. Five irrigation management strategies viz. T1: no irrigation (NI), T2: controlled deficit irrigation (CDI) and T3: full irrigation (FI) in the wet season, T4: controlled deficit irrigation (CDI) and T5: full irrigation (FI) in the dry season were assessed regarding their impact on maize yield in northern Togo. The results showed high variability in rainfall during the wet season, which led to substantial variability in the expected yield for NI. This variability was significantly lessened when optimised supplemental irrigation management strategies (CDI or FI) were applied. This also holds for the irrigation scenarios under the dry season. Finally, these findings were validated by an irrigation field experiment conducted at an agricultural research institute in northern Togo. Under a moderate level of deficit irrigation during the vegetative and reproductive growth stages, the above-ground biomass and the maize grain yield were reduced. However, a moderate level of deficit irrigation during the vegetative growth stage could result in similar values of water productivity to that of fully irrigated treatment. It was found that, based on the values of the statistical indicators, AquaCrop has accurately simulated the maize grain yield for all the irrigation strategies evaluated. The results of this study revealed that climate variability might engender a higher variability in the maize yields of northern Togo than soil variability does. Large- and smallscale water harvesting, access to groundwater, and irrigation infrastructures would be required for implementing the irrigation management strategies assessed in this study.:Declaration iii Declaration of Conformity v Dedication vii Acknowledgements ix Abstract xi Table of Contents xv List of Figures xvii List of Tables xix List of Acronyms and Abbreviations xxi 1. Introduction 1 1.1 Background and Problem Statement 1 1.1.1 Global Fresh and Agricultural Water Use 1 1.1.2 Erratic Rainfall, Rising Temperatures, and Soil fertility depletion in West Africa 2 1.1.3 Transboundary Water Issues in West Africa 3 1.1.4 Agriculture and Water Use in Togo 3 1.2 Objectives of the Study 4 2. State of the Art 6 2.1 Relevant Agroecosystems, Farming Systems and Irrigation Management in West Africa 6 2.2 Key Performance Indicators: Water productivity and Food Security 8 2.3 Common Approaches Used to Evaluate Crop Water Productivity 9 2.4 Key production Factors: Climate, Soil and Management 9 2.5 Crop Yield Modelling 12 2.6 Integrated Modelling 13 3. Novel Framework for Optimising Irrigation Systems in West Africa 15 3.1 Model-based Sensitivity Analysis of Climate and Management Impact on Crop Water Productivity, Water Demand and Food Security 15 3.2 Experimental Validation of the Farm Model and Management Strategies, Soil Data Analysis and Modelling 17 3.3 Joint Stochastic Analysis of the Impact of Climate and Soil Variability on Crop Water Productivity and Food Security 19 4. Overview of Publications 21 4.1 Potential of Deficit and Supplemental Irrigation under Climate Variability in Northern Togo, West Africa 21 4.2 Impact of Irrigation Strategies on Maize (Zea mays L.) Production in the Savannah Region of Northern Togo (West Africa) 22 4.3 Impact of climate and soil variability on maize (Zea mays L.) yield under full and deficit irrigation in the savannah region of northern Togo, West Africa 23 5. Conclusion and Outlook 26 References 28 A. Selected Publications of the Author 37 A.1 Potential of Deficit and Supplemental Irrigation under Climate Variability in Northern Togo, West Africa 39 A.2 Impact of Irrigation Strategies on Maize (Zea mays L.) Production in the Savannah Region of Northern Togo (West Africa) 61 A.3 Impact of Climate and Soil Variability on Maize (Zea mays L.) Yield under Full and Deficit Irrigation in the Savannah Region of Northern Togo, West Africa 81 B. Histograms of distributions of the expected maize yield in northern Togo (scenarios in the third paper) 121