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