Clay minerals and pedogenic metal (oxyhydr)oxides (hereafter termed ‛oxides’) significantly affect organic carbon (OC) storage and persistence in soils. Although it is known from laboratory experiments that minerals have a high variability in their reactivity, there is still a lack of fundamental knowledge how changing mineral composition under natural soil conditions works. The frequently used clay content (soil particles < 2-μm) or the dithionite-citrate-bicarbonate extractable Fe (Fed) content do not allow a mechanistic explanation of the function of clay minerals and pedogenic metal oxides. This is due to the fact that a similar relative mineral composition cannot necessarily be derived from the clay content. Therefore, the overall objective of the thesis was to identify the influence of defined mineralogical combinations on OC content, stock, and persistence under forest and cropland. The study examined highly weathered acidic soils (mainly Acrisols) from the East Usambara Mountains in NE Tanzania. The long-term weathering of geologically similar crystalline material under a humid tropical climate has produced a uniform spectrum of pedogenic minerals. Nonetheless, differences in the relative compositions of kaolinite (Al2Si2O5(OH)4), gibbsite (γ-Al(OH)3), goethite (α-FeOOH) and hematite (α-Fe2O3) were identified for the investigated soils. The soils were classified according to their mineralogical composition, based on the content of aluminous clay (kaolinite and gibbsite) and pedogenic Fe oxides (goethite and hematite). The applied approach generated a range of 149–434 g kg–1 for aluminous clay, and a range of 21–101 g kg–1 for Fed. A significant loss of 1.3 kg C m–2 was observed for the top 0–10 cm under cropland in comparison to the near-natural forests. Both the OC content and its persistence were significantly affected by the mineralogical composition in the soils. The ‛low clay‒high Fe’ combination had a similar or significantly higher OC content under both land uses, whereas significantly lower OC contents were measured for the other combinations under cropland. A more distinct effect of the mineral composition was measured for the mineral-associated OC (MAOC) content, which was determined by density fractionation and defined as OC associated with the heavy fraction (HF). A MAOC content of 37.4 g C kg–1 was measured for the ‛low clay‒high Fe’ combination, while only 19.4 g C kg–1 was found for the ‛high clay‒high Fe’ combination under cropland in the 0–10 cm depth. The lower OC and MAOC contents were accompanied by a significantly higher content of soil particles < 2-μm for the latter combination. This clearly reveals that no causal relationship between the OC and MAOC contents with the clay content can be established unless the dominant mineral phases and their composition for the soil sample are known. Analysis of the soils in the mineralogical combinations showed that with similar mineralogy, the Fed / aluminous clay ratio can be used as a metric to evaluate OC and MAOC contents and the persistence against land-use change. No significant changes were measured in the investigated soils under land-use change from forest to cropland for ratio values between 0.44 and 0.56. Results from a conducted incubation experiment and chemical oxidation of mineral-bound OC supported this result. The separation of the HF material in defined grain-size fractions along the respective mineralogical combinations revealed that the siltHF and clayHF fractions make an almost identical contribution to the total MAOC storage under undisturbed soil conditions. Thus, the MAOC content of the entire HF is mainly explained by the different mass of the grain-sizes fractions. Of particular note is the variable effect of land-use change on MAOC contents in the siltHF and clayHF fractions. While the MAOC content of the clayHF fraction showed little variation between land uses, the MAOC content of the siltHF fraction tracked the contents of the entire HF fraction along the mineralogical combinations. This result implies that small microaggregates / mineral-clusters (< 63-μm) are modified in their properties by changing mineral composition, which has a direct impact on the persistence of MAOC. Soil structure analysis revealed a high amount of very stable macroaggregates of the soils along the mineralogical combinations with small, but significant differences under both land uses. The results showed that an aluminous clay content of > 250 g kg–1 combined with Fed content of < 60 g kg–1 (‛high clay‒low Fe’) had a significantly positive effect on macroaggregation. The opposite combination (‛low clay‒high Fe’) caused a significant decrease in aggregates > 4 mm under cropland, which, however, was not associated with a significant decrease of the aggregate related OC content. Thus, the macroaggregation was influenced by the mineralogical combinations, but this had no significant effect on the related OC content. Rather, it becomes clear that interactions between minerals were particularly important in mineral clusters > 2-μm and < 63-μm, which determined the significant impact on the total (MA)OC content along the mineralogical combinations. Further studies can follow this trajectory to extend the mechanistic understanding of OC and MAOC content and persistence for highly weathered soils in the humid tropics.:1. General introduction 1.1 Soils of the humid tropics and their dominant mineral phases 1.2 Influence of mineralogy on the formation of mineral-associated OM in highly weathered soils of the humid tropics 1.3 Aggregation in weathered soils of the humid tropics: formation and associated organic carbon content 1.5 Scientific goals, main objectives, and hypotheses 2. Methodology 2.1 Characterization of the study area and the sampling strategy 2.2 Analyses to characterize mineral phases, MAOC and aggregation of the selected mineralogical combinations 2.3 Data set and applied statistical methods 3. General results and discussion 3.1 Soil mineralogical properties of the selected aluminous clay and pedogenic Fe oxide combinations 3.2 Basic considerations on bulk OC stocks and effect of mineralogical combinations on bulk OC and MAOC content and persistence 3.3 Effect of mineralogy on aggregation and aggregate-related OC contents 4. Conclusion 5. References Part II – Scientific publications 1. Impact of land use on soil organic carbon stocks in the humid tropics of NE Tanzania 1.0 Abstract 1.1 Introduction 1.2 Material and Methods 1.2.1 Study area 1.2.2 Description of the investigated land uses 1.2.3 Sampling, processing and analysis of soil 1.3 Results 1.3.1 Soil characterization 1.3.2 Content and stocks of SOC to 100 cm depth (soil profile approach) 1.3.3 SOC stocks of diagnostic soil horizons compared to fixed depth increments (satellite approach) 1.3.4 Content of clay, dithionite- and oxalate-extractable Fe and Al, and relationships to SOC content 1.4. Discussion 1.4.1 Impacts of land use on SOC stocks and potential controls of SOC stabilization 1.4.2 Effects of sampling schemes on the analysis of SOC stocks 1.5 Conclusions 1.6 References 2. Iron oxides and aluminous clays selectively control soil carbon contents and stability in the humid tropics 2.0 Abstract 2.1 Introduction 2.2 Results and Discussion 2.2.1 Mineralogical and geochemical properties of soil mineralogical combinations 2.2.2 Dependence of bulk soil carbon on aluminous clay and pedogenic Fe oxides 2.2.3 Effects of aluminous clay and pedogenic Fe oxides on mineral-associated carbon 2.2.5 Chemical and biological resistance of bulk and mineral-associated carbon 2.3 Implications 2.4. Methods 2.4.1 Study area and soil sampling 2.4.2 Basic soil properties 2.4.3 Density fractionation 2.4.4 Aluminous clay and pedogenic Fe oxide contents 2.4.5 Total element contents and weathering indicators 2.4.6 X-ray diffraction (XRD) 2.4.7 Mössbauer spectroscopy 2.4.8 Specific surface area (SSA) 2.4.9 13C Nuclear Magnetic Resonance Spectroscopy 2.4.10 Wet chemical oxidation and soil respiration 2.4.11 Statistics and calculations 2.5 References 2.6 Supplementary material 3. Aluminous clay and pedogenic Fe oxides modulate aggregation and related carbon contents in soils of the humid tropics 3.0 Abstract 3.1 Introduction 3.2 Material and methods 3.2.1 Study area and soil sampling 3.2.2 Soil analyses 3.2.2.1 Basic soil properties and terminology of selected mineralogical combinations 3.2.2.2 Aggregate size distribution and aggregate stability 3.2.2.3 Statistics and calculations 3.3 Results 3.3.1 Mineralogical composition and general soil properties 3.3.2 Aggregate size distribution 3.3.3 Aggregate stability 3.3.4 Organic carbon in soils and aggregate size fractions 3.4 Discussion 3.4.1 Aggregation and aggregate stability as controlled by aluminous clay and pedogenic Fe oxides 3.4.2 Importance of aggregation for OC persistence – effects of aluminous clay and pedogenic Fe oxides 3.5 Conclusions 3.6 References 3.7 Supplementary material 4. Appendix Erklärung zur Eröffnung des Promotionsverfahrens

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Soils are a critical resource for supporting ecosystems, agricultural systems, and human wellbeing. However, these same soils have been degraded by human activities throughout human history. Despite the rapid development of global models that include dynamic changes in land use and land cover (LULC) and biogeochemical processes to assess climate and hydrological impacts, soil properties are often assumed to be spatially or temporally constant. These assumptions can affect the results of model projections, impact assessments and underestimate the human impact on Earth systems. This study reveals the physical impacts of human-altered soil conditions on the global water balance through a meta-analysis study and soil degradation modeling. We link major global LULCs to four hydrologic soil groups: sandy (sand, sandy loam, and loamy sand), loamy (loam, silty loam, and silt)), clayey soils (clay, sandy clay, clay loam, silty clay, and silty clay loam), and sandy clay loam) from 850 to 2015 AD, and identified loamy and clayey soils as the preferred soils for most human land uses. Humans selectively use those soils for intensive agriculture and pasture activities, while grazing occurs on sandier soils. To simulate the impact of human activities on soils, several soil change models were built for soil organic carbon (SOC) content, soil texture (sand, silt, and clay), and soil bulk density from meta-analyses of site observations. The models were applied globally based on the LULC and soil relations, global environmental and soil conditions, and LULC distributions. Pedotransfer functions were applied to estimate soil water-holding capacity using those soil properties, then a Thornthwaite-type water balance model was used to assess the impacts of soil degradation on the global water balance. Results show that under a high-intensity LULC scenario (conventional tillage on croplands and heavy grazing), SOC decreases by 363 Pg and water deficit increases 78 km3 globally. The impacts on SOC and deficit are reduced to 213 Pg and 51 km3, respectively, when reducing land-use intensity by substituting animal ploughing/no-till and light grazing for conventional tillage and heavy grazing. Impacts from other LULC types are identical for these two LULC scenarios. Development of this history between LULC and soil properties allows for improved simulation of human impacts on global water, energy, and biogeochemical cycles. The results of the water balance simulations demonstrate how different soils representations in models can significantly alter the estimates of global evapotranspiration, water deficit, and surplus. This study contributes to developing a better understanding of the processes by which human-induced soil degradation impacts climate/hydrological models and providing a mechanism to better assess the impacts of humans on the Earth system. The outcome will also complement numerous ongoing global studies that evaluate the impacts of climate change on water resources and society. / Graduate / 2023-08-09

Forecasted changes to climate and land use were used to model variations in the streamflow characteristics of Occoquan watershed and water quality in the Occoquan reservoir. The combination of these two driving forces has created four themes and an integrated complexly-linked watershed-reservoir model was used to run the simulations. Two emission scenarios from the fourth assessment report of the Intergovernmental Panel on Climate Change (IPCC), along with four General Circulation Models (GCMs) by using two statistical downscaling methods, were applied to drive the Hydrological Simulation Program - Fortran (HSPF) and CE-QUAL-W2 (W2) in two future time periods (2046-2065 and 2081-2100). Incorporation of these factors yielded 68 simulation models which were compared with historical streamflow and water quality data from the late 20th century. Climate change is projected to increase surface air temperature and precipitation depth in the study area in the future. Using climate change only, an increase in high and median flows and decrease in low flows are projected. Changes in flow characteristics are more pronounced when only future land use changes are considered, with increases in high, median and low flows. Under the joint examination of the driving forces, an amplifying effect on the high flows and median flows observed. In contrast, climate change is projected to dampen the extreme increases in the low flows created by the land use change. Surface water temperatures are projected to increase as a result of climate change in the Occoquan reservoir, while these changes are not very noticeable under the effect of land use change only. It is expected that higher water temperatures will promote decreased oxygen solubility and greater heterotrophy. Moreover, longer anoxic conditions are projected at the bottom of the reservoir. Results indicate that higher water temperature will increase the denitrifying capacity of the reservoir, especially during summer months, further reducing the nitrate concentration in the reservoir. / PHD / Water resources managers are facing a new set of challenges of developing strategies to address the regional impacts of climate change and land use change, especially in metropolitan areas. Simulating climate change and land use change scenarios can shed light on mitigation and adaption approaches for water resources management as well as future designs (for example, infrastructure, agriculture, irrigation, etc. among other sectors). The focus of this study is the Occoquan watershed with an area of 1530 km² (590 square miles) which includes the 1700-acre Occoquan reservoir that yields about 40% of the drinking water supply of near 2.0 million residents in northern Virginia. The Occoquan watershed located approximately 40 km to the southwest of Washington, D.C. and is situated in the Mid-Atlantic region of the United Sates with four distinct seasons and is part of a bigger watershed known as the Potomac River Watershed. The primary aim of this research is to provide an improved, quantitative understanding of the potential impacts of climate change and land use change on the Occoquan watershed. The findings of this research can benefit future water supply reliability and mitigation strategies in the study area considering this watershed’s essential role as a water supplier in northern Virginia.

Societal developments are associated with land use change, and with urbanization in particular. Urbanization can influence hydrological processes by decreasing evapotranspiration and infiltration as well as by increasing streamflow, peak flow and overland flow. This causes higher risks of flooding. Although several studies have investigated the impacts of urbanization on streamflow over the last decades, less is known about how urbanization affects the hydrological processes in peri-urban areas characterized by a complex mosaic of different land uses. This study aimed to model the impact of land use change, or more specifically urbanization, on the hydrological responses of the small peri-urban Ribeira dos Covões catchment (6.2 km2) located in central Portugal. The catchment has undergone rapid land use change since the mid- 1950s associated with conversion of agricultural fields (decreased from 48 to 4%) into woodland and urban areas, which increased from 44 to 56% and from 8 to 40%, respectively. For the study, the hydrological modelling system MIKE SHE was used. Parameters and data of climate, vegetation and soil types were used as input. There were also land use maps and daily streamflow values available for the hydrological years 2008/09 to 2012/13, which were used to calibrate and validate the model. The statistics from the calibration and validation both indicated that the model simulated the streamflow well. The model was designed to examine both how past land use change might have affected the streamflow, and to investigate the impacts on hydrology if the urban area was to be increased to cover 50% of the catchment. It was not only the importance of the urban cover’s size that was tested, but also the placement of additional urban areas. Three future scenarios were run, all with a 50% urban cover, but distributed differently within the catchment. The study did not indicate that an increase in urbanization leads to higher peak flow or streamflow. Neither could any decrease in infiltration be seen. All three scenarios however gave an increase in overland flow of approximately 10% and a decrease in evapotranspiration by 55%, regardless of where the urban areas were added. The reliability of the models can be enhanced by additional climate, soil and vegetation data. This would improve the results and make them more useful in decision making processes in the planning and management of new urban areas. / Samhällets ständiga utveckling medför förändringar i markanvändning. Utvecklingen och förändringarna är framför allt associerade med urbanisering som kan påverka ett avrinningsområdes hydrologiska processer genom att exempelvis reducera dess evapotranspiration och infiltration samt öka vattenföringen, högsta flödet och ytavrinningen. Det i sin tur ökar risken för översvämning. Trots att många studier har undersökt urbaniseringens inverkan på vattenföring de senaste decennierna saknas viss kunskap om dess påverkan på hydrologin i stadsnära avrinningsområden, kännetecknade av flera olika typer av markanvändning. Denna studie syftade till att modellera hur förändringar i markanvändning, eller mer specifikt urbanisering, påverkar hydrologin i det lilla stadsnära avrinningsområdet Ribeira dos Covões (6,2 km2) i centrala Portugal. Avrinningsområdet har genomgått snabba markanvändningsförändringar sedan mitten av 1950-talet i samband med en omvandling av åkrar (täckningsarean har minskat från 48 till 4 %) till skogsmark och urbaniserade områden, vilkas storlek har ökat från 44 till 56 % respektive 8 till 40 %. För att uppfylla syftet har den hydrologiska modellen MIKE SHE använts. Parametrar avseende klimat samt vegetations- och jordegenskaper användes som indata till modellen. Det fanns också tillgång till en markanvändningskarta över området samt dagliga flödesvärden mellan de hydrologiska åren 2008 och 2013. Dessa användes för att kalibrera och validera modellen. Statistiken för både kalibreringen och valideringen indikerade en fullt acceptabel modell. Modellen var avsedd att undersöka dels hur tidigare förändring i markanvändning kan ha påverkat vattenföringen, dels för att studera effekten på hydrologin om urbaniseringen fortgår tills dess täckning är 50 % av avrinningsområdet. Det var inte bara betydelsen av de urbana ytornas storlek som testades, utan även placeringen av dem. Tre framtidsscenarier togs fram, alla med en urban yta på 50 % fördelad olika inom avrinningsområdet. Studien indikerade inte att ytterligare urbanisering ökar vare sig flödet eller det högsta flödet. Inte heller gav de någon minskning av infiltration. Alla tre scenarierna gav emellertid en ökning av ytavrinningen med cirka 10 % och en minskning av evapotranspirationen med 55 %, oavsett placering av de urbana ytorna. Modellernas tillförlitlighet skulle kunna förbättras med hjälp av ytterligare klimat-, vegetations- och jordindata. Det skulle förbättra resultaten och göra dem användbara i beslutsfattanden vid planering och utveckling av nya urbana områden.

In den letzten zwei Jahrzehnten wurden in Sumatra (Indonesien) große Regenwaldflächen für den Anbau von Kautschuk- und Palmölplantagen zerstört. Dies zeigt sich in der Abnahme Waldfläche in dieser Region um 36% zwischen 1990-2010. Eine solch schnelle Landnutzungsänderung hat Auswirkungen auf die Umwelt: Es ist davon auszugehen, dass die Zerstörung von Regenwald und die Etablierung von Kautschuk- und Palmölplantagen aufgrund von Einflüssen auf die Bodenoberfläche, Veränderungen von Streufall, Nährstoffverfügbarkeit und Management in den Plantagen zu erhöhter Nährstoffauswaschung und einer verminderten Nährstoffretentionseffizienz führt. Diese Arbeit stellt zwei Studien vor, die sich mit den Auswirkungen der Regenwaldzerstörung - und der einhergehenden Kultivierung von Kautschuk und Ölpalmenbäumen - auf Nährstoffauswaschung und Nährstoffretentionseffizienz beschäftigt. Außerdem untersucht sie Unterschiede in der Nährstoffauswaschung zwischen gedüngten und mit Palmwedeln bedeckten Bereichen in Palmölplantagen. Beide Studien wurden in zwei Landschaften der Provinz Jambi (Sumatra, Indonesien) mit stark verwitterten Acrisol-Böden durchgeführt, die sich in der Bodenart unterscheiden (lehm- bzw. tonhaltiger Acrisol). Die Nährstoffauswaschung im Boden wurde mit Saugkerzen-Lysimetern gemessen, die in 1,5m Tiefe im Boden installiert wurden. Beprobt wurde von Februar bis Dezember 2013 zweiwöchentlich bis monatlich. Die erste Studie beschäftigt sich mit der Nährstoffauswaschung und Nährstoffretentionseffizienz im Boden vierer verschiedener Landnutzungsarten. Dabei handelt es sich um die zwei Referenznutzungsformen Tieflandregenwald sowie Sekundärwald durchsetzt mit Kautschukbäumen, als auch um die veränderten Landnutzungsformen kleinbäuerlicher Kautschuk- und Ölpalmplantagen. Jede Landnutzung, ausgenommen der Palmölplantagen mit drei Wiederholungen, wurde durch vier Wiederholungsflächen innerhalb jeder Landschaft repräsentiert. Somit wurde die Studie auf insgesamt 30 Flächen durchgeführt. Die Ergebnisse zeigen für den lehmigen Acrisol-Boden der Referenzflächen eine höhere Auswaschung und eine niedrigere N-Retentionseffizienz für Stickstoff (N) und basische Kationen, verglichen mit dem tonigen Acrisol-Boden bestanden. In den Palmölplantagen zeigte sich, dass Düngung und Kalkung zu erhöhter Auswaschung von gelöstem N, gelöstem organischen Kohlenstoff (DOC) und basischen Kationen führte, sowie zu einer geringeren Retentionseffizienz von N und basischen Kationen im Boden. In den ungedüngten Kautschukplantagen dagegen waren die Auswaschungsverluste von gelöstem N, DOC und basischen Kationen geringer als in den Palmölplantagen. Zusammenfassend zeigten die Ergebnisse, dass Nährstoffverluste und Nährstoffretentionseffizienz in Kautschuk- und Palmölplantagen auf stark verwitterten Acrisolen primär von Tongehalt und Management abhängen. In der zweiten Studie wurde die Nährstoffauswaschung in den gedüngten und mit Palmwedeln bedeckten Bereichen in Palmölplantagen von Kleinbauern in lehm- bzw. tonhaltigen Acrisolen gemessen. Die Ergebnisse zeigten höhere Auswaschverluste (d.h. N, basische Kationen, Gesamt-Aluminium, Gesamt-Mangan, Gesamt-Schwefel und Chlor) in den gedüngten Bereichen als in den mit Palmwedeln bedeckten Bereichen aufgrund der Frequenz des Mineraldünger- und Kalkeinsatzes. Auf Landschaftsebene wurden die höheren Bodennährstoffvorräte und eine niedrigere Nährstoffauswaschung im Ton-Acrisol im Vergleich zum Lehm-Acrisol sowohl in den gedüngten als auch in den mit Palmwedeln bedeckten Bereichen durch die höhere Nährstoffretention (als Ergebnis höheren Tongehaltes) verursacht. Die Kombination von Nährstoffauswaschung und Nährstoffeintrag (d.h. Gesamtniederschlag und Dünger) mit zusätzlichen Informationen über den Nährstoffaustrag durch die Ernte, geben uns umfassendere Informationen über die Veränderungen im partiellen Nährstoffhaushalt von N, Phosphor (P), und basischen Kationen bei Waldumwandlung zu Palmöl- und Kautschukplantagen. Gedüngte Palmölplantagen hatten aufgrund der hohen jährlichen Nährstoffauswaschung und des Ernteexports das niedrigste jährliche Teilbudget an N, Kalzium (Ca) und Magnesium (Mg). Dennoch verringerten die hohen negativen Teilbudgets von N, Ca und Mg in den Palmölplantagen nicht deren Vorräte in 1m Bodentiefe verglichen mit den anderen Landnutzungsformen - außer für austauschbares Mg im Lehm-Acrisol. Obwohl ungedüngte Kautschukplantagen geringere Auswaschung zeigen als der Wald (z.B. für P), führte der Ernteexport zu einem geringeren jährlichen P-Teilbudget. Insgesamt implizieren die Ergebnisse der beiden Studien folgende verbesserte Managementverfahren für diese hochverwitterten Böden: eine Synchronisation der Düngermenge mit der Pflanzenaufnahme sowie eine Anpassung der Düngungshӓufigkeit.