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
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:82334 |
| Date | 21 November 2022 |
| Creators | Kirsten, Maximilian |
| Contributors | Kalbitz, Karsten, Veldkamp, Edzo, Dötterl, Sebastian, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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