Search

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 ran

Nanoindentation is one of the most widespread methods to measure the mechanical performance of complex materials systems. As it allows for local characterization of composite architectures with sub-micron spatial features and a large range of properties, nanoindentation is commonly used to measure the properties of biological materials. In situ nanoindentation, a further development of the approach, is a powerful tool for the analysis of plastic deformation and failure of materials. Here, samples can be mechanically manipulated using the indenter, while their behavior is monitored with the resolution of a scanning electron microscope (SEM). Indeed, numerous studies demonstrate the potential of this approach for studying the most fundamental material characteristics. However, so far, these measurements are performed in high-vacuum conditions inherent to the conventional electron microscopy method, which are irrelevant when studying biological structures that evolved to perform in hydrated conditions. In this work, the ability to conduct nanoindentation experiments under controlled humidity and temperature inside an environmental SEM is developed. This technique has the potential to become crucial for materials design and characterization in many domains where humidity has a significant impact on performance. These include organic/polymer systems, microelectronic and optoelectronic devices, materials for catalysis, batteries, and many more.

Much research focuses on increasing carbon storage in mineral-associated organic matter (MAOM), in which carbon may persist for centuries to millennia. However, MAOM-targeted management is insufficient because the formation pathways of persistent soil organic matter are diverse and vary with environmental conditions. Effective management must also consider particulate organic matter (POM). In many soils, there is potential for enlarging POM pools, POM can persist over long time scales, and POM can be a direct precursor of MAOM. We present a framework for context-dependent management strategies that recognizes soils as complex systems in which environmental conditions constrain POM and MAOM formation.

Background: Soil microbes are key drivers of carbon (C) and nutrient cycling in terrestrial ecosystems, and their properties are influenced by the relationship between resource demand and availability. - Aims: Our objective was to investigate patterns of microbial properties and their controls to understand whether they differ between soils derived from geochemically contrasting parent material in tropical montane forests. - Methods: We measured microbial biomass C (MBC/Soil), potential extracellular enzyme activity (pEEA), and assessed microbial investments in C and nutrient acquisition at the beginning and end of a 120-day laboratory incubation experiment using soils developed from three geochemically contrasting parent material (i.e., mafic, mixed sediment, and felsic) and three soil depths (0–70 cm). - Results: We found that MBC/Soil and pEEA were highest in soils developed from the mafic parent material. Microbial investment in C acquisition was highest in soils developed from mixed sedimentary rocks and lowest in soils developed from the felsic parent material. We propose that our findings are related to the strength of contrasting mineral-related C stabilization mechanisms and varying C quality. No predominant microbial investment in nitrogen (N) acquisition was observed, whereas investment in phosphorus (P) acquisition was highest in subsoils. We found lower microbial investment in C acquisition in subsoils indicating relatively high C availability, and that microbes in subsoils can substantially participate in C cycling and limit C storage if moisture and oxygen conditions are suitable. Geochemical soil properties and substrate quality were important controls on MBC/Soil per unit soil organic C (MBC/SOC), particularly after the exhaustion of labile and fast cycling C, that is, at the end of the incubation. - Conclusion: Although a laboratory incubation experiment cannot reflect real-world conditions, it allowed us to understand how soil properties affect mi

Microorganisms interact with different soil components, such as varying substrates and soil minerals, to drive soil processes and functionality. They can be influenced by the environment, but they themselves can influence their environment by their activities, for example through the production of extracellular enzymes and extracellular polymeric substances (EPS). The formation and stability of aggregates as the backbone of the soil structure, for instance, are thought to be largely influenced by soil microorganisms, or vice versa. There remain, however, open questions as to whether and how microorganisms can influence soil aggregation. While microbes are influencing their environment their interaction with the soil minerals could also change their responses upon adsorption - affecting their influence on soil aggregation. Therefore, the overarching goal of this thesis was to investigate the effect of soil minerals, in particular clay content, on the composition and activity of soil microbial community, with a specific focus on enzyme activities and EPS. Finally, the microbial control of soil aggregation through the influence of substrate availability was explored. In total, two adsorption experiments and two incubation experiments were conducted using soils manipulated experimentally with increasing clay content. The sandy soil was amended with different amounts of soil minerals (i.e. montmorillonite) to achieve a gradient in clay content. For the first incubation experiment, organic substrates differing in decomposability (i.e., starch and cellulose) were added to the soil to stimulate microbial activities and incubated for 80 days. Soil samples from the first incubation experiment were analysed after 0, 3, 10, 20, 40 and 80 days for enzyme activities, microbial community composition, biomass C, EPS-protein and polysaccharide. Additionally, the geometric mean diameter and mean weight diameter of the soil aggregates were determined as measures of aggregate formation

Soil structural stability plays a pivotal role in landscape preservation when a protective vegetation cover is lacking. For example, under semiarid climates seasonal rainfall cannot sustain a full vegetation cover, but still causes soil erosion. With the loss of (fertile) soil material, ecosystem productivity reduces and less C can be stored. In natural semiarid systems, soil erosion is a spatially heterogeneous process, whereby local highly erodible spots are alternated by improved soil structure under the sparse canopy cover, creating a very heterogeneous landscape. Although the physical protection by the plant canopy is well understood, the potential influence of soil archaea and bacteria on soil structural stability in relation to plants and parent material is less well known. Mainly from studies under controlled conditions, we know that certain archaeal and bacterial species have the ability to produce extracellular polymeric substances (EPS), forming an extracellular matrix. As the formed matrix connects soil particles, EPS seem to have the potential of playing a substantial role in soil aggregation, thereby controlling soil erodibility. Little is known of this gluing process by EPS and its importance under natural conditions as most evidence is derived from controlled conditions in the laboratory. This dissertation aims to unravel the role of EPS from soil archaea and bacteria in soil aggregate stabilization in semiarid grasslands by considering the potential role of plant species and parent material in this process. The sparse vegetation in semiarid grasslands provide a useful gradient in soil organic C contents to study these processes. Improved conditions for soil microbes producing EPS can be found at the root surface, while the bare canopy interspaces lack in C/resources. Two sites were selected in southeast Spain, mainly differing in graphitic C, inorganic C and nitrogen contents. On both sites, soil adjacent to the widely occurring Anthyllis cytisoides, Die Stabilität der Bodenstruktur spielt eine entscheidende Rolle in der Erhaltung der Landschaft, insbesondere wenn keine schützende Vegetationsbedeckung vorhanden ist. So ist beispielsweise unter semiariden Klimabedingungen wegen der Saisonalität der Niederschläge keine vollständige Vegetationsbedeckung vorhanden, was Bodenerosion verursacht. Durch den Verlust von (fruchtbarem) Bodenmaterial verringert sich die Produktivität des Ökosystems. Dadurch kann weniger Kohlenstoff (C) im Boden gespeichert werden. In natürlichen semiariden Systemen ist die Bodenerosion ein räumlich heterogener Prozess, bei dem sich stark erosionsanfällige Stellen mit solchen Bereichen abwechseln, welche durch günstige Bodenstruktur unter der spärlichen Pflanzendecke gekennzeichnet sind. Hierdurch entsteht eine sehr heterogene Landschaft. Während zum physikalischen Schutz durch Vegetationsüberschirmung viele Erkenntnisse vorliegen, ist über den möglichen Einfluss von Archaeen und Bakterien auf die strukturelle Stabilität des Bodens in Bezug auf Pflanzen und Ausgangsmaterial weit weniger bekannt. Hauptsächlich aus Studien unter kontrollierten Bedingungen wissen wir, dass bestimmte Archaen- und Bakterienarten die Fähigkeit besitzen, extrazelluläre polymere Substanzen (EPS) zu produzieren, die eine extrazelluläre Matrix bilden. Da die gebildete Matrix Bodenpartikel verbindet, scheint EPS das Potenzial für eine maßgebliche Beeinflussung der Bodenaggregation zu haben und dadurch die Erosionsanfälligkeit zu steuern. Über solche Klebemechanismen von EPS und deren Bedeutung unter natürlichen Bedingungen ist aber wenig bekannt; die meisten Hinweise stammen aus kontrollierten Bedingungen im Labor. Diese Dissertation zielt darauf ab, die Bedeutung von EPS von Archaeen und Bakterien hinsichtlich der Stabilisierung von Bodenaggregaten in semiariden Graslandschaften unter Berücksichtigung der möglichen Rolle von Pflanzenarten und Ausgangsmaterial in diesem Prozess aufzuklären. Zur Untersuchung solcher Pro

In the past years, extracellular polymeric substances (EPS) produced by soil microorganisms received an increasing interest, as they not only protect microbes against environmental stresses, but seem to play a pivotal role in soil structure formation as well. Within soils, root deposits provide an important source of easily accessible energy and nutrients, stimulating microbial growth to produce EPS. Especially under semiarid climates, where a full vegetation cover cannot be sustained, large gradients in living conditions for microbes can be found between the root-soil interface and barren intercanopy spaces.In this study, we aimed to elucidate the plant-specific effects on microbes, EPS production and soil aggregation. At two sites in southern Spain, differing in carbonate and graphite content, legume shrubs of Anthyllis cytisoides and grass tussocks of Macrochloa tenacissima were selected. Soil samples were taken in the adjacent bare interspace, under the canopy and of the rhizosphere. From these samples the microbial community (here bacteria and archaea), EPS(-saccharide) content and soil aggregation (The rhizoplane microbial communities differed strongly between plant species and sites, whereby site was the most important factor shaping the communities. The plant species effect on microbial communities diminished strongly with distance to the root surface. At the carbonate-poor Rambla Honda site (site 1), plant species-specific effects were observed in the rhizoplane and rhizosphere, whereas in the carbonate-rich Alboloduy site (site 2) almost no plant species-specific effects were found at the genus level. The larger heterogeneity in microbial communities at site 1 was reflected in EPS-saccharide contents and subsequent soil aggregation, while no difference in soil aggregation was found at site 2. Both parameters increased strongest in the Anthyllis cytisoides rhizosphere at site 1.Despite the lack of a strong gradient with distance from

Subsoils are characterized by low concentrations of organic carbon (OC). Nevertheless, they contain more than half of the global soil OC because of their large volume. This discrepancy suggests that subsoils might further sequester carbon (C), thus acting as potential sinks for atmospheric C. Plant roots and associated rhizodeposits are a major OC input source to subsoils. However, whether and how increased OC inputs via plant roots to subsoils affect soil C sequestration mechanisms remains unclear. Here we set up a pot experiment with European Beech (Fagus sylvatica L.) seedlings to investigate the effect of tree roots and associated rhizosphere development on soil aggregation and C allocation in topsoil vs. subsoil material collected from three forest sites of different parent materials. Over a 5-month growth period, the seedlings developed a dense root system transforming the whole soil volume into root-affected (i.e., rhizosphere) soil. We found that roots and the associated rhizosphere development increased the amount of macroaggregates in the two finest-textured subsoils. The most C-poor and fine-textured subsoil had a 15% increase in bulk OC concentration, indicating a potential for C sequestration in subsoils by enhanced macroaggregation. Across subsoils, rooting strongly enhanced microbial abundance and was especially correlated with fungal abundance and a shift in the fungal-to-bacterial- ratio. The strong fungal growth was likely the cause for the enhanced macroaggregation in these subsoils. In topsoils, however, rooting treatment decreased macroaggregate abundance, potentially through the disruption of preexisting aggregates, as indicated by the concomitant increase in microaggregates. Our study supports the growing awareness that OC dynamics may be governed by different mechanisms in top- and subsoils, respectively. It demonstrates that the enhanced addition of OM via plant roots to subsoils boosts fungal growth and thereby increases macroaggregate f

In the barren semiarid landscape individual plant species improve soil structure, reducing the erosion risks, whereby microaggregates form the most fundamental soil structural components. Extracellular polymeric substances (EPS) are considered an important glue determining aggregation in addition to inorganic binding agents such as carbonates. However, the role of the prokaryotic community in EPS formation and consequently for microaggregation in natural environments is not substantially clarified yet. EPS should be particularly important under semiarid conditions as it forms a protection mechanism of the prokaryotes against desiccation. Therefore, we examined the influence of the prokaryotic community on soil EPS content and subsequently on soil microaggregation in semiarid grasslands, depending on the parent material, common plant species and the distance to the plant. Soil samples were taken over a distance gradient from two major semiarid grassland plant species in Southern Spain, the legume shrub Anthyllis cytisoides and the grass tussock Macrochloa tenacissima, to the surrounding bare soil at two sites, rich and poor in carbonates. Total community DNA and EPS were extracted, followed by quantification of EPS-saccharide, bacterial abundance and examination of the prokaryotic community composition. Further, the particle size distribution of the microaggregate fraction was determined as an indication of microaggregation. We found that the overall prokaryotic community composition differed between the two sites, but not between plant species. Nonetheless, a link between the community composition and EPS content was established, whereby soil organic matter (OM) seems to be a regulating factor as increasing soil OM contents resulted in more EPS-saccharide. Furthermore, microaggregation was enhanced by the canopy, especially at the edge of Macrochloa tussocks. Contrary to the expectation that increasing inorganic C contents would diminish importance of EPS, the pa