Featured projects (1)
Within the soil aggregate system, microaggregates are supposed to be of primary importance, as they are strongly linked with essentially all processes which control interaction, transport and turnover of soil constituents. As such, they are intimately connected with the major energy and biogeochemical cycles. During pedogenesis, microaggregates are formed by a complex interplay of physical, chemical and biological aggregation mechanisms, the quantitative role of which, although progressively more investigated, is still poorly understood. Soil microaggregates are generally all compound soil structures <250 µm, which are composed of mineral and organic components arranged in a heterogeneous but rather unknown pattern. Soil microaggregates are considered the fundamental building blocks for aggregate structure in almost all soils, including soils with an aggregate hierarchy where they are the subunits for increasingly larger aggregates. Although soil properties and functions are to a large degree controlled by the formation of an aggregate structure, still very little is known about the rates and underlying deterministic or stochastic controls on soil microaggregate formation in space and in time. Yet, this knowledge is mandatory to functionally link the microarchitecture of soils to fluid flow and transport processes, activity of soil microorganisms, the turnover and interactions of elements, as well as to the stability of the soil microaggregates themselves.
Featured research (8)
Soil aggregation and the translocation of clay and organic matter are significant pedogenic processes that manifest in diagnostic horizons in mature soil. Yet, their onset might date to much earlier stages of soil development where host rock weathering is dominant and litter from pioneer vegetation is the only input of organic matter. We present a time-lapse experimental-pedogenesis study on early host rock weathering that shows the formation of aggregates and clay translocation in response to irrigation with and without organic matter released from a litter layer. The presence of organic matter increases total carbonate dissolution capacity and results in a characteristic surface morphology, while simultaneously slowing down the dissolution rate. With the dissolution of carbonates, clay minerals of the host rock and iron from pyrite are released. Controlled by the presence of organic matter, both are either transported with the seepage water or form crusts and aggregates from clay minerals and freshly precipitated secondary iron oxides. Our study shows that the interplay of dissolution, neoformation of secondary minerals, translocation, and aggregation of organic matter and clay-sized minerals shape soil structure evolution during early pedogenesis in carbonate host rocks.
Earthworms and (tap-)roots impact the soil structure by creating large biopores, affecting infiltration capacity, seepage, nutrient cycling, and soil aeration. Despite the importance of biopores for the functions of soils and the fact that several hundreds of biopores >2 mm in diameter may occur on one square meter of soil, knowledge on the interdependence of soil properties, land-use intensity, and biopore number is still rudimentary. In this study, we investigate the linkage of the number of biopores (>2 mm i.d.) with the earthworm community, root biomass, and soil properties, including pH, water content, soil organic carbon (SOC), as well as the land-use intensity (pasture vs. cropland) as a function of the soil depth (15, 30 and 50 cm). Hypothesized causal relationships among these factors were analyzed by piecewise structural equation modelling (SEM). We found various and novel linkages between roots, earthworms, biopores, and soil properties depending on soil depth. In topsoil (at 15 cm depth), roots directly affected the number of small-sized biopores, and anecic earthworms were related to medium-sized biopores. These effects diminished with depth. We identified land-use intensity as the factor preponderating the relations between biopores, root biomass, and earthworm number in the topsoil horizons, thereby masking other interactions among variables. This appeared as high multicollinearity among variables in the SEM of the topsoil. Land-use intensity effects were found to impact the whole soil profile but decreased with soil depth. To further elucidate the single effects of soil properties on biopore-forming biota and number of biopores in the topsoil, we excluded land-use intensity as a variable in subsequent analyses. Biopores increased with soil pH and soil water content but decreased with increasing SOC. Based on our SEM analysis, we conclude that the occurrence, frequency, and persistence of biopores are the consequence of intricate interdependencies between earthworm communities, roots, and site-specific soil properties, governed by land-use intensity.
Soil organisms are recognized as ecosystem engineers and key for aggregation in soil due to bioturbation, organic matter (OM) decomposition, and excretion of biogenic OM. The activity of soil organisms is beneficial for soil quality, functions, and nutrient cycling. These attributions are based on field-scale observations that link the presence and activity of organisms to spatiotemporal changes in soil properties and can be traced back to the formation of biogenic aggregates. This biogenic formation pathway encompasses a cascade of processes so far discussed not comprehensively. A more general approach needs to consider the activity and feedback loops between soil biota, the active release of biogenic OM by excretion, the interaction of biogenic OM with soil constituents, the formation of organo-mineral associations, and how these become incorporated in aggregated structures. Especially the function of biogenically excreted OM, which is quite complex in composition, is controversial as it permits or inhibits aggregation. This review analyzes the various roles of biogenically excreted OM may take as an aggregation agent. We will show that its function depends on the interplay of numerous factors, including environmental conditions, variety of OM producers, composition and availability of biogenically excreted OM, and type of interacting mineral phase. We consider biogenically excreted OM to affect aggregate formation in three different ways: (I) as a bridging agent which promotes the aggregation due to surface modifications and attraction, (II) as a separation agent which favors the formation, mobility, and transport of organo-mineral associations and inhibits their further inclusion into aggregates, and (III) as a gluing agent which mediates aggregate stability, after an external force provokes a close approach of soil particles. We conclude that biogenically excreted OM takes these functional roles simultaneously and to a varying extent across spatiotemporal scales. Hence, biogenically excreted OM is involved in the surface modification of soil particles, in the enmeshment and gluing of particles into soil aggregates, in the (im-)mobilization, and in facilitating the transport of particles. All that depends on the interplay of a hierarchy of factors comprising the local soil community's composition, the properties of biogenically excreted OM, and the conditions of the immediate environment.
Colloidal settlement in natural aqueous suspensions is effectively compensated by diffusive movement if particles resist aggregation – a state known as colloidal stability. However, if the settling velocity increases upon aggregation, complex structural features emerge from the directional movement induced by gravity. We present a comprehensive modeling study on the evolution of an aggregated three-dimensional structure due to diffusion, surface interactions, and gravity. The systematic investigation of particle geometry and size revealed three mechanisms: (I) aggregation due to spatial confinement of settled particles, (II) aggregation due to differential settling, whereby fast and slow particles collide, (III) inhibition of aggregation due to fractionation of particles with different settling velocity. A 3D visualization tool allowed us to follow the subtle interplay of these mechanisms and the highly dynamic hierarchical self-assembly of aggregates. It revealed how the balance of the different interactions determines the actual rate of aggregation.
Surface-sourced organic compounds in infiltrating waters and percolates are transformed during their belowground passage. Biotic and abiotic processes thereby lead to continuously changing chemical environments in subsurface compartments. The investigation of such transformations of organic compounds aims for tracing subsurface fluxes as well as biotic and abiotic activity. To collect samples of soil solution, different kinds of lysimeters are available, spanning simple free-draining devices that sample water based on gravimetric flow and tension lysimeters allowing for approximating natural hydraulic conditions. Protocols for untargeted analytical profiling of organic soil solution constituents are scarce. We report here a solid phase extraction followed by GC–MS analysis, utilizing two long-term sampling devices in the Hainich Critical Zone Exploratory in Thuringia, Germany. In addition, we introduce a new lysimeter constructed exclusively from inert materials that allows for obtaining samples with little background signals in GC–MS. Polyvinylchloride (PVC)-based lysimeters introduce substantial background signals from plasticizers. We show how signals from these contaminants can be lowered during data analysis using chemometric background removal. Applying multivariate statistics for data analysis, we demonstrate the ability for monitoring of several sugars, fatty acids and phenolic acids at the topsoil-subsoil boundary and even beyond, via an untargeted analytical approach. Statistical tools facilitated the detection of differences in chemical signatures at three different land use sites. Data mining methods for metabolomics led to the identification of 3-carboxyphenylalanin as marker for a pasture site. The combined approach is suitable for the collection and extraction of topsoil and subsoil solution for untargeted metabolomics under near-natural flow conditions.
- Institute of Geoscience, Department of Hydrogeology
About Kai Uwe Totsche
- Kai Uwe Totsche is Professor and head of the Department of Hydrogeology at the Institute of Geoscience, Friedrich Schiller University Jena, Germany. Kai and his group does research in Nanobiogeochemistry, Environmental Chemistry, Subsurface Critical Zone Research and (Ground-) Water Science. The research approach comprises field (in-situ), lab (in-vitro) and numerical (in-silico) methods. Focus is on structure-function relationships, interaction and reaction at biogeochemical interfaces in natural porous media, origin and fate of mobile (organic) matter including microorganimsms. Special emphasis is put on the vastly ignored colloidal and suspended particle fraction of organic, mineral and biotic origin. Current projects are among others 'AquaDiva' and 'MAD Soil'