Participating Projects
P01 – Water storage and redistribution in the forest floor affect percolation, evaporation and DOM loss
Concerning the overall hypotheses of the Research Unit we in Project 1 investigate the Forest Floor (FF) related dynamics of water and dissolved organic matter (DOM). A detailed knowledge of the hydrological processes of the FF is urgently needed to link above and below ground processes in forests as well as to predict the effects of changes in the FF on hydrology and carbon cycling.
This project aims to link the spatial and temporal variability of water fluxes and water storage in the FF with the fluxes of DOM and their feedback to the atmosphere and the underlying mineral soil.
We will achieve this by developing a novel weighing Grid-Lysimeter with continuous measurements of dissolved organic carbon (DOC) and electrical conductivity (EC). The DOC concentration will be quantified by optical fluorescence and absorbance measurements directly below the lysimeter using a combination of LEDs, photodiodes and filters. This will allow us to observe and analyze water and DOM fluxes at a high spatio-temporal resolution at all study sites and to have several replicates within the forest.
Additionally, we will measure the temporal change of hydrophobicity and its effects on preferential flow by including experiments using wetting agents to eliminate hydrophobicity. Since hydrophobicity may have a pronounced effect on the hydrological functions of the FF, we plan to observe the temporal changes of hydrophobicity in the field. As no sensor is available to measure hydrophobicity continuously, we will use the simple water drop penetration time test (WDPT) to observe the changes in water repellency of the FF over time.
Altogether we will provide detailed information about the storage capacity and flow pathways of the FF and its relation to FF properties. The spatio-temporal pattern of water and solute fluxes from the FF to the mineral soil will be analyzed on their temporal persistence, seasonal dynamics in relation to hydrophobicity, and their influence on infiltration patterns. The variability of storage properties and leaching potential will be derived. In addition, the influence of FF thickness and type on soil evaporation will be analyzed. Thereby P1 will strongly contribute to the overall understanding of FF processes and thus the impact of FF on soil hydrology, trees, soil fauna, microorganisms as well as SOM and nutrient supply.
P02 – Organic matter cycling in forest floors of temperate mixed forests
Our project is centered on improving our understanding of organic matter cycling in temperate mixed forest floors. We aim to examine the key roles of different input and output pathways. Organic matter comes from various sources like foliage, roots, or mycorrhiza. This organic matter is either mineralized, leached, transformed into stable soil organic matter, or transferred into mineral soil by soil fauna. While turnover rates and source contributions (e.g., for mineral-associated organic matter) have been studied extensively within the mineral soil, similar knowledge for the forest floors is still scarce.
We will study forest floors of temperate mixed forests across four altitudinal gradients in Germany and Switzerland. These sites vary in their parent material and mean annual temperature. We plan to measure carbon fluxes as CO2 and dissolved organic matter regularly. Using isotopic analyses of 13C, 15N, and 2H, we aim to uncover how different sources contribute to carbon pools in both forest floor and mineral soil.
To better understand turnover rates, we’ll use isotopic tracer experiments with labeled beech and maple litter in both lab and field settings. The lab work will highlight the role of earthworm species in organic matter processing. In the field, we’ll partner with researchers from the University of Göttingen to explore how different soil fauna affect the forest floor.
Further cooperations include linking isotope signals with organic matter quality measurements done by the Technical University Munich as well as links to nutrient cycling investigated by the University of Freiburg.
P03 – Interrelation of biochemical and physicochemical forest floor properties: Effect of SOM-mineral association and interaction
Our project focuses on investigating the biochemical properties of Soil Organic Matter (SOM) and exploring the extent and nature of organo-mineral associations within the forest floor (FF) of temperate mixed forests. Our aim is to improve our understanding of SOM formation, stability, and vulnerability to external changes in FF, while uncovering the influence of site-specific factors such as climate, phosphorus (P) status, and parent material (calcareous vs silicates) on FF properties.
The state-of-the-art methodology we employ primarily focus on the application of density fractionation, enabling us to discriminate SOM pools based on their density. This method, traditionally used to differentiate Particulate Organic Matter (POM) from Mineral-Associated Organic Matter (MAOM) has been exclusively applied in mineral soils, but the adaptation and application of this method to FF reveals different forms of SOM that before were overlooked. For instance, fractions < 1 g cm-3 predominantly comprise non-associated SOM, while fractions ranging from 1 to 1.6 g cm-3 exhibit SOM partially associated with minerals or complexed with carboxylic functional groups at different extent. Moreover, heavier fractions exceeding 1.6 g cm-3 signify more stable mineral-associated SOM, with an increased mineral contribution present in organic layers through processes such as bioturbation or atmospheric deposition.
Our analytical approach integrates chemical analysis to quantify the total concentration of main soil inorganic constituents and nutrients (Al, Fe, Ca, Mg, N, P, S) alongside synchrotron-based spectroscopy (XANES) analysis to elucidate the speciation of crucial components such as Al, Fe, Ca, and S within the forest floor. Complementarily, Nuclear Magnetic Resonance (NMR) spectroscopy aids in discerning C speciation, additionally, the measurement of biomarkers such as non-cellulose polysaccharides allows us to evaluate the concentration and source of sugars—an essential carbon source—in SOM, thereby distinguishing between plant and microbial origins. Furthermore, information of isotope signals such as 14C provide information on SOC/SOM age turnover which will be performed in cooperation with Swiss Federal Institute for Forest, Snow and Landscape Research (WSL). Micro and nano scale analysis (µ-XRF and Nano SIMS) are planned to be performed to study element composition and distribution on aggregates form different parent materials (Of, Ah) to elucidate SOM-mineral association patterns.
The data obtained on different fractions and bulk soils in FF and mineral soils will serve as important explanatory variables for the phenomena investigated in the other RU projects (e.g., nutrient availability, tree regeneration dynamics, microorganisms’ abundance and communities, hydrophobicity, DOC, water storage and architecture in FF).
P04 – Nutrient dynamics along the forest floor-mineral soil continuum
Until to date, the role of the forest floor for forest nutrition is not fully understood.
Slow turnover of the forest floor (FF) is often assumed to be related to immobilisation of nutrients within the organic matter. This is in contrast to the overall hypotheses of the main project, i.e. that thick FF represents an adaptation to low nutrient supply by mineral soils. The project will investigate under which conditions slow turnover of FF material might be an advantage. The objective is to determine which processes control the relevance of the FF for tree nutrition as compared to the mineral topsoil.
For this purpose, mobilization kinetics of the macronutrients from FF and A horizons are studied performing soil dialysis and using an infinite sink approach with ion exchange resins. To study the effects of minerals in the FF on nutrient mobilization, a novel method based on litter and OF bags with admixture of the respective minerals is used. How tree species affect nutrient mobilization in the FF and in the upper mineral soil, mesocosms with labeled litter at controlled conditions of a Danish broadleaf tree species experiment are analyzed.
All in all, novel methods to study phytoavailable macronutrients in the forest soil will reveal the role of the forest floor for tree nutrition and contribute to a solid basis for sustainable forest management.
Soil dialysis probe in a stirred solution containing nutrients to test its performance
P05 – The architecture of the forest floor and consequences for connectivity
The aim of the project is to investigate the effects of climate change on the forest soil and its water balance and to develop a new method using photogrammetry to model the structure of the soil in detail in a 2.5D representation. The storage and flux of carbon dioxide and water with dissolved nutrients should also be included in the model and thus be easier to analyse. The main objective of modelling the structure of the different soil layers is to gain a better understanding of the distribution of water, nutrients and carbon dioxide in the forest soil.
Photogrammetry is a technology used to create models of objects or environments by analysing photographs. It involves taking photographs of the object, here soil, from different angles and perspectives and processing them with the help of special software and algorithms.
To measure the availability of nutrients in the different soil layers, redox plates are used. These plates are inserted into the soil and monitor oxygen availability at different depths. This will improve the understanding of nutrient distribution and movement in the forest ecosystem.
By attempting to analyse the forest soil in a new way, a new method for effective forest management can be created. Modelling the structure of the soil, with the further incorporation of other research results, can also contribute to a fundamental understanding of the influence of climate change on the forest ecosystem and the distribution of water, nutrients and carbon dioxide in the forest soil in the future, and to more forward-looking action for the future.
Overall, the project can help improve forest management and protection and support climate change mitigation.
P06 – Soil fauna as drivers of the architecture of forest floors
Early research suggests that the configuration of the organic layer in forests is primarily influenced by soil animal activity. Yet, research in this area has long relied on descriptive methods, offering only restricted insights into the underlying mechanisms. In recent years, significant strides have been made in comprehending the structure and operation of decomposer communities. However, these advancements have seen limited application in studying the development and fluctuations of forest floors (FFs).
Project P6 aims to investigate the impact of soil fauna on the structure and dynamics of forest floors (FF), while also examining how their effects correlate with various functional groups and feeding guilds of decomposer invertebrates. Detritivore soil animals including Oribatida, Enchytraeidae, Collembola, Diplopoda, Isopoda and Lumbricidae will be determined to species level. Compound-specific amino acid analysis and bulk stable isotope analysis are employed to investigate the flow of energy through the detritivore food web. The study also examines the influence of soil meso- and macrofauna on the structure and dynamics of forest floors (FFs), as well as the spatial distribution of carbon (C) and nitrogen (N) in litter facilitated by detritivores, utilizing mesocosms containing labeled litter of varying mesh sizes.
In summary, the study utilizes all 12 observational sites of the RU for both the mesocosm experiment and the FF labeling experiment. By integrating state-of-the-art methodologies and experimental approaches, it aims to thoroughly investigate the contribution of soil detritivores to the formation and dynamics of FFs.
P07 – Drivers of microbial nutrient turnover in mineral soil, rhizosphere, and forest floors
In Project 7, we investigate the microbiological aspects of the forest floor (FF). Although forest soils are renowned for their vast diversity of microbes and have been extensively researched, the forest floor remains a relatively understudied ecosystem. FF properties influence nutrient concentrations and quality as well as oxygen and water availability. The turnover of nutrients by microorganisms is regulated on the level of nutrient stoichiometry of ecosystems and genetic operon structure of single microorganisms who pursue different strategies to regulate efficient nutrient use. Thus, nutrient availability and community composition are closely interlinked. Consequently, changes in nutrient availability and habitat structure because of increasing temperature feedback on microbial communities in terms of their taxonomic and functional composition as well as trophic interactions.
The aim of P7 is to understand the interplay of abiotic FF properties with the microbial community composition under different P and temperature regimes.
We aim to identify the adaptations of microbial communities involved in phosphorus and nitrogen turnover along gradients of phosphorus availability and temperature. We hypothesize that the quality and quantity of soil organic matter and root exudates as well as nutrient stoichiometry (C, N, P, cations) drives the microbial potential to transform nitrogen and phosphorus and determines trophic interaction with other biota (fungi, fauna, and trees).
To explore forest floor microbiomes and their potentials, we will apply qPCR and amplicon sequencing, as well as metagenomic shotgun sequencing, microbial biomass determination, fluorescence in situ hybridization (FISH) and isolation techniques. Next to sampling of forest floor layers and compartments at high resolution, we will utilize mesocosm and litterbag approaches for our experimental designs.
Results from P7 will widen the understanding about microbial nutrient turnover in the forest floor and the impacts of climate, litter quality and phosphorus availability.
P08 Mycorrhizal functions in the forest floor
The mycorrhizal symbiosis is a central component of plant-soil feedbacks and carbon (C) cycles of forest ecosystems. Yet even though it is known that the two major mycorrhizal association types influence litter decomposition and soil organic matter formation differently, it remains unresolved whether this also influences their preference for the forest floor (FF) as a habitat. We hypothesize that mycorrhizal fungi are more dependent on FF C in cold or warm nutrient-poor forest stands, since less photosynthates are produced and invested into the fungal symbiont. As a result, the diversity of the mycorrhizal fungal community in the FF increases.
The aim of my doctoral project is to analyse the response of the mycorrhizal fungal community structure and its functions in the forest floor and mineral soil of temperate mixed beech forests to gradients of temperature and phosphorus availability. I combine functional root trait analyses, an isotopic fractionation analysis, a 13C- and 15N-labeling experiment, and extracellular enzyme analyses with next generation sequencing of the mycorrhizal fungal community in the FF and mineral soil of nine silicate forests composed of European beech (Fagus sylvatica L.; ectomycorrhizal host) admixed with sycamore maple (Acer pseudoplatanus L.; arbuscular mycorrhizal host). My measurements will assess the fungal habitat in roots, the C investment by trees into their mycorrhizal symbionts and the consequences for litter decomposition across the investigated gradients. In addition, I investigate the influence of the microbial and mycorrhizal fungal community composition on root exudation in close cooperation with P7 and P9. My project will refine our understanding and prediction of the ecology of mycorrhizal fungi in the FF of mixed beech forests, which are increasingly exposed to rising temperature and P limitation.
P09 – Effect of tree nutrient strategies and carbon exudation by root-mycorrhizal associations on forest floor turnover
In this project, we investigate the influence of different tree species on the Forest Floor (FF) with a focus on three specific aspects:
First, the nutrient cycling and tree-internal recycling strategies of the three focal tree species (European beech, Norway spruce, sycamore maple) will be investigated. This includes the sampling of different tissues, analyse their content of carbon (C), nitrogen (N), phosphorus (P) and their isotopic composition (δ13C and δ15N) at different time points throughout the vegetation period.
Second, root exudation plays an important role not only by representing a source of carbon to the FF and the mineral soil, but also by altering the distribution, potentially the composition, and activity of microorganisms, investigated by project P7. The focus in this project will be on the exudation of sugars and organic acids of the above-mentioned species in the FF and the mineral soil across different seasons. This pattern is dependent on the mycorrhizal association of the tree, which is investigated in project P8
Third, the trees’ contribution to lateral displacement of nitrogen (N) and hydrogen (H) will be investigated in a litter dispersal experiment in co-operation with projects P2, P3, P4, P6 and P8. For this purpose, beech and maple litter with traceable 15N and 2H will be applied at two sites and its displacement over the following years will be studied by sampling roots and branches of the surrounding trees and understory vegetation.
Results gained in this project might help understand the complex forest ecosystem and predict the consequences of forest change.
P10 – The influence of forest floor changes on the success and composition of tree regeneration
Climate change will greatly impact tree species composition and hence biodiversity and ecosystem functioning of forests. So far indirect effects such as changes in regeneration success as a result of altered substrate conditions for seed germination, modifications in nutrient availability or seed predation and subsequent shifts in the competition among tree species have received very little attention, although they may be crucial for future forest resilience and adaptive capacity. In central Europe, drivers such as increasing temperatures, increasing proportions of deciduous species and ongoing eutrophication may lead to a reduction in forest floor (FF) mass and other FF properties with unknown consequences for the regeneration of tree species. Most research on the influence of FF on tree regeneration dates back to the 1960-1970s, long before widespread P deficiency and climate change were important considerations.
Our project aims to close this knowledge gap by studying the influence of changes in FF properties on the regeneration success of important tree species in temperate mixed European beech forests. Specifically, we will address the combined influence of climate change and FF changes on the regeneration success of three target species, Fagus sylvatica, Picea abies, Acer pseudoplatanus. In addition to the direct influence of these two factors, we will analyse indirect influences through changes in the availability of nutrients and water or mycorrhization, changes in biotic factors such as fungal pathogens and seed predation, and changes in the competition among tree species as well as between seedlings and other understorey vegetation. For that purpose, we will assess the germination rates as well as the mortality, growth (e. g., density, height, biomass of roots and shoots), the competitive status as well as mycorrhization and nutrition of seedlings in an experimental approach. In mesocosm experiment with soil columns from sites differing in soil P status and hence FF mass, we focus on the germination and initial establishment process. Here we will test whether decreasing FF thickness influences the susceptibility of seeds and germinants to 1) fungal infestation with changes in seasonal distribution of precipitation (wetter winters) and 2) to desiccation in dry periods. We further test whether 3) the establishment success depends on FF structure. In a field experiment, where seeds of the three species are sown at six sites contrasting in P availability mean annual temperature, we test whether decreasing FF thickness promotes 4) competition for tree seedlings from understorey and 5) increases competition for P and other nutrients between mature trees and seedlings. If successful, our project will provide an important contribution to understanding the regeneration dynamics in forest ecosystems under changing environmental conditions.
