Refractory and labile substances play a significant role in the dynamics of soil organic matter and have key ecological functions. Only little information, however, is available on the interactions between labile substrates and biochar – a lack which hampers any prognoses of soil processes and critical appraisals of biochar applications for improving soil quality. This proposal aims at identifying transformation pathways for substrates of increasing complexity in the order low molecular weight organic substances, mucilage, fine roots and coarse roots in contact with biochar in incubations. A mechanistic understanding of the resulting biological, biophysical and biochemical interactions and reactions will be achieved by employing dual isotopic labelling for biochar (C-13) and substrates (C-14) and following the pathways using biological analyses and soil partitioning into density and aggregate fractions. The factorial experimental designs in combination with soil biological, biophysical and biochemical analyses will allow elucidation of the effects of substrates, substrate application rate, biochar age and soil moisture on the interactions and pathways. Near and mid-infrared spectroscopy in different measurement modes will be employed to characterize the substrates, i.e. pure (freshly produced) biochar, light fractions of the mixtures of soils and biochar during biochar ageing and mixtures of soils, mucilage and fresh or aged biochar using band assignments. Additionally, improved quantitative determinations using the full spectra with foci on estimation accuracies for pure, coated and aged substances will be achieved using an optimization of chemometric approaches. The results of the dual isotopic labelling approaches in combination with the spectroscopic approaches will give a mechanistic understanding of the key processes explaining C stabilization by biochars. Providing the tools to differentiate the impact of pure and aged biochars on soil processes, this project will allow an improved quantification and evaluation of the use of biochars in agriculture.
DFG: Biochar-phosphorus interaction after joint application: assessing the impact of direct (im)mobilization and microbial-mediated P transformation on maize P nutrition
As a result of an extremely fast rate of phosphate rock mining along with environmental risks associated with ongoing application of high P rates, current P management has to be thoroughly revised and new approaches require not only a more detailed understanding of mechanisms of P mobilization and solubilization, but also should jointly be designed with an elaborated management of soils as C sinks. Therefore, an improved understanding on using biochar application to enhance the P use efficiency of fertilizer P is of key importance.
The aim of this work is to improve mechanistic understanding of the biochar effect on the availability of applied P and soil P especially during the early plant P uptake. Strategies for large areas of arable land considering biochar require the application of much lower rates (less than 1-2 Mg ha-1), which then can, besides their C stabilizing nature, be used as nutrient carrier to increase nutrient use efficiency (NUE) and decrease nutrient losses from agroecosystems. The band application concentrates biochar and P in the zone reached rapidly by the seedlings and may unfold its positive effects on plant nutrition even without, yet, showing a general effect on the bulk soil. Fluid formulation of biochar allows adjustment of different characteristics and impregnation with beneficial additives, such as nutrients or microorganisms. Although, understanding mobilizing and immobilizing processes for soil P and fertilizer P need to be disentangled and a mechanistic, process-based understanding for direct and indirect interaction of biochar with P in interaction with the growing root need to be gained.
The fundamental channels of matter and energy in soils are the reactions of microbial metabolism, which conform to thermodynamic principles such as negative Gibbs free energy change. Consequently, catabolic exergonic reactions are needed to fuel the energy-demanding reactions of biomass synthesis in addition to extracellular enzymes to access substrates. Therefore, bioenergetic and cell economic principles will underlie the thermodynamic-metabolic approach developed by EcoEnergeticS. We aim at understanding microbial use of simple and complex substrates in soils by characterizing five proxies of microbial efficiency: 1) substrate use efficiency, 2) carbon use efficiency, 3) biochemical efficiency, 4) calorespirometric ratio, and 5) thermodynamic efficiency. We expect a trade-off between the efficiency of biomass growth on diverse monomeric precursors that require little metabolic conversion, and the inefficiencies of producing complex enzyme systems to liberate these monomers from diverse polymers. We therefore hypothesize an optimum of microbial efficiency at intermediate substrate complexity, modulated by the cooperative functional diversity of microbial communities and their soil habitats’ boundary conditions. By extension, we hypothesize that the complexity of microbial necromass accounts for its entombing in SoilSystems.In WP1, we implement thermodynamic constraints in soil metabolic flux analysis (MFA) to analyze and model microbial growth on chemically diverse substrates entering central C metabolism at different levels. The impact of monomer diversity on microbial growth and use efficiencies will be quantified in WP2, considering the advantages of precursor ratios that meet the demand of biosynthetic pathways for cell replication. WP3 investigates an alternative substrate use – storage compound formation – as an energy storing strategy under energetically unfavorable growth conditions. In WP4 we shed light on the additional costs of exoenzyme production arising from the predominance of polymers as C sources in soils, using MFA and enzyme economic approaches. Cooperative energy gains by “exoenzyme sharing” among functionally diverse microbial communities will be targeted in WP5. WP6 implements spatio-temporal boundary conditions of microbial habitats as crucial factors of exoenzyme economics, coupling the bioenergetic dimension of substrate use to the joint CERES experiment on various levels of soil complexity as predictors of matter and energy-use channels. Summarizing, a systems-ecology-based bioenergetic concept on substrate use and channeling in soils will be developed, resolving matter and energy fluxes down to the level of biochemical pathways. Thermodynamic constraints of microbial metabolism will be linked with resource economy, considering complex factors of habitat boundary conditions in soils. Thus, this project will provide a unifying concept of bioenergetics and resource economics to SoilSystems.
Transformation of matter and energy in soils depends on the activity state of microorganisms and their metabolism. Microbial mining of phosphorus (P) may be driven not only by P, but also carbon (C) and energy limitation, as high-energy phosphoester bondings serve as major energy-transfer and storage molecules in cells. In this project we will deepen the thermodynamic-metabolic approach developed within ‘EcoEnergeticS’ towards the bioenergetics of P mining from organic sources. We aim to evaluate the energy costs of enzyme production for P mining to acquire C, energy and nutrients from substrates of contrasting complexity. We aim to combine the parameters of microbial P immobilization (Pmic formation), ATP level, dynamics of storage compound (PHBs), PLFA and DNA contents and ratios and enzyme activity with heat dissipation and CO2 emission as proxies of metabolic activity. We hypothesize that energy costs of P-hydrolyzing enzyme production by microorganisms increase with increasing substrate complexity. Besides, at either C or P limitation, microorganisms utilize compounds with high-energetic phosphoester bonds (e.g. ATP) rather as a source of either C or P, respectively. In the absence of P limitation, C from ATP can be accumulated to storage compounds (i.e. PHB) to save the energy, while hydrolyzed P is released to the soil. In WP1, we will assess the investment of microorganisms into enzyme production to mine energy and nutrients from organic P substances of various complexity, that requires hydrolysis by one or several phosphatases (phosphomono- and phosphodiesterases, phytases). Further in WP2, we will disentangle the energy costs of i) inorganic P uptake and anabolic use, ii) mineralization of organic P compounds of increasing complexity. The fate of high energy, C and P-containing organo-phosphoesters used by microorganisms with nutrient and/or energy limitation will be investigated in WP3. Specifically, we aim to define ATP incorporation into cell components as an indicator of the metabolic state of cell being able to differentiate i) energy limited, ii) C limited or iii) P limited microbial communities in soils. Overall, this project will investigate how nutrient deficiencies and the requirement of phosphatase formation is interfering with energy and C storage and how the microbial activity state of microorganisms affects the control of these P and C pathways. The understanding of these mechanisms will help to increase the microbial efficiency in P transformation from SOM especially considering the future P crisis.
Soil microorganisms are important players in terrestrial carbon and nutrient cycles. Many microorganisms produce intracellular storage compounds (PHB, trehalose, glycogen, triacylglycerides), but the implications of storage for microbial growth and C turnover are unknown. Our project investigates microbial storage compounds in soil to understand their occurrence, roles, and drivers in the context of a temperate pasture.
Laboratory incubation experiments and field sampling will be carried out with isotopic labelling and compound-specific chemical analyses to reveal microbial storage dynamics. Experimental results will be combined with stoichiometric modelling to integrate the findings into a broad understanding of soil carbon cycling.
Monocropping and inefficient resource utilization, accompanied by soil degradation and soil biodiversity loss in conventionally managed agricultural systems, pose challenges to stable food production. Sustainable strategies are needed to ensure food production without (further) compromising the health and functionality of agroecosystems. Arbuscular mycorrhizal fungi (AMF) are the most widespread symbionts among crops and have received increasing attention for their beneficial effects on plant nutrition, promotion of crop resistance, and improving soil stability. Most studies on the benefits of AMF for crop production, however, have focused on AMF in the topsoil, i.e., the thin plow layer in conventionally managed systems. The potential of AMF in agricultural subsoils (i.e., the mineral soil below the plow layer) for sustainable agriculture is rarely studied. While the abundance of AMF decreases from top- to subsoil, it has been shown that subsoils can host a unique set of AMF taxa that are assumed to differ in traits and functionality from topsoil AMF. The main objective of this project is the functional differentiation of top- and subsoil AMF communities with respect to their nutrient acquisition and allocation strategies. The project aims to answer the question of (i) whether subsoils harbor AMF communities with functional traits that are distinct from, or even more diverse than, AMF communities in topsoil, and (ii) whether these traits can be beneficial for sustainable management. DNA amplicon sequencing and metatranscriptome analysis of AMF communities associated to plants of different plant functional groups will allow a first assessment of the functional diversity of subsoil AMF communities, based on the assumption of a trait-based partner selection between plants and fungi. For a further functional characterization, the uptake and transfer of readily available N and K by subsoil AMF communities is assessed using 15N-enriched compounds and trace elements that act as K analogs during nutrient uptake. The stimulation of N and P mobilization from organic matter by AMF subsoil communities is investigated using dual isotopic labeled (15N, 33P) organic matter. With 13CO2-pulse labeling, the carbon allocation of crop plants to different subsoil AMF communities will be analyzed. The project will provide insights into the potential of subsoils as reservoirs for functionally diverse AMF communities and their potential to improve nutrient utilization in agricultural systems. Thereby, the project will help to clarify whether an adequate subsoil management is suitable to promote functional diversity of AMF in agricultural systems, or even target specific functional traits, to help stabilize food production.
BMBF: Deep-rooting cover crop mixtures: Creating highways to subsoil water and nutrient resources (RootWayS)
Organic farming systems traditionally consider subsoil resources for crop production. As increasing drought frequencies and intensities will reduce nutrient availability in dry topsoils, future conventional agriculture has to manage subsoils as valuable water and nutrient sources. This project aims at optimization of deep rooting winter cover crop mixtures as a strategy for fast access to subsoil resources by root channel re-use in conventional cropping systems. The niche complementarity principle will be used by combining a shallow and a deep-rooting species of one functional cover crop group (Brassicaceae, grasses, legumes) to form deep-reaching root channels even during short winter growing season. Species-specific root-channel re-use of cover crop mixtures by maize will be quantified.
(Physico-)chemical and microbiological characterization of cover crop root channels and their interactions with the maize rhizosphere will explain the preferential use of species-specific root channels by the cash crop maize. Tracer approaches (13C, 15N, D2O, Cs, Rb, Sr) will unravel the fast and efficient access of maize to subsoil nutrients and water using cover crop root channels. Drone-based thermography will allow upscaling of water and nutrient use by maize to the field scale. Implementation of these data in crop models will allow prediction of maize yield depending on cover crop management. The relevance of cover cropping as subsoil-exploring management practice will be determined depending on two key factors: 1) Soil properties by conducting experiments on the three key agricultural soil types in Germany and 2) Soil water supply by simulated drought events by rainout shelters. We will deliver clear recommendations on soil type-specific cover crop mixtures for maize cultivation facing increasing drought risks. We will bring these outcomes to practice by direct cooperation with cover crop seed suppliers and professional agricultural organizations.
Agropolis foundation: Determining root mucilage exudation as a key adaptive trait mitigating drought impacts
Intensified and frequent drought has severely increased the scarcity of water limitation. This situation has posed greater risk in crop production worldwide and more so in sub-Saharan region. One of the strategies is to understand and exploit the below ground (root and rhizosphere) traits such as root mucilage and the dynamic interaction with their microbial environment. Mucilage, a polymeric gel exuded at the root tip and capable of absorbing large volumes of water, is a strategy that plants deploy to dynamically alter the gradients in water potential at the root–soil interface. The premise is that mucilage keeps the rhizosphere wet, connected to the root surface and hydraulically well conductive, especially in drying soil. The quantity and composition of the mucilage can in turn be affected by the soil moisture conditions. Furthermore, the soil moisture condition affects the microbial activity and physiology in the rhizosphere. Conversely, it is shown that mucilage maintain moisture around the rhizosphere that affects the root-microbial interaction and thereby the microbial community such as arbuscular mycorrhizal fungi (AMF). The interaction between the soil moisture condition, mucilage and the soil AMF are postulated to have impact on cycling of soil organic matter especially the dissolvable organic matter. Hence, elucidating and disentangling the complex dynamics between these important key factors is paramount in the understanding of the rhizosphere.
Thus, the objectives of this project are: 1) to characterize the sugar monomer composition of mucilage extracted from root tips and tropical soils on which Barley and Sorghum varieties are grown under different soil moisture conditions; 2) to characterize AMF communities in the rhizosphere and specifically in the mucilage-covered biofilm area of the rhizosphere; 3) to analyse the extractable organic matter using untargeted soil metabolomics method from these conditions.
Program for the Promotion of Junior Researchers: Effect of nitrogen availability on microbial necromass formation and carbon use efficiency
The "4 per 1000" Initiative of the United Nations Climate Change Conference 2015 now even politically claims the essential demand for an increase of carbon (C) stabilization in agricultural soils for both – the mitigation of climate change and the increase in soil fertility combating the global food crisis. The meanwhile widely accepted concept of the microbial C pump states that the major pathway forming persistent organic C in soils is the transformation of plant residues to microbial biomass, which after cell death becoming microbial necromass is prone to stabilization by its intense interaction with mineral surfaces in soils. However, this process of stabilization as well as the process in opposite direction – the priming effect (PE = microbially enhanced soil organic matter mineralization) are strongly controlled by the availability of N in the soil solution. Thus, post-harvest N levels might not only be key in controlling the potential stabilization of residue C but they are also a relatively easy to manipulate parameter in cropland management. Therefore, this proposal aims to generate an improved understanding on how post-harvest N levels affect microbial transformation of plant residue-C to soil microbial necromass and the physiological control of the priming affect. Laboratory incubations with highly enriched 13C litter C will allow tracing of the 13C in microbial groups and pools as well as key microbial functions of the microbial C pump under the influence of increasing soil N solutions. Thus, we will provide important knowledge promoting agricultural practices towards increasing C sequestration in agricultural soils.
Program for the Promotion of Junior Researchers: Diffusive 18O labelling to relate microbial growth and phosphorus solubilization in the rhizosphere
Phosphorus (P) is a growth-limiting nutrient for plants and microorganisms. Microbial transformations of P in soils plays an important role in increasing its availability for plants and reducing leaching of P as an environmental pollutant, while reducing the dependence on mining nonrenewable phosphate rocks for P fertilizer. Rhizosphere microorganisms are the key drivers of these transformations, but their activity depends on the carbon (C) from root exudates and the prevailing moisture conditions. Microbial activity can be assessed as microbial growth by applying 18O-labelled water to soil, which results in incorporation 18O into all newly synthetized DNA. However, until recently this labeling method has necessarily involved a disturbance of the moisture conditions to introduce the label. The recent development of diffusive 18O labeling makes it possible to apply this method in both moist and dry soil without disturbance. This promising approach has not yet been applied to the rhizosphere. In this proposal we intend to conduct experiments within two working packages (WPs). WP1 will adapt diffusive 18O labeling for rhizosphere studies and verify that it can be combined with 13CO2 plant labelling. WP2 will implement the new technique in combination with 13C labeling of root exudates and potential enzyme activity measurements to relate C utilization, microbial growth and enzymatic transformation of organic P in the rhizosphere. This work will deliver a powerful new method and novel insights into rhizosphere P transformations. Furthermore, the methodological advancement will establish the basis for coupling 18O rhizosphere labelling to DNA sequencing in a subsequent proposal, with the ultimate aim of identifying responsive organisms active in rhizosphere P solubilization.
Data availability statement
We strive to publish all relevant data sets in data journals or repositories after the manuscripts have been published or at the end of the projects.
All other data including metadata collected and analysed during these projects can be made available to other researchers and the general public upon request.
Please contact: Prof. Michaela Dippold or Dr. Callum Banfield
Finished projects:
TERRA pre-study: A joint project on the subject of Africa at the Universities of Hohenheim and Tübingen 'Belowground traits of Kenyan grasslands: Establishing and maintaining functional diversity'
Grasslands are ecologically as well as agronomically of key importance for African countries. Their restoration is essential to combat desertification and maintaining their productivity is essential for mitigating the food crisis of the African continent. For both purposes, their productivity needs to be ensured, often under conditions of multiple abiotic stresses (droughts, unfertile soils, heat stress, etc.).
The project aims to study two already intensively (and ongoingly) monitored grassland trials in Eastern and Western Kenya which are already characterized for their aboveground traits and species composition.
In Eastern Kenyan University (SEKU) Restoration Trial will be established to study 1) the relevance of rooting depth and root traits associated with resource conservation for biomass production grown in monoculture and 2) the influence of rhizosphere traits such as high microbially-mediated nutrient mobilization along with acquisitive root traits for the competition effects of grasses grown in mixtures.
In Western Kenyan Experiment on Use Intensity of native grasslands 1) the effect of trimming frequency on plant biodiversity and by that diversity in belowground traits; 2) the effect of belowground diversity (in root and rhizosphere traits) on drought stress tolerance.
This project establishes a new collaboration between Hohenheim and Tübingen and also serves as a basis for a long-term collaboration with SEKU (South Eastern Kenyan University) and ITCER, where existing experimental platforms and infrastructure can be the basis for future projects.
Vector-Stiftung: Factors of plant phosphorus use efficiency in unfertile soils
The project aims to gain a basic understanding of the changing factors to optimize phosphorus uptake in young plants. Various phosphorus species are applied either in combination with biostimulants, other micronutrients or plant growth-promoting microorganisms. The project aims specifically to study plants in early growth stages, as there is an increased sensitivity to their dependence on macronutrients such as phosphorus. In particular, particularly infertile, low-phosphorus soils are to be examined in order to determine their potential for renaturation as well as for agricultural use. In rhizoboxes, phosphor imaging of the 33P-labeled phosphorus species is used to visualize their mobilization with µm precision and quantify uptake by the roots. The phosphorus forms are freely available or applied in special “mesh bags” (hyphae but not root-permeable membranes) in order to differentiate uptake by mycorrhizal fungi. It is to be investigated whether the co-applications that potentially improve the phosphorus use efficiency (PUE) can significantly improve the P use under extreme conditions, which will occur frequently in the context of climate change. The young plants on the P-poor soils are to be exposed to both heat stress (heat waves) and extreme droughts. Following the incubation and imaging phase, the rhizotrons are harvested and the phosphorus uptake by the plants is quantified. This research is extremely important, especially with regard to the factors of global change, because: 1) Increased atmospheric CO2 concentrations and N deposits "fertilize" the ecosystems globally and increasingly shift the primary deficiency nutrient towards phosphorus, which - because it is rock-borne - cannot be increased in natural ecosystems and in agricultural ecosystems due to the depletion of global, degradable phosphorus deposits will also soon no longer be available to a sufficient extent; 2) climatic extremes such as heat waves and droughts will have a massive impact on plant phosphorus uptake capacity and microbial capacity to mobilize phosphorus in soils Together, these two factors will contribute to the so-called “global phosphorus crisis” that will affect natural and agroecosystems. The potential measures used here (application of special microorganisms, biostimulants, etc.) can represent a sustainable strategy to mitigate this phosphorus crisis and are therefore of global relevance.
