Research Projects

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DFG, 2023 - 2026

The accuracy of visual and near infrared (Vis/NIR) and mid-infrared (MIR) spectroscopy for soil organic matter (SOM) characterization and quantification has been very variable in a large number of observational studies and there is insufficient knowledge on the underlying reasons for this variation in performance. The reasons are mainly related to IR-active properties of SOM and SOM fractions being affected by interactions of SOM with (mineral-bound) ions that result in shifts of bands in the Vis/NIR and MIR spectra relative to those without cations, minerals and soils. This proposal aims by means of designed experiments (i) to improve the mechanistic understanding of the interaction between organic matter (OM) and cations using Vis/NIR, MIR and micro-MIR; (ii) to subsequently develop an adjustment procedure to quantify effects of cations on shifts and intensities of OM bands; and (iii) to finally optimize linear and non-linear chemometric regression approaches for the prediction of contents of bulk soil organic carbon (SOC) and SOC fractions using the adjustment procedure and optimized spectral variable selections. Salts, minerals, and soils will be added to predefined organic components to obtain different OM-cation associations. The complexity of OM in the experiments will increase from low molecular weight organic substances to commercial humic acid, pre-incubated wheat straw and forest floor Oa material. The Vis/NIR and MIR spectra (including micro-MIR) of the mixtures will be compared with those of the OM to advance the mechanistic understanding of the relations between OM-cation interactions and band shifts. The adjustment procedure, which quantifies the effects of cations on band shifts and intensities, will be based on the Lambert-Beer law and will use the data of the OM-cation experiments to calculate wavenumber-dependent weighting factors that reflect the cation effects on the band intensities. The weighting coefficients will be used to recalculate the band intensities of the OM components. The accuracy of re-calculated OM spectra will be checked by comparing them with the spectra of the original OM and in case of native OM by a spectral subtraction procedure after chemical oxidation. Linear and non-linear chemometric regression approaches for the prediction of contents of bulk SOC and SOC fractions in soils at field and regional scales will be subsequently optimized. For the optimizations, the usefulness of band-shift corrections and improved chemometric regressions that consider advanced spectral variable selection approaches and cation-/mineral-related information will be explored. Overall, the spectral data from experiments designed at scales of increasing complexity are expected to advance the understanding of effects of OM-cation interactions on band shifts which can then be applied for improving the signal to noise ratios and optimizing linear and non-linear chemometric regression approaches.

DFG, 2022-2024

Sensor-based soil monitoring, which uses infrared spectroscopic (IR) approaches (Vis/NIRS, MIRS) and energy dispersive X-ray fluorescence spectroscopy (XRF), can quantify soil properties that are essential to derive and evaluate soil functions in high spatial and temporal resolutions. However, collection and evaluation in laboratory-based and portable sensor-based methods have not yet been optimised, making it difficult to critically assess possibilities and limitations of these methods.This proposal aims at optimising the IR and XRF data collection for spectrally-active soil properties using laboratory-based and portable methods for representative soil type complexes of selected soil landscapes. The individual methods are optimised with regard to measuring modes (diffuse reflectance infrared Fourier transform vs. attenuated total reflection for MIRS, powder vs. pellets for XRF), sample preparation (in-situ, dried, dried and sieved, dried and ground), measuring conditions and required sample numbers. Specific depth functions for the different soil properties will be defined in the studied sample profiles.A systematic comparison of calibration algorithms for the quantification of spectrally-active soil properties by Vis/NIRS, MIRS and XRF will be performed. Linear and non-linear calibration approaches and spectral deconvolution will be employed as functions of different sample sizes in multiple partitions. Spectral variable selection and spiking using existing databases will be used. The accuracy of soil profile characterisation by portable and laboratory-based spectroscopic methods as well as a potential transferability of calibrated estimation models will be systematically investigated on individual profiles of the studied soil type complexes. The analysis of site-specific relationships between spectrally-active and spectrally-inactive or spectrally poorly-defined soil properties will be performed both chemometrically and by regression analyses using the spectrally-active soil properties as predictors. The synergistic IR and XRF data analysis using the laboratory-based and mobile approaches will be based on low-, mid- and high-level as well as hybrid data fusion approaches with an emphasis on outer-product analysis, low-level fusion with subsequent automatic wavelength range selection, mid-level fusion using the principal components and high-level fusion using the approaches of Bates-Granger and Granger-Ramanathan.This project provides the tools with which to optimize the data collection of sensor-based soil monitoring, to improve the determination accuracy of differently complex soil properties by data fusion approaches, and to evaluate the applicability of a spatial and profile-related sensor-based monitoring for selected soil type complexes.

(DFG, 2020-2023)

Projektkurzbeschreibung: 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, 2018-2020)

Total contents of soil organic carbon (SOC), nitrogen (N) and phosphorus (P) are only of limited use for studies of management (e.g. fertilizations or tillage) on soil fertility; SOC and N fractions as well as soil microbial properties are much more sensitive indicators. However, a high spatial and temporal density of samples can only be achieved with non-destructive sampling techniques. In this context, the project studies the potentials of spectroscopic techniques to determine key soil properties (SOC, N, pH, fractions of SOC and N, P, sulphur, potassium, iron, cation exchange capacity, soil texture, microbial and hot water-soluble C and N) with high accuracy by combining non-imaging spectroscopy in the near and middle (vis-NIR and MIR) domain with hyperspectral imaging. In addition to the lab scale, we focus on the field scale with on-site spectroscopic measurements, which is favoured by new instrumental developments, a portable MIR spectrometer and a portable hyperspectral frame camera. The MIR range is essential for soil spectroscopy, as fundamental bands of chemical groups can be measured (different from the NIR range with only combination bands and overtones). For a total of eight arable sites with soils of differing textures, top soils and soil profiles will be sampled to investigate the potentials of lab spectroscopy compared to on-site spectroscopy by combining the different spectroscopic techniques for the estimation of the soil properties mentioned above. To improve obtained accuracies, methods of multivariate calibration will be optimized by using e.g. Support Vector Machines or Random Forest instead of PLSR, by applying spectral variable selection techniques, by substituting global by local calibrations (i.e., a sample-wise selection of appropriate calibration samples is performed) and by using the approach of spiking to locally adapt calibration models. The optimized techniques will then be validated on existing data sets. Additionally, it will be analysed, whether and to what extent disturbances originating from different soil surface roughness or from different soil water contents can be compensated. Already existing soil spectral libraries (LUCAS, ICRAF-ISRIC) are evaluated to select appropriate samples which may support the definition and optimization of calibration models. In addition, the underlying spectral predictive mechanisms will be analysed (e.g., by 2D-correlation spectroscopy) to elucidate whether a direct or only indirect spectral prediction is feasible for each of the studied soil properties. This is fundamental to clarify whether a prediction model, once calibrated, may be in principle transferred in space and time.

(DFG, 2016-2019)

Scarcity of land, water, and labour are the main constraints to agricultural production along the rural-urban interface of Bangalore. This has led to the widespread replacement of diverse dryland farming systems by intensive irrigated vegetable production on open lands at the city fringe and in scattered city areas. The consequences of such changes in traditional Indian farming systems on the nutrient dynamics and soil fertility are largely unknown. This project therefore addresses transformation-related changes in the intensity of agricultural production systems at the micro(plot)- and meso-scale (field and household) by analysing horizontal and vertical flows of nutrients and carbon in field and laboratory experiments, turnover of soil organic carbon and soil nitrogen in different pools, and their effects on nutrient and water use efficiency under rainfed and irrigated conditions on-station and on 72 selected farmer fields. It combines expertise in agronomy, plant nutrition and non-destructive measurements of soil physical properties and plant growth (Bürkert) with modelling of water dynamics, crop growth, and carbon (C) and nutrient fluxes (Ludwig). The project will contribute to the overall goals of FOR2432 by providing spatially explicit data on the effects of intensification-related land use changes on nutrient and water use efficiencies for major food crops in the study region. It also aims at assessing the impact of these changes on key chemical soil parameters (pools of C and nitrogen N, phosphorus P, potassium K, and sulphur S) that govern crop output and ecosystem services. It thereby provides data for the calibration and validation of models to predict medium- and long-term changes of key soil parameters determining crop productivity and ESS. To this end we will investigate the plant uptake and turnover of C and N, P, K, and S including gaseous (N2O, NH3, CO2) and leaching losses (NO3-, dissolved organic carbon, organic P, and SO4[2-]), by photo-acoustic spectroscopy, gas chromatography, and resin cartridges /micro-lysimeters, respectively, and relate them to the soil hydrological data of A02 and the multi-spectral cropping / land use signatures of C01 and C02. Using 15N in the central experiment will allow to determine the fate of added fertilizer N. The data will also be used to parameterize the DNDC model. High resolution aerial photography and multispectral canopy scanning from hexacopters will be used to non-destructively monitor, in cooperation with C01, plant growth in the on-station experiments and the spatial variability of final biomass on-farm and to collect high resolution, low altitude multispectral crop signature data. Thereby this project will contribute to the estimation of the current productive value of agricultural lands which partly explain a decisions of farmers to intensify crop production or give up land for urban development.

(DFG, 2013-2016)

It is well established that reduced supply of fresh organic matter, interactions of organic matter with mineral phases and spatial inaccessibility affect C stocks in subsoils. However, quantitative information required for a better understanding of the contribution of each of the different processes to C sequestration in subsoils and for improvements of subsoil C models is scarce. The same is true for the main controlling factors of the decomposition rates of soil organic matter in subsoils. Moreover, information on spatial variabilities of different properties in the subsoil is rare. The few studies available which couple near and middle infrared spectroscopy (NIRS/MIRS) with geostatistical approaches indicate a potential for the creation of spatial maps which may show hot spots with increased biological activities in the soil profile and their effects on the distribution of C contents. Objectives are (i) to determine the mean residence time of subsoil C in different fractions by applying fractionation procedures in combination with 14C measurements; (ii) to study the effects of water content, input of 13C-labelled roots and dissolved organic matter and spatial inaccessibility on C turnover in an automatic microcosm system; (iii) to determine general soil properties and soil biological and chemical characteristics using NIRS and MIRS, and (iv) to extrapolate the measured and estimated soil properties to the vertical profiles by using different spatial interpolation techniques. For the NIRS/MIRS applications, sample pretreatment (air-dried vs. freeze-dried samples) and calibration procedures (a modified partial least square (MPLS) approach vs. a genetic algorithm coupled with MPLS or PLS) will be optimized. We hypothesize that the combined application of chemical fractionation in combination with 14C measurements and the results of the incubation experiments will give the pool sizes of passive, intermediate, labile and very labile C and N and the mean residence times of labile and very labile C and N. These results will make it possible to initialize the new quantitative model to be developed by subproject PC. Additionally, we hypothesize that the sample pretreatment “freeze-drying” will be more useful for the estimation of soil biological characteristics than air-drying. The GA-MPLS and GA-PLS approaches are expected to give better estimates of the soil characteristics than the MPLS and PLS approaches. The spatial maps for the different subsoil characteristics in combination with the spatial maps of temperature and water contents will presumably enable us to explain the spatial heterogeneity of C contents.

(DFG, 2013-2016)

Near-infrared (NIRS) and mid-infrared (MIRS) reflectance infrared spectroscopy has high potential for determining soil chemical and biological characteristics, but research is still needed in terms of predictive accuracy and understanding of underlying relationships. Project objectives are to: (i) To optimize the accuracy of NIRS and MIRS to predict soil organic C and N content and soil organic matter composition. This will involve classifying the population by soil type, texture and mineralogical composition. Sub-samples will be chemically or thermally oxidized and a modified PLS method, a genetic algorithm, will be tested. (ii) General relationships between the amounts of labile, intermediate, and passive C and N (to be obtained from incubation experiments and Na2S2O8 treatments) and the significant wavelengths of the NIRS and MIRS cross-validations will be derived from spectra taken before and after the incubations. (iii) To determine the predictive quality of soil constituents using NIRS and MIRS for open populations.

(DFG, 2011)

When organic material is available, soil microorganisms may either take up simple organic molecules directly and release the surplus nitrogen (N) into the soil in the form of ammonium (NH4+), or they may mineralize organic N first and take it up in the form of NH4+. The route of N uptake has implications for the competition for N between microorganisms and plants and therefore for the N nutrition of crops. The objective of the proposed study is to gain a better understanding of the factors controlling the relative importance of the two N utilization routes by soil microorganisms. In a first step, an assay to determine the extent of the extracellular deamination of amino acids in soil shall be developed and tested. In a second step, the new assay shall be applied in laboratory incubations in combination with other methods, namely gross N mineralization, mineralization of amino acid-N, and natural 15N abundance of the microbial biomass, to determine the factors affecting the relative importance of the two N uptake routes. In a third step, the same methods shall be applied in a field study, which will focus of the temporal effects of fertilization management, including organic and mineral fertilizers, and the interactions with crops.

(ZFF, 2009)

The aim of the project is to verify whether ExpertN can be used to simulate N dynamics in northern Chinese soils. For this purpose, microcosm experiments will be used to collect data needed to calibrate the ExpertN model, and test simulations will be performed. Soil samples from northern China and 15N-labeled materials (feces, urea, and nitrate) will be used for the microcosm experiments. Data regarding climate important for model parameterization will be obtained from literature sources.

(ZFF, 2008)

The aim of this project is to verify whether pool sizes and their turnover rates as well as isotope ratios can be predicted by near-infrared spectroscopy (NIRS) and mid-infrared spectroscopy (MIRS). For this purpose, soils used in previous studies for modeling soil organic matter dynamics are investigated by NIRS and MIRS in the field-fresh and dried states. Based on the spectra, calibrations and predictions are made regarding the modeled pool sizes and their turnover rates. The pools considered here are those of the RothC, CANDY and DAISY models. Samples from various forest and agricultural sites under C3 and C4 vegetation are used to quantify the suitability of NIRS and MIRS for estimating 13C/12Cand 15N/14Nisotope ratiosin the natural abundance range.

(Graduiertenkolleg DFG 2007 - 2015)

The regulation of the budget of soil organic matter (SOM) and nutrients by management is a central focus in agriculture, especially in organic agriculture. The budgets of SOM and nutrients determine soil fertility, i.e. the long-term productivity of soils. However, the underlying processes of soil fertility in organic agriculture are not well understood. The interdisciplinary Research Training Group, therefore, deals within the scope of twelve PhD theses on the possibilities to regulate soil organic matter and nutrient budgets by soil management, crop rotation and by feeding strategies via the quality of the manure.
The focus is to be on:
(1) the quality of inputs (crop residues, organic fertilisers),
(2) the turnover of litter and SOM, especially carbon, nitrogen and other relevant nutrients (Ca, K, Mg, PO4),
(3) gaseous (CO2, CH4, N2O, NH3) and liquid (NO3, dissolved organic carbon, cations) outputs and
(4) the coupling of these aspects by modelling.
Field experiments are carried out on seven sites under temperate and subtropical climatic conditions, which differ in respect of dominating soil unit (Haplic or Orthic Luvisol, Irragric Cambisol, Irragric Anthrosol) and/or soil texture (silt, sand). They are complemented by laboratory experiments under controlled conditions, semi-controlled microcosm experiments in the laboratory and field and modelling studies.

(DFG, Ludwig & Flessa)

Modeling of C sequestration in soils is necessary for improved process understanding and prediction. However, validations of existing models showed serious deviations from measured values, unless adjustable parameters were used, or implausibly low C inputs. Building on the previous results, the following topics will be addressed for the Halle, Rotthalmünster and Bad Lauchstädt sites in the proposed project phase: (i) extension of the Rothamsted Carbon Model; (ii) regression analysis between the fraction sizes or turnover rates determined in the SPP and the model results of the Roth C modeling and inclusion of the 14C data in the modeling; and (iii) implementation of C storage modeling for subsoils of the three sites.

(DFG, Flessa & Ludwig, 01.10.04-30.09.06)

Quantitative knowledge of the incorporation rates of litter carbon into soil organic matter (SOM) fractions of varying stability is an important prerequisite for understanding the regulation of soil organic carbon (SOC) stabilization. Building on the results of the first two phases of the project, the following issues will be addressed in the proposed project section: (i) different methods to characterize stable SOM pools (oxidation processes, hydrolysis with trifluoroacetic acid and HCl, organomineral association dissolution processes, and "black carbon" recovery) will be reviewed and optimized based on 13Cand 14C analysesand the existing modeling results; (ii) SOC turnover rates in aggregate size classes of different land use systems in incubation experiments will be determined and in cooperation with AG Bachmann the wettability of the aggregate classes will be determined; (iii) using 13C-and 15N-labeledlitter, the influence of litter quality and fungal biomass for aggregate formation and stabilization of organic matter in the course of litter decomposition will be determined, using NIR laser spectrometry for 13CO2 and 12CO2determinationin cooperation with AG Löhmannsröben; (iv) modeling of SOC dynamics will be performed considering above results and improved modeling approaches (fractionation concepts according to Six et al. (2002) and Skjemstad (2003)) will be performed without adjustable parameters.

 (DFG, Ludwig, 01.08.04-31.07.06)

Projektkurzbeschreibung: Quantitative Kenntnisse über die Zusammensetzung, Eigenschaften und Umsetzbarkeit organischer Abfälle (Komposte und Klärschlamme) und der organischen Bodensubstanz (OBS) sind für eine nachhaltige Landnutzung bedeutsam. Analytische Methoden und Experimente hierzu sind häufig kosten- und arbeitsintensiv, während Modellberechnungen nicht allgemein akzeptiert sind. Ziel ist es, die Eignung der Spektroskopie im nahen (NIRS) und mittleren Infrarotbereich (MIRS) zur Bestimmung der Zusammensetzung, Eigenschaften und Umsetzbarkeit organischer Abfälle  und der OBS zu überprüfen und Optimierungen durchzuführen.

(DFG, Flessa, Ludwig & Beese, 01.10.02 - 30.09.04)

Project abstract: Quantitative knowledge of the incorporation rates of litter carbon into differently stable fractions of soil organic matter (SOM) is an important prerequisite for understanding the regulation of organic matter stabilization in soils. Determining the rates of formation and turnover of differentially stable fractions of soil organic matter during litter decomposition requires that the origin of organic carbon in the fractions can be traced. The objective of this project is to record the long-term turnover and stabilization rates of corn-derived carbon in the soils of corn monoculture plots (since 1961) of the permanent experiment "perpetual rye". The analysis is based on the natural 13C distributionin different fractions (physical fractionation, extractions, pyrolysis) of soil organic matter. Incubation experiments will quantify the importance of SOM stocks of different ages (corn-born or formed before 1961) as substrate for DOC production and soil respiration. Furthermore, the influence of mineral nutrient supply on the turnover and stabilization rates of corn litter will be recorded. The results will be used to model the dynamics of C turnover processes using the Rothamsted C model.

(BMBF, Beese & Ludwig, 01.05.01 - 30.04.04)

Projektkurzbeschreibung: Das Verhalten oberflächennah abgelagerter, gering kontaminierter "Abfälle zur Verwertung" in oder auf Böden sowie der Transport der gelösten Reaktionsprodukte wird durch komplexe Wechelwirkungen mit diesem Umweltmedium geprägt. Die Projektziele beinhalten eine Optimierung der Verfahren zur Ermittlung der Quellstärken am Beispiel diverser Schlacken der Metallherstellung und Aschen. Die potentielle Quellstärke soll in modifizierten Batchexperimenten bei Anwendung verschiedener Wasser-Feststoff-Kontaktzeiten, Dränungszeiten und Extraktionslösungen ermittelt werden. Die effektive Quellstärke soll in Kurz-Säulen-Fließexperimenten mit Fließunterbrechungen bestimmt werden. Weitere Ziele beinhalten die Überprüfung der Anwendbarkeit des Modells PHREEQC2 für die Ermittlung der potentiellen und effektiven Quellstärke unterschiedlicher "Abfälle zur Verwertung" und die Überprüfung des Modells PHREEQC2 für die Sickerwasserprognose an repräsentativen Bodensäulen unterschiedlicher Texturen und pH-Pufferbereiche.