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Research themes

MOLTER funds scientific exchanges, workshops, and conferences which address one of the following 5 themes:

1. Molecular composition and turnover of soil organic matter

2. Plant molecular structures as drivers for C stabilisation in soils

3. Fire transformations of plant and soil molecular structures

4. Molecular markers in soils

5. Dissolved organic molecules in soils: origin, functionality and transport

These are explained more in detail below.

Theme 1: Molecular composition and turnover of soil organic matter

themeone.jpg (Bildekarusell (BF))

Theme 1 supports activities aimed at unravelling the molecular composition and respective turnover times of the soil organic matter.

Molecularly-uncharacterized organic matter comprises most organic carbon in soils and other terrestrial systems. Contrasting molecular structures display contrasting properties, which ultimately govern their fate in natural environments, but also control major soil functions.

SOM molecular properties with relation to soil functions are not only the concentration of organic matter, but also its hydrophobic or hydrophilic character that controls the soil water holding capacity. The affinity of SOM to the soil mineral matrix is decisive for the formation of soil structure. The origins, reactions and fates of these ubiquitous organic materials are relatively obscure, in large part because the rich vein of chemical information that typically derives from detailed structural and stereochemical analysis is yet to be tapped. But molecular investigations have recently played a key role in the understanding of SOM by providing molecular-level details about its composition.

New advanced techniques in organic and isotopic chemistry, both spectrometric and spectroscopic, are now giving us the capacity to probe for specific molecular structures of specific origin in soils and other natural environments. For example, 13C nuclear magnetic resonance (NMR) spectroscopy provides unsurpassed detail on the distribution of molecular functional groups within soils. Pyrolysis (Py) coupled to mass spectrometry (MS) and isotopic mass spectrometry (IRMS) allow us to analyze the distribution and turnover time of molecular structures and moieties within soil samples. Thermogravimetry and differential scanning calorimetry (TG/DSC) coupled to IRMS allow us to explore the turnover time of SOM across operationally-defined fractions.

Theme 2: Plant molecular structures as drivers for C stabilisation in soils

soilmolter.JPG (Bildekarusell (BF))

Theme 2 supports activities aimed at deriving a comprehensive picture of molecular C fluxes in the plant-soil continuum, which includes frontier methods in organic chemistry as well as modelling and database development.

Plant molecular structures from above-ground residues and root activities represent the largest flux of C to the soil system. Only a small fraction of this large C input is eventually stabilised in soils. Understanding the molecular origin of this stabilised C is crucial to help maximize energy output from plant residues while increasing C storage in soils.

For a long-time, the lack of appropriate in situ methods meant that the fate of C derived from specific molecular structures was probed through laboratory incubations. These studies have heavily emphasized short-term mineralization rather than long-term C stabilization, while the latter is the relevant information in terms of C storage. This over-emphasis on short term mineralization has led us to misinterpretations. For example, lignin has long been considered as a major contributor to stable soil organic C because of its short term chemical recalcitrance to mineralization, while recent studies suggest that its long-term stabilization is not especially high. These recent studies are based on combined chemical (MS) and isotopic (IRMS) determination of molecular structures, which are isolated from soils either by wet chemistry or Py. Models of the molecular-C flux at the field scale are being adapted to these emerging datasets.

As a result, a new paradigm for the stabilisation in soils of plant C derived from specific molecular structures is currently emerging. Recent investigations have addressed, among others, macromolecules such as lignin, polysaccharides, tannins, cutins and suberins, and simpler molecules such as sugars and amino acids.

Theme 3: Fire transformations of plant and soil molecular structures

themethree_clip_fire.jpg (Bildekarusell (BF))

Theme 3 supports activities aimed at studying refractory molecular structures formed during fire events and the contribution of these structures to long-term soil C storage.

Wildfires have a substantial bearing on the long-term sequestration of C and N in forest soils. These processes should be better understood, since small changes in the C storage potential of the soil may alter the global C cycle. Some fire-affected ecosystems may act as sinks for atmospheric C because vegetation fires and fuel-wood combustion transfer C from the relatively fast biological-atmosphere C turnover to the long-term geological one in the form of refractory C.

Most studies probing the formation of refractory C have focused on thermal degradation of lignocellulosic material. Laboratory experiments have proved that forest fires or controlled burnings of crop residues induce a series of inter- and intra-molecular reactions leading to condensed materials with chaotic structure and close resemblance with humic or kerogen-like matter. In addition, other biomolecules such as peptides or lipids may experience major alterations during heat treatments.

The simultaneous occurrence of different processes during forest fires makes it extremely difficult to accurately describe SOM transformations in burned areas. These may range from the total loss of SOM in the top soil or the surface of swamps and peat deposits, to an increase of the passive SOM pool due to the incorporation of necromass from fire affected vegetation, but also partially and heavily charred plant remains (highly refractory organic matter collectively referred to as "black carbon") on the ground.

Nevertheless, laboratory simulation experiments have unambiguously documented the selective destruction of humus fractions, the chemical alteration in SOM properties, and external inputs of charcoal and charred lingo-cellulose from the vegetation.

Theme 4: Molecular markers in soils

Theme 4 supports activities on soil biomarkers aimed at unravelling the microbial transformations and decomposition processes affecting soil organic matter, as well as tracing its origin in terms of plant species and organ type.

Biomarkers are a powerful tool to unravel the processes that govern the soil organic matter transformation in soils. Among these processes, microbial transformations and activities are central. Knowing which microorganisms are active is indispensable for biogeochemical research.

Since most microorganisms cannot be cultured (easily) in the laboratory, alternative molecular techniques have been developed, based on observations that specific biomarker molecules, characteristic for a specific type of microorganism present in an ecosystem, can be isolated and quantified. Linking these molecular biomarker techniques with component specific stable isotope techniques (e.g. GC-c-IRMS and LC-c-IRMS) provide additional advantages. Via stable isotopes probing (SIP) labeled substrates can be introduced in an ecosystem e.g. via (13CO2) pulse labeling of the vegetation. Transfer of 13C via root exudates to active microorganisms permits to identify metabolically active microorganisms via the isotopic composition of their respective biomarkers.

Because of the great structural diversity and specificity of the phospholipid fatty acid (PLFA) building blocks of cell membranes and because of their instability after necrosis, PLFA's are an ideal proxy for the microbial biomass. Cell walls of fungi, bacteria and actinomycetes are partially constructed of amino sugars (AS) which can be used as biomarkers for microbial necromass. The relative and absolute amounts of AS in soil, together with their individual δ13C values can be used to determine turnover times and contribution to C-sequestration of microbial soil communities.

These state of the art approaches in isotope and environmental ecology are essential to link microbiological ecology to biogeochemistry.

Biomarkers are also used to probe the origin and alteration degree of soil organic matter. For example, the biomarker potential of root and shoot derived aliphatic macromolecules (suberin and cutin) is currently under investigation in order to trace the origin of soil organic carbon. The acid-to-aldehyde ratio of soil lignin structures is a function of its alteration degree, which can be characteristic of given soil formations and thereby help determine the origin of SOM in sedimentary systems.

Theme 5: Dissolved organic molecules in soils: origin, functionality and transport

DOM.jpg (Bildekarusell (BF)) 

Theme 5 supports activities aiming at identifying, quantifying and fate-tracing of dissolved organic molecular structures in terrestrial environments.

Dissolved organic molecules present in soil solution represent a wide variety of compounds from high to low molecular mass being both hydrophilic and hydrophobic, some of which probably bear significant ecosystem functions, such as complex-forming ligands and siderophores.

High molecular mass molecules, operationally defined as humic acids, have been suggested to be supramolecular associations. The sources of dissolved organic molecules are mainly litterfall, root litter, root exudates and microbial production. The sinks are adsorption and precipitation in the soil, mineralization to CO2 and leaching out of the soil. Investigating molecular structures helps us evaluate interactions in the plant-mycorrhiza-bacteria-soil system, and their impact on the ecosystem C balance.

By using molecular techniques such as Liquid Chromatography coupled to Tandem Mass Spectrometry (LC-MS/MS) or Inductively Coupled Plasma Mass Spectrometry (LC-ICP-MS) for studying complexation with metals, molecules of significance can be determined. Also near-edge structure spectroscopy (XANES) is used to study complexation with sulphur containing molecules, and NMR techniques are used for identification of functional groups. By these and other techniques, the fate of dissolved organic molecules in the plant-soil-water continuum can be followed, which helps us understand its significance for terrestrial ecosystem functioning. 




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