Autor:innen:
Dr. Carsten Simon | Helmholtz - Zentrum für Umweltforschung GmbH - UFZ | Germany
Paul Pietsch | Helmholtz - Zentrum für Umweltforschung GmbH - UFZ | Germany
Konstantin Stumpf | Helmholtz - Zentrum für Umweltforschung GmbH - UFZ | Germany
Dr. Klaus Kaiser | Martin-Luther-Universität Halle-Wittenberg, Bodenkunde und Bodenschutz | Germany
Dr. Oliver Lechtenfeld | Helmholtz - Zentrum für Umweltforschung GmbH - UFZ | Germany
Organic matter is an important component of soils that contributes to manifold aspects of soil reactivity, fertility, and physics. The molecular structure of soil organic matter (SOM), however, remains largely elusive. Ultrahigh resolution mass spectrometry (like FT-ICR-MS) has contributed new impulses to the study of molecular properties of the soluble fraction of soil organic matter, i.e., dissolved organic matter (DOM) via electrospray ionization (ESI), but remains “blind” to the much larger non-soluble fraction of SOM. Traditionally, this problem was tackled by employing more potent solvents, such as alkaline extractants. Alternative ionization sources such as laser desorption ionization (LDI) have shown promising results to extend the analytical window of FT-ICR-MS and to achieve a more complete insight into SOM chemistry through direct analysis of soil surfaces. This also reduces the risk of structural changes by avoiding extraction steps. We here present outcomes of such parallel analyses of soluble and non-soluble SOM fractions based on a set of arable topsoils from long-term experiments under no fertilization and farmyard manure fertilization regimes. Additionally, soil DOM (water extracts), reference DOM samples (Suwannee River Fulvic Acid, SRFA), model compounds (syringic acid, sinapic acid, syringaldehyde, vanillic acid and tannic acid) and model mineral phases (goethite, illite, quartz sand) were used to study potentials and limitations of the LDI method. Sensitivity and matrix effects were assessed by dilution series and mixing of DOM with reference mineral phases, whereas conditions for intact ionization were assessed with model compounds. Direct analysis of soil particles was conducted and compared to aqueous DOM extracts considering molecular trends in e.g., aromaticity or nominal oxidation state. In general, ESI ionized a very different fraction of the SOM mixture, being more polar and more saturated, while LDI yielded rather low-to-mid polar, less saturated ions. Besides clear differences in directly analyzed soil particles and DOM, molecular trends such as aromaticity or nominal oxidation state corresponded well. Our results therefore suggest that direct analysis of soil particles is a fast, reproducible, sensitive and less labor-intense alternative to routine DOM analyses employing FTMS detection. However, for a maximum molecular insight, a combination of both ionization methods is recommended.