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).

 Scheme of the main techniques to be applied for SOM characterization in Forest floor