Spatial patterns of organic matter formation in subsoil: Hierarchical preferential flow pathways

• Applicants: Professor Dr. Jörg Bachmann; Dr. Stefanie Heinze, since 5/2024; Professor Dr. Markus Weiler
• Subject Area: Soil Sciences
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 452510514

Previous results of the Research Unit suggest that decoupling of biological and physical processes by spatial separation of organic matter (OM), microorganisms and extracellular enzyme activity is apparently one of the most important mechanisms leading to the protection and stabilization of litter- and topsoil-derived dissolved organic carbon (DOC) in deeper subsoil layers. The occurrence of preferential flow (PF) paths is supposed to be a major driver for the formation of hot spots of microbial activity and OM transformation and stabilization in subsoil. Consequently, employing the globally unique subsoil observatories at Grinderwald, our new research initiative aims at investigating the spatial patterns of OM formation in subsoil. The following aspects will be covered in the complementary research activity:- Hierarchical preferential flow pathways - Microbial carbon retention and turnover - Nutritional control of microbial hot spot formation and dynamics - Formation of mineral-associated organic matter Our project is the attempt of a multi-scale approach to capture active and less active transport zones in soil. Experimental evidence already confirmed a stable network of PF pathways on the cm to dm scale over years. We assume that this meso-scaled flow heterogeneity can be captured with fluorescent dye irrigation experiments in the field. Subsequent dye-guided sampling will identify locations with systematic differences regarding active or less active flow domains. The newly developed technique to extract undisturbed flow cells (FC) allows a comprehensive analysis of the in situ state by highly resolved measurements and imaging of important physicochemical and microbiological surface properties (e.g. wettability, functional groups, exoenzyme activity on the mm- to cm-scale). Corresponding sampling strategy will allow to quantify nested PF structures. To assess DOC transport and transformation we will simulate flow processes using 13C-labeled DOC. The percolation experiments will be conducted with two undisturbed serial FC under realistic in situ moisture, DOC concentration, flow rate and temperature conditions, simulating the average flux rate at the sampling site during the winter drainage season. The microbial status of soils sampled from active vs. less active transport domains will be compared to investigate differences in microbial community patterns (in cooperation with Kandeler et al.). Here, we aim to achieve a complete 13C mass balance recovery by quantifying 13C (i) adsorbed to the solid phase, (ii) incorporated into microbial biomass, (iii) released as CO2, and (iv) transported as DOC. This approach (in cooperation with all projects) will finally allow to evaluate the importance of micro-PF patterns versus field scale PF pathways to elucidate the mechanisms triggering the spatial pattern of microbial hot spots.

• DFG Programme: Research Grants
• Ehemaliger Antragsteller: Professor Dr. Bernd Marschner, until 4/2024