Spatial patterns of organic matter formation in subsoil: Nutritional control of microbial hot spot formation and dynamics

• Applicants: Dr. Stefanie Heinze; Professor Dr. Bernd Marschner
• Subject Area: Soil Sciences
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 452509846

Subsoils are characterized by highly heterogeneous distribution of soil organic matter and microbial life due to very localized inputs of substrate and nutrients. While the rhizosphere isknown to be a soil domain with higher C inputs, we also detected numerous hotspots of enzyme activities in undis-turbed subsoil samples of our main research site Grinderwald without any visible association with roots. These microbial hotspots may therefore be related to preferential flow pathways serving as highways for dissolved organic carbon (DOC) and microorganisms into the subsoil, building spatial-ly restricted hotspots of biochemical cycling.Based on the tracer experiments to be conducted by Bachmann/Weiler/Marschner we will identify input trails and quantify the microbial active zones in subsoils, in order to test the following hypothe-ses: i) within each soil depth, microbial and enzyme activities are higher in flow paths than in the matric soil; ii) flow paths are persistent hot spots of microbial C-turnover impacted by duration and frequency of C and nutrient input; iii) flow paths show higher abundance of microorganisms adapted to labile C input than matric soil; iv) due to the repeated input of easily available C and nu-trients into the flow paths, negative priming will take place increasing SOC stocks in subsoils; v) the soil volume marked with the dye tracer is larger than the flow path volume with elevated microbial activity because transport of cells is limited to larger pores and substrate diffusion is lower than tracer diffusion. In order to test these hypotheses, samples from flow paths and from the matric soil, as well as from soil segments above the suction plates (project Guggenberger/Kalbitz/Mikutta) will be analyzed for enzyme activities, basal respiration, substrate utilization and priming effects. In order to determine the spatial extent of flow path associated hotspots and its persistence, undisturbed soil samples from the tracer-stained profile faces will be subjected to zymography, i.e. mapping enzyme activi-ties at high spatial resolution, combined with analysis of the microbial community composition on cooperation with Kandeler/Marhan/Poll. By combining this approach with nutrient and substrate ad-ditions, we will determine limiting factors for microbial activity within and outside of the flow paths. Selected undisturbed samples will further be analyzed by hyperspectral imaging in order to obtain highly resolved data related to soil properties. In this way, we have the unique opportunity to charac-terize and spatially delineate flow path affected active soil areas in undisturbed soil samples. Addi-tionally, the undisturbed soil samples will be incubated to gain insights into the persistence of micro-bial hotpots with and without substrate additions. This project will enhance our understanding of the contribution of the microbial active soil volume and the controlling mechanisms for SOC mineralization in subsoils.

• DFG Programme: Research Grants