Organic matter stabilization processes in subsoils: Impact of small-scale physical heterogeneity of soil particle interfaces

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The spatial separation between degraders and substrate is considered as a major factor controlling subsoil organic carbon variability, stability and dynamics. Processes in soils take place at solid particle surfaces, emphasizing the relevance of interfacial properties for physical, chemical and biological processes due to large solid-surface to mass ratios. However, mostly ignored is at present the importance of the solid interfaces that may enhance spatial and temporal heterogeneity in soils. Accordingly, it still needs to be answered which soil-specific factors are governing the carbon budget in subsoils and what the respective scales for capturing stabilizing mechanisms are.One innovative element of subproject P4 is the development of a flow cell soil sampling technique to assess small-scale soil properties and processes and to reduce 3-dimensional transport processes to quasi 2-d which allows a direct link between soil heterogeneity and processes of interest. Flow cell sampling provide the option to sample within one frame (i.e. 8 x 12 cm) simultaneously up to 5 undisturbed soil slices as a sandwich. Due to the small thickness of a single sandwich layer (5 to 10 mm), undisturbed samples can be used as quasi-replicates. Two approaches will be made to link interfacial properties with chemical and biological processes in order to identify biological hot spots. Using a cascade of flow cells of different depths, a virtual soil profile will be simulated by connected cells and depth-dependent breakthrough behavior of infiltrating DO13C solution can be analyzed. Using single flow cells, physical, chemical, and biological properties can be mapped on the undisturbed soil particle interfaces. Two central questions are addressed: (i) whether and how are hotspots of microorganisms activity related to either physical or physicochemical properties of the spore space and (ii) what are the specific physical and interfacial interactions between soil and roots with special focus on root stage (root density, morphology) and rhizosphere wettability. To answer these questions, mapping of various parameters will be done in close cooperation with partner projects. Physicochemical behavior of the interfaces is assessed by wettability measurements (sessile drop contact angle determination, ESEM condensation experiments), whereas soil organic matter content and quality, i.e. hydrophobicity index derived by DRIFT-mapping,is assessed by calibrated MIR spectroscopy. Biological parameters (respiration, exo-enzyme activity, decay of 14C labeled glucose) will be measured to relate interfacial properties with respective biological responses. Flow cells will be sampled on 3 sites with different parent material in close cooperation with other projects. Overall, this study aims to offer new insights on the assessment and the relevance of small-scale structural and interfacial heterogeneity for SOM stabilization processes in less structured forest subsoils.