Spatial patterns of organic matter formation in subsoil: Formation of mineral-associated organic matter

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Within the research unit, we provided strong evidence from subsoil observatories and field manipulation experiments that dissolved organic matter (DOM) is translocated through soils by multiple sorption-desorption events. We showed that highly organic matter (OM)-loaded minerals are microbial hotspots, and microbial activity is strongly involved in the exchange processes of OM on mineral surfaces. Results obtained from segmented suction plates revealed a large and stable spatial heterogeneity of water and dissolved organic carbon (DOC) fluxes as well as of DOM com-position. The differentiation between preferential flow paths and matric soil suggests a horizontal control of OM retention. Based on these findings, we here aim at investigating how the small-scale horizontal heterogeneity of the DOM dynamics translates into a small-scale horizontal heterogeneity in the formation and properties of mineral-associated organic matter (MAOM). We hypothesize that in preferential flow paths, the large input of easily available OM fuels microbial hot spots on mineral surfaces, thus creating conditions for a dominating in vivo microbial turnover pathway in the for-mation of MAOM. In contrast, under matric flow conditions with less OM input and minerals of low C loading, direct sorption including more plant-derived compounds prevails. The proposed project bases on the small-scale pattern of DOC fluxes in the segmented suction plates of the subsoil ob-servatories identified within. Soil located directly above each segment will be sampled and related to domains of preferential or matric flow. These different domains will be analyzed for physical and mineralogical characteristics that are important drivers for OM mobility and retention. A comparison of 13C in the soil above the individual segments of suction plates with the cumulative 13C flux/retention in these domains will provide information on the spatially distinct retention of the litter-derived DOM. The analysis of nutrients and biomarkers (lignin, amino sugars, extracellular polymeric substances) as well as spectroscopic data based on X-ray photoelectron spectroscopy will inform on the source and degree of microbial transformation of the OM in the different flow domains. These data on the pathways of MAOM formation will be linked to the bioavailability of MAOM and to information on different microbial parameters, provided by partner projects. The proposed project directly addresses the overarching hypothesis of the proposal package that matric and preferential flow path domains are major determinants in the formation of subsoil OM as they control the spatiotemporal patterns of water and matter fluxes and provide habitats for microbial OM turnover that differ from matric soil.